Develop Reference

Comparison method violates its general contract! while sorting [duplicate]

I saw many questions about this, and tried to solve the problem, but after one hour of googling and a lots of trial & error, I still can't fix it. I hope some of you catch the problem.
This is what I get:
java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.ComparableTimSort.mergeHi(ComparableTimSort.java:835)
at java.util.ComparableTimSort.mergeAt(ComparableTimSort.java:453)
at java.util.ComparableTimSort.mergeForceCollapse(ComparableTimSort.java:392)
at java.util.ComparableTimSort.sort(ComparableTimSort.java:191)
at java.util.ComparableTimSort.sort(ComparableTimSort.java:146)
at java.util.Arrays.sort(Arrays.java:472)
at java.util.Collections.sort(Collections.java:155)
...
And this is my comparator:
#Override
public int compareTo(Object o) {
if(this == o){
return 0;
}
CollectionItem item = (CollectionItem) o;
Card card1 = CardCache.getInstance().getCard(cardId);
Card card2 = CardCache.getInstance().getCard(item.getCardId());
if (card1.getSet() < card2.getSet()) {
return -1;
} else {
if (card1.getSet() == card2.getSet()) {
if (card1.getRarity() < card2.getRarity()) {
return 1;
} else {
if (card1.getId() == card2.getId()) {
if (cardType > item.getCardType()) {
return 1;
} else {
if (cardType == item.getCardType()) {
return 0;
}
return -1;
}
}
return -1;
}
}
return 1;
}
}
Any idea?

The exception message is actually pretty descriptive. The contract it mentions is transitivity: if A > B and B > C then for any A, B and C: A > C. I checked it with paper and pencil and your code seems to have few holes:
if (card1.getRarity() < card2.getRarity()) {
return 1;
you do not return -1 if card1.getRarity() > card2.getRarity().
if (card1.getId() == card2.getId()) {
//...
}
return -1;
You return -1 if ids aren't equal. You should return -1 or 1 depending on which id was bigger.
Take a look at this. Apart from being much more readable, I think it should actually work:
if (card1.getSet() > card2.getSet()) {
return 1;
}
if (card1.getSet() < card2.getSet()) {
return -1;
};
if (card1.getRarity() < card2.getRarity()) {
return 1;
}
if (card1.getRarity() > card2.getRarity()) {
return -1;
}
if (card1.getId() > card2.getId()) {
return 1;
}
if (card1.getId() < card2.getId()) {
return -1;
}
return cardType - item.getCardType(); //watch out for overflow!

You can use the following class to pinpoint transitivity bugs in your Comparators:
/**
* #author Gili Tzabari
*/
public final class Comparators
{
/**
* Verify that a comparator is transitive.
*
* #param <T> the type being compared
* #param comparator the comparator to test
* #param elements the elements to test against
* #throws AssertionError if the comparator is not transitive
*/
public static <T> void verifyTransitivity(Comparator<T> comparator, Collection<T> elements)
{
for (T first: elements)
{
for (T second: elements)
{
int result1 = comparator.compare(first, second);
int result2 = comparator.compare(second, first);
if (result1 != -result2)
{
// Uncomment the following line to step through the failed case
//comparator.compare(first, second);
throw new AssertionError("compare(" + first + ", " + second + ") == " + result1 +
" but swapping the parameters returns " + result2);
}
}
}
for (T first: elements)
{
for (T second: elements)
{
int firstGreaterThanSecond = comparator.compare(first, second);
if (firstGreaterThanSecond <= 0)
continue;
for (T third: elements)
{
int secondGreaterThanThird = comparator.compare(second, third);
if (secondGreaterThanThird <= 0)
continue;
int firstGreaterThanThird = comparator.compare(first, third);
if (firstGreaterThanThird <= 0)
{
// Uncomment the following line to step through the failed case
//comparator.compare(first, third);
throw new AssertionError("compare(" + first + ", " + second + ") > 0, " +
"compare(" + second + ", " + third + ") > 0, but compare(" + first + ", " + third + ") == " +
firstGreaterThanThird);
}
}
}
}
}
/**
* Prevent construction.
*/
private Comparators()
{
}
}
Simply invoke Comparators.verifyTransitivity(myComparator, myCollection) in front of the code that fails.

It also has something to do with the version of JDK.
If it does well in JDK6, maybe it will have the problem in JDK 7 described by you, because the implementation method in jdk 7 has been changed.
Look at this:
Description: The sorting algorithm used by java.util.Arrays.sort and (indirectly) by java.util.Collections.sort has been replaced. The new sort implementation may throw an IllegalArgumentException if it detects a Comparable that violates the Comparable contract. The previous implementation silently ignored such a situation. If the previous behavior is desired, you can use the new system property, java.util.Arrays.useLegacyMergeSort, to restore previous mergesort behaviour.
I don't know the exact reason. However, if you add the code before you use sort. It will be OK.
System.setProperty("java.util.Arrays.useLegacyMergeSort", "true");

Consider the following case:
First, o1.compareTo(o2) is called. card1.getSet() == card2.getSet() happens to be true and so is card1.getRarity() < card2.getRarity(), so you return 1.
Then, o2.compareTo(o1) is called. Again, card1.getSet() == card2.getSet() is true. Then, you skip to the following else, then card1.getId() == card2.getId() happens to be true, and so is cardType > item.getCardType(). You return 1 again.
From that, o1 > o2, and o2 > o1. You broke the contract.

if (card1.getRarity() < card2.getRarity()) {
return 1;
However, if card2.getRarity() is less than card1.getRarity() you might not return -1.
You similarly miss other cases. I would do this, you can change around depending on your intent:
public int compareTo(Object o) {
if(this == o){
return 0;
}
CollectionItem item = (CollectionItem) o;
Card card1 = CardCache.getInstance().getCard(cardId);
Card card2 = CardCache.getInstance().getCard(item.getCardId());
int comp=card1.getSet() - card2.getSet();
if (comp!=0){
return comp;
}
comp=card1.getRarity() - card2.getRarity();
if (comp!=0){
return comp;
}
comp=card1.getSet() - card2.getSet();
if (comp!=0){
return comp;
}
comp=card1.getId() - card2.getId();
if (comp!=0){
return comp;
}
comp=card1.getCardType() - card2.getCardType();
return comp;
}
}

I had the same symptom. For me it turned out that another thread was modifying the compared objects while the sorting was happening in a Stream. To resolve the issue, I mapped the objects to immutable temporary objects, collected the Stream to a temporary Collection and did the sorting on that.

The origin of this exception is a wrong Comparator implementation. By checking the docs, we must implement the compare(o1, o2) method as an equivalence relation by following the rules:
if a.equals(b) is true then compare(a, b) is 0
if a.compare(b) > 0 then b.compare(a) < 0 is true
if a.compare(b) > 0 and b.compare(c) > 0 then a.compare(c) > 0 is true
You may check your code to realize where your implementation is offending one or more of Comparator contract rules. If it is hard to find it by a static analysis, you can use the data which cast the exception to check the rules.

If you try to run this code you will meet the kind this exception:
public static void main(String[] args) {
Random random = new Random();
List<Integer> list = new ArrayList<>();
for (int i = 0; i < 50000; i++) {
list.add(random.nextInt());
}
list.sort((x, y) -> {
int c = random.nextInt(3);
if (c == 0) {
return 0;
}
if (c == 1) {
return 1;
}
return -1;
});
}
Exception in thread "main" java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.util.TimSort.mergeLo(TimSort.java:777)
at java.util.TimSort.mergeAt(TimSort.java:514)
at java.util.TimSort.mergeCollapse(TimSort.java:441)
at java.util.TimSort.sort(TimSort.java:245)
at java.util.Arrays.sort(Arrays.java:1512)
at java.util.ArrayList.sort(ArrayList.java:1462)
at Test.main(Test.java:14)
The reason is when implementing the Comparator, it may meet the case of A > B and B > C and C > A and the sort method will be run around to be broken. Java prevent this case by throw exception this case:
class TimSort<T> {
.
.
.
else if (len1 == 0) {
throw new IllegalArgumentException(
"Comparison method violates its general contract!");
.
.
.
In conclusion, to handle this issue. You have to make sure the comparator will not meet the case of A > B and B > C and C > A.

I got the same error with a class like the following StockPickBean. Called from this code:
List<StockPickBean> beansListcatMap.getValue();
beansList.sort(StockPickBean.Comparators.VALUE);
public class StockPickBean implements Comparable<StockPickBean> {
private double value;
public double getValue() { return value; }
public void setValue(double value) { this.value = value; }
#Override
public int compareTo(StockPickBean view) {
return Comparators.VALUE.compare(this,view); //return
Comparators.SYMBOL.compare(this,view);
}
public static class Comparators {
public static Comparator<StockPickBean> VALUE = (val1, val2) ->
(int)
(val1.value - val2.value);
}
}
After getting the same error:
java.lang.IllegalArgumentException: Comparison method violates its general contract!
I changed this line:
public static Comparator<StockPickBean> VALUE = (val1, val2) -> (int)
(val1.value - val2.value);
to:
public static Comparator<StockPickBean> VALUE = (StockPickBean spb1,
StockPickBean spb2) -> Double.compare(spb2.value,spb1.value);
That fixes the error.

I ran into a similar problem where I was trying to sort a n x 2 2D array named contests which is a 2D array of simple integers. This was working for most of the times but threw a runtime error for one input:-
Arrays.sort(contests, (row1, row2) -> {
if (row1[0] < row2[0]) {
return 1;
} else return -1;
});
Error:-
Exception in thread "main" java.lang.IllegalArgumentException: Comparison method violates its general contract!
at java.base/java.util.TimSort.mergeHi(TimSort.java:903)
at java.base/java.util.TimSort.mergeAt(TimSort.java:520)
at java.base/java.util.TimSort.mergeForceCollapse(TimSort.java:461)
at java.base/java.util.TimSort.sort(TimSort.java:254)
at java.base/java.util.Arrays.sort(Arrays.java:1441)
at com.hackerrank.Solution.luckBalance(Solution.java:15)
at com.hackerrank.Solution.main(Solution.java:49)
Looking at the answers above I tried adding a condition for equals and I don't know why but it worked. Hopefully we must explicitly specify what should be returned for all cases (greater than, equals and less than):
Arrays.sort(contests, (row1, row2) -> {
if (row1[0] < row2[0]) {
return 1;
}
if(row1[0] == row2[0]) return 0;
return -1;
});

A variation of Gili's answer to check if the comparator satisfies the requirements described in the compare method's javadoc - with a focus on completeness and readability, e.g. by naming the variables the same as in the javadoc. Note that this is O(n^3), only use it when debugging, maybe just on a subset of your elements, in order to be fast enough to finish at all.
public static <T> void verifyComparator(Comparator<T> comparator, Collection<T> elements) {
for (T x : elements) {
for (T y : elements) {
for (T z : elements) {
int x_y = comparator.compare(x, y);
int y_x = comparator.compare(y, x);
int y_z = comparator.compare(y, z);
int x_z = comparator.compare(x, z);
// javadoc: The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y, x))
if (Math.signum(x_y) == -Math.signum(y_x)) { // ok
} else {
System.err.println("not holding: sgn(compare(x, y)) == -sgn(compare(y, x))" //
+ " | x_y: " + x_y + ", y_x: " + y_x + ", x: " + x + ", y: " + y);
}
// javadoc: The implementor must also ensure that the relation is transitive:
// ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0.
if (x_y > 0 && y_z > 0) {
if (x_z > 0) { // ok
} else {
System.err.println("not holding: ((compare(x, y)>0) && (compare(y, z)>0)) implies compare(x, z)>0" //
+ " | x_y: " + x_y + ", y_z: " + y_z + ", x_z: " + x_z + ", x: " + x + ", y: " + y + ", z: " + z);
}
}
// javadoc: Finally, the implementor must ensure that:
// compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z.
if (x_y == 0) {
if (Math.signum(x_z) == Math.signum(y_z)) { // ok
} else {
System.err.println("not holding: compare(x, y)==0 implies that sgn(compare(x, z))==sgn(compare(y, z)) for all z" //
+ " | x_y: " + x_y + ", x_z: " + x_z + ", y_z: " + y_z + ", x: " + x + ", y: " + y + ", z: " + z);
}
}
}
}
}
}

I had to sort on several criterion (date, and, if same date; other things...). What was working on Eclipse with an older version of Java, did not worked any more on Android : comparison method violates contract ...
After reading on StackOverflow, I wrote a separate function that I called from compare() if the dates are the same. This function calculates the priority, according to the criteria, and returns -1, 0, or 1 to compare(). It seems to work now.

What about doing something simpler like this:
int result = card1.getSet().compareTo(card2.getSet())
if (result == 0) {
result = card1.getRarity().compareTo(card2.getRarity())
}
if (result == 0) {
result = card1.getId().compareTo(card2.getId())
}
if (result == 0) {
result = card1.getCardType().compareTo(card2.getCardType())
}
return result;
You just need to order the comparisons in order of preference.

How to change edittext string sum to int [duplicate]

I'm trying to write a Java routine to evaluate math expressions from String values like:
"5+3"
"10-4*5"
"(1+10)*3"
I want to avoid a lot of if-then-else statements.
How can I do this?

With JDK1.6, you can use the built-in Javascript engine.
import javax.script.ScriptEngineManager;
import javax.script.ScriptEngine;
import javax.script.ScriptException;
public class Test {
public static void main(String[] args) throws ScriptException {
ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavaScript");
String foo = "40+2";
System.out.println(engine.eval(foo));
}
}

I've written this eval method for arithmetic expressions to answer this question. It does addition, subtraction, multiplication, division, exponentiation (using the ^ symbol), and a few basic functions like sqrt. It supports grouping using (...), and it gets the operator precedence and associativity rules correct.
public static double eval(final String str) {
return new Object() {
int pos = -1, ch;
void nextChar() {
ch = (++pos < str.length()) ? str.charAt(pos) : -1;
}
boolean eat(int charToEat) {
while (ch == ' ') nextChar();
if (ch == charToEat) {
nextChar();
return true;
}
return false;
}
double parse() {
nextChar();
double x = parseExpression();
if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);
return x;
}
// Grammar:
// expression = term | expression `+` term | expression `-` term
// term = factor | term `*` factor | term `/` factor
// factor = `+` factor | `-` factor | `(` expression `)` | number
// | functionName `(` expression `)` | functionName factor
// | factor `^` factor
double parseExpression() {
double x = parseTerm();
for (;;) {
if (eat('+')) x += parseTerm(); // addition
else if (eat('-')) x -= parseTerm(); // subtraction
else return x;
}
}
double parseTerm() {
double x = parseFactor();
for (;;) {
if (eat('*')) x *= parseFactor(); // multiplication
else if (eat('/')) x /= parseFactor(); // division
else return x;
}
}
double parseFactor() {
if (eat('+')) return +parseFactor(); // unary plus
if (eat('-')) return -parseFactor(); // unary minus
double x;
int startPos = this.pos;
if (eat('(')) { // parentheses
x = parseExpression();
if (!eat(')')) throw new RuntimeException("Missing ')'");
} else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();
x = Double.parseDouble(str.substring(startPos, this.pos));
} else if (ch >= 'a' && ch <= 'z') { // functions
while (ch >= 'a' && ch <= 'z') nextChar();
String func = str.substring(startPos, this.pos);
if (eat('(')) {
x = parseExpression();
if (!eat(')')) throw new RuntimeException("Missing ')' after argument to " + func);
} else {
x = parseFactor();
}
if (func.equals("sqrt")) x = Math.sqrt(x);
else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));
else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));
else if (func.equals("tan")) x = Math.tan(Math.toRadians(x));
else throw new RuntimeException("Unknown function: " + func);
} else {
throw new RuntimeException("Unexpected: " + (char)ch);
}
if (eat('^')) x = Math.pow(x, parseFactor()); // exponentiation
return x;
}
}.parse();
}
Example:
System.out.println(eval("((4 - 2^3 + 1) * -sqrt(3*3+4*4)) / 2"));
Output: 7.5 (which is correct)
The parser is a recursive descent parser, so internally uses separate parse methods for each level of operator precedence in its grammar. I deliberately kept it short, but here are some ideas you might want to expand it with:
Variables:
The bit of the parser that reads the names for functions can easily be changed to handle custom variables too, by looking up names in a variable table passed to the eval method, such as a Map<String,Double> variables.
Separate compilation and evaluation:
What if, having added support for variables, you wanted to evaluate the same expression millions of times with changed variables, without parsing it every time? It's possible. First define an interface to use to evaluate the precompiled expression:
#FunctionalInterface
interface Expression {
double eval();
}
Now to rework the original "eval" function into a "parse" function, change all the methods that return doubles, so instead they return an instance of that interface. Java 8's lambda syntax works well for this. Example of one of the changed methods:
Expression parseExpression() {
Expression x = parseTerm();
for (;;) {
if (eat('+')) { // addition
Expression a = x, b = parseTerm();
x = (() -> a.eval() + b.eval());
} else if (eat('-')) { // subtraction
Expression a = x, b = parseTerm();
x = (() -> a.eval() - b.eval());
} else {
return x;
}
}
}
That builds a recursive tree of Expression objects representing the compiled expression (an abstract syntax tree). Then you can compile it once and evaluate it repeatedly with different values:
public static void main(String[] args) {
Map<String,Double> variables = new HashMap<>();
Expression exp = parse("x^2 - x + 2", variables);
for (double x = -20; x <= +20; x++) {
variables.put("x", x);
System.out.println(x + " => " + exp.eval());
}
}
Different datatypes:
Instead of double, you could change the evaluator to use something more powerful like BigDecimal, or a class that implements complex numbers, or rational numbers (fractions). You could even use Object, allowing some mix of datatypes in expressions, just like a real programming language. :)
All code in this answer released to the public domain. Have fun!

For my university project, I was looking for a parser / evaluator supporting both basic formulas and more complicated equations (especially iterated operators). I found very nice open source library for JAVA and .NET called mXparser. I will give a few examples to make some feeling on the syntax, for further instructions please visit project website (especially tutorial section).
https://mathparser.org/
https://mathparser.org/mxparser-tutorial/
https://mathparser.org/api/
And few examples
1 - Simple furmula
Expression e = new Expression("( 2 + 3/4 + sin(pi) )/2");
double v = e.calculate()
2 - User defined arguments and constants
Argument x = new Argument("x = 10");
Constant a = new Constant("a = pi^2");
Expression e = new Expression("cos(a*x)", x, a);
double v = e.calculate()
3 - User defined functions
Function f = new Function("f(x, y, z) = sin(x) + cos(y*z)");
Expression e = new Expression("f(3,2,5)", f);
double v = e.calculate()
4 - Iteration
Expression e = new Expression("sum( i, 1, 100, sin(i) )");
double v = e.calculate()
Found recently - in case you would like to try the syntax (and see the advanced use case) you can download the Scalar Calculator app that is powered by mXparser.

The correct way to solve this is with a lexer and a parser. You can write simple versions of these yourself, or those pages also have links to Java lexers and parsers.
Creating a recursive descent parser is a really good learning exercise.

HERE is another open source library on GitHub named EvalEx.
Unlike the JavaScript engine this library is focused in evaluating mathematical expressions only. Moreover, the library is extensible and supports use of boolean operators as well as parentheses.

You can also try the BeanShell interpreter:
Interpreter interpreter = new Interpreter();
interpreter.eval("result = (7+21*6)/(32-27)");
System.out.println(interpreter.get("result"));

You can evaluate expressions easily if your Java application already accesses a database, without using any other JARs.
Some databases require you to use a dummy table (eg, Oracle's "dual" table) and others will allow you to evaluate expressions without "selecting" from any table.
For example, in Sql Server or Sqlite
select (((12.10 +12.0))/ 233.0) amount
and in Oracle
select (((12.10 +12.0))/ 233.0) amount from dual;
The advantage of using a DB is that you can evaluate many expressions at the same time. Also most DB's will allow you to use highly complex expressions and will also have a number of extra functions that can be called as necessary.
However performance may suffer if many single expressions need to be evaluated individually, particularly when the DB is located on a network server.
The following addresses the performance problem to some extent, by using a Sqlite in-memory database.
Here's a full working example in Java
Class. forName("org.sqlite.JDBC");
Connection conn = DriverManager.getConnection("jdbc:sqlite::memory:");
Statement stat = conn.createStatement();
ResultSet rs = stat.executeQuery( "select (1+10)/20.0 amount");
rs.next();
System.out.println(rs.getBigDecimal(1));
stat.close();
conn.close();
Of course you could extend the above code to handle multiple calculations at the same time.
ResultSet rs = stat.executeQuery( "select (1+10)/20.0 amount, (1+100)/20.0 amount2");

Another way is to use the Spring Expression Language or SpEL which does a whole lot more along with evaluating mathematical expressions, therefore maybe slightly overkill. You do not have to be using Spring framework to use this expression library as it is stand-alone. Copying examples from SpEL's documentation:
ExpressionParser parser = new SpelExpressionParser();
int two = parser.parseExpression("1 + 1").getValue(Integer.class); // 2
double twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double.class); //24.0

This article discusses various approaches. Here are the 2 key approaches mentioned in the article:
JEXL from Apache
Allows for scripts that include references to java objects.
// Create or retrieve a JexlEngine
JexlEngine jexl = new JexlEngine();
// Create an expression object
String jexlExp = "foo.innerFoo.bar()";
Expression e = jexl.createExpression( jexlExp );
// Create a context and add data
JexlContext jctx = new MapContext();
jctx.set("foo", new Foo() );
// Now evaluate the expression, getting the result
Object o = e.evaluate(jctx);
Use the javascript engine embedded in the JDK:
private static void jsEvalWithVariable()
{
List<String> namesList = new ArrayList<String>();
namesList.add("Jill");
namesList.add("Bob");
namesList.add("Laureen");
namesList.add("Ed");
ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine jsEngine = mgr.getEngineByName("JavaScript");
jsEngine.put("namesListKey", namesList);
System.out.println("Executing in script environment...");
try
{
jsEngine.eval("var x;" +
"var names = namesListKey.toArray();" +
"for(x in names) {" +
" println(names[x]);" +
"}" +
"namesListKey.add(\"Dana\");");
}
catch (ScriptException ex)
{
ex.printStackTrace();
}
}

if we are going to implement it then we can can use the below algorithm :--
While there are still tokens to be read in,
1.1 Get the next token.
1.2 If the token is:
1.2.1 A number: push it onto the value stack.
1.2.2 A variable: get its value, and push onto the value stack.
1.2.3 A left parenthesis: push it onto the operator stack.
1.2.4 A right parenthesis:
1 While the thing on top of the operator stack is not a
left parenthesis,
1 Pop the operator from the operator stack.
2 Pop the value stack twice, getting two operands.
3 Apply the operator to the operands, in the correct order.
4 Push the result onto the value stack.
2 Pop the left parenthesis from the operator stack, and discard it.
1.2.5 An operator (call it thisOp):
1 While the operator stack is not empty, and the top thing on the
operator stack has the same or greater precedence as thisOp,
1 Pop the operator from the operator stack.
2 Pop the value stack twice, getting two operands.
3 Apply the operator to the operands, in the correct order.
4 Push the result onto the value stack.
2 Push thisOp onto the operator stack.
While the operator stack is not empty,
1 Pop the operator from the operator stack.
2 Pop the value stack twice, getting two operands.
3 Apply the operator to the operands, in the correct order.
4 Push the result onto the value stack.
At this point the operator stack should be empty, and the value
stack should have only one value in it, which is the final result.

This is another interesting alternative
https://github.com/Shy-Ta/expression-evaluator-demo
The usage is very simple and gets the job done, for example:
ExpressionsEvaluator evalExpr = ExpressionsFactory.create("2+3*4-6/2");
assertEquals(BigDecimal.valueOf(11), evalExpr.eval());

It seems like JEP should do the job

It's too late to answer but I came across same situation to evaluate expression in java, it might help someone
MVEL does runtime evaluation of expressions, we can write a java code in String to get it evaluated in this.
String expressionStr = "x+y";
Map<String, Object> vars = new HashMap<String, Object>();
vars.put("x", 10);
vars.put("y", 20);
ExecutableStatement statement = (ExecutableStatement) MVEL.compileExpression(expressionStr);
Object result = MVEL.executeExpression(statement, vars);

Try the following sample code using JDK1.6's Javascript engine with code injection handling.
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
public class EvalUtil {
private static ScriptEngine engine = new ScriptEngineManager().getEngineByName("JavaScript");
public static void main(String[] args) {
try {
System.out.println((new EvalUtil()).eval("(((5+5)/2) > 5) || 5 >3 "));
System.out.println((new EvalUtil()).eval("(((5+5)/2) > 5) || true"));
} catch (Exception e) {
e.printStackTrace();
}
}
public Object eval(String input) throws Exception{
try {
if(input.matches(".*[a-zA-Z;~`#$_{}\\[\\]:\\\\;\"',\\.\\?]+.*")) {
throw new Exception("Invalid expression : " + input );
}
return engine.eval(input);
} catch (Exception e) {
e.printStackTrace();
throw e;
}
}
}

This is actually complementing the answer given by #Boann. It has a slight bug which causes "-2 ^ 2" to give an erroneous result of -4.0. The problem for that is the point at which the exponentiation is evaluated in his. Just move the exponentiation to the block of parseTerm(), and you'll be all fine. Have a look at the below, which is #Boann's answer slightly modified. Modification is in the comments.
public static double eval(final String str) {
return new Object() {
int pos = -1, ch;
void nextChar() {
ch = (++pos < str.length()) ? str.charAt(pos) : -1;
}
boolean eat(int charToEat) {
while (ch == ' ') nextChar();
if (ch == charToEat) {
nextChar();
return true;
}
return false;
}
double parse() {
nextChar();
double x = parseExpression();
if (pos < str.length()) throw new RuntimeException("Unexpected: " + (char)ch);
return x;
}
// Grammar:
// expression = term | expression `+` term | expression `-` term
// term = factor | term `*` factor | term `/` factor
// factor = `+` factor | `-` factor | `(` expression `)`
// | number | functionName factor | factor `^` factor
double parseExpression() {
double x = parseTerm();
for (;;) {
if (eat('+')) x += parseTerm(); // addition
else if (eat('-')) x -= parseTerm(); // subtraction
else return x;
}
}
double parseTerm() {
double x = parseFactor();
for (;;) {
if (eat('*')) x *= parseFactor(); // multiplication
else if (eat('/')) x /= parseFactor(); // division
else if (eat('^')) x = Math.pow(x, parseFactor()); //exponentiation -> Moved in to here. So the problem is fixed
else return x;
}
}
double parseFactor() {
if (eat('+')) return parseFactor(); // unary plus
if (eat('-')) return -parseFactor(); // unary minus
double x;
int startPos = this.pos;
if (eat('(')) { // parentheses
x = parseExpression();
eat(')');
} else if ((ch >= '0' && ch <= '9') || ch == '.') { // numbers
while ((ch >= '0' && ch <= '9') || ch == '.') nextChar();
x = Double.parseDouble(str.substring(startPos, this.pos));
} else if (ch >= 'a' && ch <= 'z') { // functions
while (ch >= 'a' && ch <= 'z') nextChar();
String func = str.substring(startPos, this.pos);
x = parseFactor();
if (func.equals("sqrt")) x = Math.sqrt(x);
else if (func.equals("sin")) x = Math.sin(Math.toRadians(x));
else if (func.equals("cos")) x = Math.cos(Math.toRadians(x));
else if (func.equals("tan")) x = Math.tan(Math.toRadians(x));
else throw new RuntimeException("Unknown function: " + func);
} else {
throw new RuntimeException("Unexpected: " + (char)ch);
}
//if (eat('^')) x = Math.pow(x, parseFactor()); // exponentiation -> This is causing a bit of problem
return x;
}
}.parse();
}

import java.util.*;
public class check {
int ans;
String str="7 + 5";
StringTokenizer st=new StringTokenizer(str);
int v1=Integer.parseInt(st.nextToken());
String op=st.nextToken();
int v2=Integer.parseInt(st.nextToken());
if(op.equals("+")) { ans= v1 + v2; }
if(op.equals("-")) { ans= v1 - v2; }
//.........
}

I think what ever way you do this it's going to involve a lot of conditional statements. But for single operations like in your examples you could limit it to 4 if statements with something like
String math = "1+4";
if (math.split("+").length == 2) {
//do calculation
} else if (math.split("-").length == 2) {
//do calculation
} ...
It gets a whole lot more complicated when you want to deal with multiple operations like "4+5*6".
If you are trying to build a calculator then I'd surgest passing each section of the calculation separatly (each number or operator) rather than as a single string.

You might have a look at the Symja framework:
ExprEvaluator util = new ExprEvaluator();
IExpr result = util.evaluate("10-40");
System.out.println(result.toString()); // -> "-30"
Take note that definitively more complex expressions can be evaluated:
// D(...) gives the derivative of the function Sin(x)*Cos(x)
IAST function = D(Times(Sin(x), Cos(x)), x);
IExpr result = util.evaluate(function);
// print: Cos(x)^2-Sin(x)^2

package ExpressionCalculator.expressioncalculator;
import java.text.DecimalFormat;
import java.util.Scanner;
public class ExpressionCalculator {
private static String addSpaces(String exp){
//Add space padding to operands.
//https://regex101.com/r/sJ9gM7/73
exp = exp.replaceAll("(?<=[0-9()])[\\/]", " / ");
exp = exp.replaceAll("(?<=[0-9()])[\\^]", " ^ ");
exp = exp.replaceAll("(?<=[0-9()])[\\*]", " * ");
exp = exp.replaceAll("(?<=[0-9()])[+]", " + ");
exp = exp.replaceAll("(?<=[0-9()])[-]", " - ");
//Keep replacing double spaces with single spaces until your string is properly formatted
/*while(exp.indexOf(" ") != -1){
exp = exp.replace(" ", " ");
}*/
exp = exp.replaceAll(" {2,}", " ");
return exp;
}
public static Double evaluate(String expr){
DecimalFormat df = new DecimalFormat("#.####");
//Format the expression properly before performing operations
String expression = addSpaces(expr);
try {
//We will evaluate using rule BDMAS, i.e. brackets, division, power, multiplication, addition and
//subtraction will be processed in following order
int indexClose = expression.indexOf(")");
int indexOpen = -1;
if (indexClose != -1) {
String substring = expression.substring(0, indexClose);
indexOpen = substring.lastIndexOf("(");
substring = substring.substring(indexOpen + 1).trim();
if(indexOpen != -1 && indexClose != -1) {
Double result = evaluate(substring);
expression = expression.substring(0, indexOpen).trim() + " " + result + " " + expression.substring(indexClose + 1).trim();
return evaluate(expression.trim());
}
}
String operation = "";
if(expression.indexOf(" / ") != -1){
operation = "/";
}else if(expression.indexOf(" ^ ") != -1){
operation = "^";
} else if(expression.indexOf(" * ") != -1){
operation = "*";
} else if(expression.indexOf(" + ") != -1){
operation = "+";
} else if(expression.indexOf(" - ") != -1){ //Avoid negative numbers
operation = "-";
} else{
return Double.parseDouble(expression);
}
int index = expression.indexOf(operation);
if(index != -1){
indexOpen = expression.lastIndexOf(" ", index - 2);
indexOpen = (indexOpen == -1)?0:indexOpen;
indexClose = expression.indexOf(" ", index + 2);
indexClose = (indexClose == -1)?expression.length():indexClose;
if(indexOpen != -1 && indexClose != -1) {
Double lhs = Double.parseDouble(expression.substring(indexOpen, index));
Double rhs = Double.parseDouble(expression.substring(index + 2, indexClose));
Double result = null;
switch (operation){
case "/":
//Prevent divide by 0 exception.
if(rhs == 0){
return null;
}
result = lhs / rhs;
break;
case "^":
result = Math.pow(lhs, rhs);
break;
case "*":
result = lhs * rhs;
break;
case "-":
result = lhs - rhs;
break;
case "+":
result = lhs + rhs;
break;
default:
break;
}
if(indexClose == expression.length()){
expression = expression.substring(0, indexOpen) + " " + result + " " + expression.substring(indexClose);
}else{
expression = expression.substring(0, indexOpen) + " " + result + " " + expression.substring(indexClose + 1);
}
return Double.valueOf(df.format(evaluate(expression.trim())));
}
}
}catch(Exception exp){
exp.printStackTrace();
}
return 0.0;
}
public static void main(String args[]){
Scanner scanner = new Scanner(System.in);
System.out.print("Enter an Mathematical Expression to Evaluate: ");
String input = scanner.nextLine();
System.out.println(evaluate(input));
}
}

A Java class that can evaluate mathematical expressions:
package test;
public class Calculator {
public static Double calculate(String expression){
if (expression == null || expression.length() == 0) {
return null;
}
return calc(expression.replace(" ", ""));
}
public static Double calc(String expression) {
String[] containerArr = new String[]{expression};
double leftVal = getNextOperand(containerArr);
expression = containerArr[0];
if (expression.length() == 0) {
return leftVal;
}
char operator = expression.charAt(0);
expression = expression.substring(1);
while (operator == '*' || operator == '/') {
containerArr[0] = expression;
double rightVal = getNextOperand(containerArr);
expression = containerArr[0];
if (operator == '*') {
leftVal = leftVal * rightVal;
} else {
leftVal = leftVal / rightVal;
}
if (expression.length() > 0) {
operator = expression.charAt(0);
expression = expression.substring(1);
} else {
return leftVal;
}
}
if (operator == '+') {
return leftVal + calc(expression);
} else {
return leftVal - calc(expression);
}
}
private static double getNextOperand(String[] exp){
double res;
if (exp[0].startsWith("(")) {
int open = 1;
int i = 1;
while (open != 0) {
if (exp[0].charAt(i) == '(') {
open++;
} else if (exp[0].charAt(i) == ')') {
open--;
}
i++;
}
res = calc(exp[0].substring(1, i - 1));
exp[0] = exp[0].substring(i);
} else {
int i = 1;
if (exp[0].charAt(0) == '-') {
i++;
}
while (exp[0].length() > i && isNumber((int) exp[0].charAt(i))) {
i++;
}
res = Double.parseDouble(exp[0].substring(0, i));
exp[0] = exp[0].substring(i);
}
return res;
}
private static boolean isNumber(int c) {
int zero = (int) '0';
int nine = (int) '9';
return (c >= zero && c <= nine) || c =='.';
}
public static void main(String[] args) {
System.out.println(calculate("(((( -6 )))) * 9 * -1"));
System.out.println(calc("(-5.2+-5*-5*((5/4+2)))"));
}
}

How about something like this:
String st = "10+3";
int result;
for(int i=0;i<st.length();i++)
{
if(st.charAt(i)=='+')
{
result=Integer.parseInt(st.substring(0, i))+Integer.parseInt(st.substring(i+1, st.length()));
System.out.print(result);
}
}
and do the similar thing for every other mathematical operator accordingly ..

It is possible to convert any expression string in infix notation to a postfix notation using Djikstra's shunting-yard algorithm. The result of the algorithm can then serve as input to the postfix algorithm with returns the result of the expression.
I wrote an article about it here, with an implementation in java

Yet another option: https://github.com/stefanhaustein/expressionparser
I have implemented this to have a simple but flexible option to permit both:
Immediate processing (Calculator.java, SetDemo.java)
Building and processing a parse tree (TreeBuilder.java)
The TreeBuilder linked above is part of a CAS demo package that does symbolic derivation. There is also a BASIC interpreter example and I have started to build a TypeScript interpreter using it.

External library like RHINO or NASHORN can be used to run javascript. And javascript can evaluate simple formula without parcing the string. No performance impact as well if code is written well.
Below is an example with RHINO -
public class RhinoApp {
private String simpleAdd = "(12+13+2-2)*2+(12+13+2-2)*2";
public void runJavaScript() {
Context jsCx = Context.enter();
Context.getCurrentContext().setOptimizationLevel(-1);
ScriptableObject scope = jsCx.initStandardObjects();
Object result = jsCx.evaluateString(scope, simpleAdd , "formula", 0, null);
Context.exit();
System.out.println(result);
}

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
public class test2 {
public static void main(String[] args) throws ScriptException {
String s = "10+2";
ScriptEngineManager mn = new ScriptEngineManager();
ScriptEngine en = mn.getEngineByName("js");
Object result = en.eval(s);
System.out.println(result);
}
}

I have done using iterative parsing and shunting Yard algorithm and i have really enjoyed developing the expression evaluator ,you can find all the code here
https://github.com/nagaraj200788/JavaExpressionEvaluator
Has 73 test cases and even works for Bigintegers,Bigdecimals
supports all relational, arithmetic expression and also combination of both .
even supports ternary operator .
Added enhancement to support signed numbers like -100+89 it was intresting, for details check TokenReader.isUnaryOperator() method and i have updated code in above Link

Can Xposed log current method name?

I want to test an application but it's too large to do by static approach.
I started googling. I found a interesting tool called Xposed Framework.
I read a lot of documents / examples. But I cant find the theads about getting method name of application.
My purpose is to log current method name when I press a button in app. Which parameters are sent when the methods are called?
For more information, the application I want to test is a chat application. I want to check that is it secured to use ? Is it true that developers claims the application uses blah blah blah encryption ? Is it real end-to-end encryption?.
According to the large of application, I need some tools to help me analyse this. When I send a message what methods are called ? what values are sent along with ?

If it is opensource you can easily insert a few logs in the source code and recompile. Some code coverage tools allow you to log the executed methods but i am unsure about the parameters (e.g. EMMA coverage).
If it is closed-source then you can do it with Xposed, but it has some challenges. Xposed allows you to hook Java methods, if it is opensource you can lookup the specific methods you want to intercept and print their parameters. If it is closed source, you can always check the method names through decompiling the app with apktool.
Check the Xposed tutorial on how to register hooks. Assuming you created your class that extends XC_MethodHook, the following methods should do the trick for primitive parameters:
#Override
protected void afterHookedMethod(MethodHookParam param) throws Throwable {
String threadname = Thread.currentThread().getName();
String d = _dateFormat.format(new Date());
Class<?> cls = param.this.getClass();
//Extend XC_MethodHook to receive the mthName in the constructor
//and store it as a static field.
Log.v("MethodInvocation", "[A][" + d + "]"
+ "[" + cls.getName() + "."
+ mthName
+ "(" + args(param) + ")" + " = " + result(param)
+ "]" + " [" + threadname + "]");
}
public boolean shouldPrintContent(Object o) {
if (o.getClass().isPrimitive()
|| o.getClass().getName().contains("java.lang"))
return true;
return false;
}
public String args(MethodHookParam param) {
String args = "";
int counter = 0;
if (param != null) {
for (Object o : param.args) {
if (counter > 0)
args += ",";
// argument can be null
if (o == null) {
args += "null";
} else { // if it is an object lets print its type and content.
args += printclean(o.getClass().getName());
if (shouldPrintContent(o)) {
args += ":" + printclean(o.toString());
} else
args += ":nonPrimitiveOrJavaLang";
}
counter++;
}
}
return args;
}
//avoid identation chars in strings
public String printclean(String str) {
char[] res = str.toCharArray();
for (int i = 0; i < str.length(); i++) {
if (res[i] == '\n' || res[i] == '\r' || res[i] == '\t'
|| res[i] == '[' || res[i] == ']') {
res[i] = '*';
}
}
return String.valueOf(res);
}
public String result(MethodHookParam param) {
String res = "";
Object retobj = param.getResult();
if (retobj == null) {
res += "null";
} else {
res += printclean(retobj.getClass().getName());
if (shouldPrintContent(retobj)) {
res += printclean(retobj.toString());
} else
res += "(nonPrimitiveOrJavaLang)";
}
return res;
}
Note that printing any object is not trivial. Here I printed only known types like primitives or other java.lang objects. More complex objects (including collections) you can try using Gson to represent them but this also comes with limitations (e.g. can't often handle reference loops).
Finally, be careful with what method you hook as hooking and logging methods that are called often will impact the performance of the app.
Good luck!

A new Xposed module called Inspeckage does (among many other useful inspections) what you want to do: you can use it to hook any method and view and even change its input and output.

How do I use this boolean. - isBlank

I'm a complete programming noob so go easy...
So I'm wondering how I would go about checking the edittext string to see if it "isBlank" using this isblank Boolean.
I know its probably a very easy answer but I just can't seem to get my head around it.
Any help appreciated.
public static boolean isBlank(String string) {
if (string == null || string.length() == 0)
return true;
int l = string.length();
for (int i = 0; i < l; i++) {
if (!Character.isWhitespace(string.codePointAt(i)))
return false;
}
return true;
}
**Thanks Heaps guys all helped alot!!.. If I could +1 I would.

You can write your method much shorter, like so:
static boolean isBlank(String string) {
return string == null || string.trim().length() == 0;
}
The trim() method removes all whitespace characters from beginning and end of a string. If what remains has length == 0, the whole string must have consisted of whitespace only.
The usage in your code depends on your need, but generally you'll use it in if() statements to make the code more readable:
String foo = "... some string ...";
if (isBlank(foo)) {
// foo is empty or only contains whitespace
} else {
// foo contains some text.
}

You can do this in single line.
if(edittext.getText().toString().trim().length()>0){
Syste.out.println("Not Blank");
}else{
Syste.out.println("Blank");
}

Like this:
if (isBlank(edittext.getText().toString())) {
// Blank
} else {
// Not blank
}

You can do it in this way:
Declare Class level variable:
boolean blank = false;
public static boolean isBlank(String string) {
if (string == null || string.trim().length() == 0){
blank = true;
}
else{
blank = false;
}
return blank;
}

Html List tag not working in android textview. What can I do?

Html List tag not working in android TextView. This is my string content:
String str="A dressy take on classic gingham in a soft, textured weave of stripes that resembles twill. Take a closer look at this one.<ul><li>Trim, tailored fit for a bespoke feel</li><li>Medium spread collar, one-button mitered barrel cuffs</li><li>Applied placket with genuine mother-of-pearl buttons</li><li>;Split back yoke, rear side pleats</li><li>Made in the U.S.A. of 100% imported cotton.</li></ul>";
I loaded it in a text view like this:
textview.setText(Html.fromHtml(str));
The output looks like a paragraph. What can I do? Is there any solution for it?
Edit:
webview.loadData(str,"text/html","utf-8");

As you can see in the Html class source code, Html.fromHtml(String) does not support all HTML tags. In this very case, <ul> and <li> are not supported.
From the source code I have built a list of allowed HTML tags:
br
p
div
em
b
strong
cite
dfn
i
big
small
font
blockquote
tt
monospace
a
u
sup
sub
So you better use WebView and its loadDataWithBaseURL method. Try something like this:
String str="<html><body>A dressy take on classic gingham in a soft, textured weave of stripes that resembles twill. Take a closer look at this one.<ul><li>Trim, tailored fit for a bespoke feel</li><li>Medium spread collar, one-button mitered barrel cuffs</li><li>Applied placket with genuine mother-of-pearl buttons</li><li>;Split back yoke, rear side pleats</li><li>Made in the U.S.A. of 100% imported cotton.</li></ul></body></html>";
webView.loadDataWithBaseURL(null, str, "text/html", "utf-8", null);

I was having the same problem, and what I did is overriding the default TagHandler. This one worked for me.
public class MyTagHandler implements TagHandler {
boolean first = true;
String parent = null;
int index = 1;
#Override
public void handleTag(boolean opening, String tag, Editable output, XMLReader xmlReader) {
if (tag.equals("ul")) {
parent = "ul";
} else if (tag.equals("ol")) {
parent = "ol";
}
if (tag.equals("li")) {
if (parent.equals("ul")) {
if (first) {
output.append("\n\t•");
first = false;
} else {
first = true;
}
} else{
if (first) {
output.append("\n\t"+index+". ");
first = false;
index++;
} else {
first = true;
}
}
}
}
}
and for displaying the text...
myTextView.setText(Html.fromHtml("<ul><li>I am an Android developer</li><li>Another Item</li></ul>", null, new MyTagHandler()));
[Edit]
Kuitsi has also posted an really good library that does the same, got it from this SO link.

Full sample project is located at https://bitbucket.org/Kuitsi/android-textview-html-list.
Sample picture is available at https://kuitsi.bitbucket.io/stackoverflow3150400_screen.png
This solution is closest to masha's answer. Some code is also taken from inner class android.text.Html.HtmlToSpannedConverter. It supports nested ordered and unordered lists but too long texts in ordered lists are still aligned with item number rather than text. Mixed lists (ol and ul) needs some work too. Sample project contains implementation of Html.TagHandler which is passed to Html.fromHtml(String, ImageGetter, TagHandler).
Edit: For wider HTML tag support, https://github.com/NightWhistler/HtmlSpanner might also be worth trying.

A small fix to Aman Guatam code. The function above has problem of rendering newline character. For example: if before <li> tag is a <p> tag, 2 newline characters are rendered. Here is upgraded code:
import org.xml.sax.XMLReader;
import android.text.Editable;
import android.text.Html.TagHandler;
public class ListTagHandler implements TagHandler {
boolean first = true;
#Override
public void handleTag(boolean opening, String tag, Editable output, XMLReader xmlReader) {
// TODO Auto-generated method stub
if (tag.equals("li")) {
char lastChar = 0;
if (output.length() > 0)
lastChar = output.charAt(output.length() - 1);
if (first) {
if (lastChar == '\n')
output.append("\t• ");
else
output.append("\n\t• ");
first = false;
} else {
first = true;
}
}
}
}

WARNING
As of Android 7 android.text.Html actually supports li and ul tags and uses a basic BulletSpan(), which means in the latest versions of Android the Html.TagHandlersolutions posted here will be ignored
Make sure your code handles this change. In case you want a BulletSpan with a larger gap than the default, you can can replace it with another span:
val html = SpannableStringBuilder(HtmlCompat.fromHtml(source, HtmlCompat.FROM_HTML_MODE_COMPACT))
val bulletSpans = html.getSpans<BulletSpan>(0, html.length)
bulletSpans.forEach {
val spanStart = html.getSpanStart(it)
val spanEnd = html.getSpanEnd(it)
html.removeSpan(it)
val bulletSpan = BulletSpan(gapWidthInDp, context.getColor(R.color.textColorBlack))
html.setSpan(bulletSpan, spanStart, spanEnd, Spanned.SPAN_EXCLUSIVE_EXCLUSIVE)
}

Different solution using LeadingMarginSpan. Handles ordered and unordered lists as well as nesting.
public class ListTagHandler implements TagHandler
{
private int m_index = 0;
private List< String > m_parents = new ArrayList< String >( );
#Override
public void handleTag( final boolean opening, final String tag, Editable output, final XMLReader xmlReader )
{
if( tag.equals( "ul" ) || tag.equals( "ol" ) || tag.equals( "dd" ) )
{
if( opening )
{
m_parents.add( tag );
}
else m_parents.remove( tag );
m_index = 0;
}
else if( tag.equals( "li" ) && !opening ) handleListTag( output );
}
private void handleListTag( Editable output )
{
if( m_parents.get(m_parents.size()-1 ).equals( "ul" ) )
{
output.append( "\n" );
String[ ] split = output.toString( ).split( "\n" );
int lastIndex = split.length - 1;
int start = output.length( ) - split[ lastIndex ].length( ) - 1;
output.setSpan( new BulletSpan( 15 * m_parents.size( ) ), start, output.length( ), 0 );
}
else if( m_parents.get(m_parents.size()-1).equals( "ol" ) )
{
m_index++ ;
output.append( "\n" );
String[ ] split = output.toString( ).split( "\n" );
int lastIndex = split.length - 1;
int start = output.length( ) - split[ lastIndex ].length( ) - 1;
output.insert( start, m_index + ". " );
output.setSpan( new LeadingMarginSpan.Standard( 15 * m_parents.size( ) ), start, output.length( ), 0 );
}
}
}

If you only need to format a list, keep it simple and copy/paste a unicode character in your TextView to achieve the same result.
• Unicode Character 'BULLET' (U+2022)

I came here looking for TagHandler implementations. Both Truong Nguyen and Aman Guatam answers are very nice, but I needed a mixed version of both: I needed my solution not to overformat it and to be able to ressolve <ol> tags, since I'm parsing something like <h3>title</h3><ol><li>item</li><li>item</li><li>item</li></ol>.
Here's my solution.
import org.xml.sax.XMLReader;
import android.text.Editable;
import android.text.Html.TagHandler;
public class MyTagHandler implements TagHandler {
boolean first = true;
String parent = null;
int index = 1;
public void handleTag(final boolean opening, final String tag,
final Editable output, final XMLReader xmlReader) {
if (tag.equals("ul")) {
parent = "ul";
index = 1;
} else if (tag.equals("ol")) {
parent = "ol";
index = 1;
}
if (tag.equals("li")) {
char lastChar = 0;
if (output.length() > 0) {
lastChar = output.charAt(output.length() - 1);
}
if (parent.equals("ul")) {
if (first) {
if (lastChar == '\n') {
output.append("\t• ");
} else {
output.append("\n\t• ");
}
first = false;
} else {
first = true;
}
} else {
if (first) {
if (lastChar == '\n') {
output.append("\t" + index + ". ");
} else {
output.append("\n\t" + index + ". ");
}
first = false;
index++;
} else {
first = true;
}
}
}
}
}
Note that, since we are resetting the index value whenever a new list starts, it WILL NOT work if you nest lists like in <ol><li>1<ol><li>1.1</li><li>1.2</li></ol><li>2</li></ol>11.11.22
With that code, you would get 1, 1, 2, 3 instead of 1, 1, 2, 2.

You can simply replace the "li" with unicodes
#Override
public void handleTag(boolean opening, String tag, Editable output, XMLReader xmlReader) {
if (tag.equalsIgnoreCase("li")) {
if (opening) {
output.append("\u2022 ");
} else {
output.append("\n");
}
}
}

Sure, there ise a way of showing bullets in Android TextView. You can replace <li> tags with • (which is HTML code for bullet).
If you want to try other list icons, use the preferred one from the table is this link;
http://www.ascii-code.com/

You can use Html.TagHandler. Below can be used for kotlin
class UlTagHandler : Html.TagHandler {
override fun handleTag(
opening: Boolean, tag: String, output: Editable,
xmlReader: XMLReader
) {
if (tag == "ul" && !opening) output.append("\n")
if (tag == "li" && opening) output.append("\n\t•")
}
}
and
textView.setText(Html.fromHtml(myHtmlText, null, UlTagHandler()));

Lord Voldermort's answer is a good starting point. However I required ol tag to display ordered list 1. 2. 3. .... instead of bullets. Also, nested tags need special handling to work properly.
In my code, I have maintained stack(parentList) to keep track of opened and closed ul and ol tags and also to know the current open tag.
Also, a levelWiseCounter is used to maintain different counts in case of nested ol tags.
myTextView.setText(Html.fromHtml("your string", null, new CustomTagHandler()));
.
.
.
private static class CustomTagHandler implements TagHandler
{
int level = 0;
private LinkedList<Tag> parentList = new LinkedList<DetailFragment.CustomTagHandler.Tag>();
private HashMap<Integer, Integer> levelWiseCounter = new HashMap<Integer, Integer>();
#Override
public void handleTag(boolean opening, String tag, Editable output, XMLReader xmlReader)
{
if (tag.equalsIgnoreCase("ul") || tag.equalsIgnoreCase("ol"))
{
if (opening)
{
if (tag.equalsIgnoreCase("ul"))
{
parentList.push(Tag.UL);
}
else
{
parentList.push(Tag.OL);
}
level++;
}
else
{
if (!parentList.isEmpty())
{
parentList.pop();
//remove counter at that level, in any present.
levelWiseCounter.remove(level);
}
level--;
if (level < 0)
{
level = 0;
}
}
}
else if (tag.equalsIgnoreCase("li"))
{
if (opening && level > 0)
{
//new line check
int length = output.toString().length();
if (length > 0 && (output.toString().charAt(length - 1) == '\n'))
{
}
else
{
output.append("\n");
}
//add tabs as per current level of li
for (int i = 0; i < level; i++)
{
output.append("\t");
}
// append dot or numbers based on parent tag
if (Tag.UL == parentList.peek())
{
output.append("•");
}
else
{
//parent is OL. Check current level and retreive counter from levelWiseCounter
int counter = 1;
if (levelWiseCounter.get(level) == null)
{
levelWiseCounter.put(level, 1);
}
else
{
counter = levelWiseCounter.get(level) + 1;
levelWiseCounter.put(level, counter);
}
output.append(padInt(counter) + ".");
}
//trailing tab
output.append("\t");
}
}
}
/**
* Add padding so that all numbers are aligned properly. Currently supports padding from 1-99.
*
* #param num
* #return
*/
private static String padInt(int num)
{
if (num < 10)
{
return " " + num;
}
return "" + num;
}
private enum Tag
{
UL, OL
}
}

How about the next code (based on this link) :
public class TextViewHtmlTagHandler implements TagHandler
{
/**
* Keeps track of lists (ol, ul). On bottom of Stack is the outermost list
* and on top of Stack is the most nested list
*/
Stack<String> lists =new Stack<String>();
/**
* Tracks indexes of ordered lists so that after a nested list ends
* we can continue with correct index of outer list
*/
Stack<Integer> olNextIndex =new Stack<Integer>();
/**
* List indentation in pixels. Nested lists use multiple of this.
*/
private static final int indent =10;
private static final int listItemIndent =indent*2;
private static final BulletSpan bullet =new BulletSpan(indent);
#Override
public void handleTag(final boolean opening,final String tag,final Editable output,final XMLReader xmlReader)
{
if(tag.equalsIgnoreCase("ul"))
{
if(opening)
lists.push(tag);
else lists.pop();
}
else if(tag.equalsIgnoreCase("ol"))
{
if(opening)
{
lists.push(tag);
olNextIndex.push(Integer.valueOf(1)).toString();// TODO: add support for lists starting other index than 1
}
else
{
lists.pop();
olNextIndex.pop().toString();
}
}
else if(tag.equalsIgnoreCase("li"))
{
if(opening)
{
if(output.length()>0&&output.charAt(output.length()-1)!='\n')
output.append("\n");
final String parentList=lists.peek();
if(parentList.equalsIgnoreCase("ol"))
{
start(output,new Ol());
output.append(olNextIndex.peek().toString()+". ");
olNextIndex.push(Integer.valueOf(olNextIndex.pop().intValue()+1));
}
else if(parentList.equalsIgnoreCase("ul"))
start(output,new Ul());
}
else if(lists.peek().equalsIgnoreCase("ul"))
{
if(output.charAt(output.length()-1)!='\n')
output.append("\n");
// Nested BulletSpans increases distance between bullet and text, so we must prevent it.
int bulletMargin=indent;
if(lists.size()>1)
{
bulletMargin=indent-bullet.getLeadingMargin(true);
if(lists.size()>2)
// This get's more complicated when we add a LeadingMarginSpan into the same line:
// we have also counter it's effect to BulletSpan
bulletMargin-=(lists.size()-2)*listItemIndent;
}
final BulletSpan newBullet=new BulletSpan(bulletMargin);
end(output,Ul.class,new LeadingMarginSpan.Standard(listItemIndent*(lists.size()-1)),newBullet);
}
else if(lists.peek().equalsIgnoreCase("ol"))
{
if(output.charAt(output.length()-1)!='\n')
output.append("\n");
int numberMargin=listItemIndent*(lists.size()-1);
if(lists.size()>2)
// Same as in ordered lists: counter the effect of nested Spans
numberMargin-=(lists.size()-2)*listItemIndent;
end(output,Ol.class,new LeadingMarginSpan.Standard(numberMargin));
}
}
else if(opening)
Log.d("TagHandler","Found an unsupported tag "+tag);
}
private static void start(final Editable text,final Object mark)
{
final int len=text.length();
text.setSpan(mark,len,len,Spanned.SPAN_MARK_MARK);
}
private static void end(final Editable text,final Class<?> kind,final Object... replaces)
{
final int len=text.length();
final Object obj=getLast(text,kind);
final int where=text.getSpanStart(obj);
text.removeSpan(obj);
if(where!=len)
for(final Object replace : replaces)
text.setSpan(replace,where,len,Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
return;
}
private static Object getLast(final Spanned text,final Class<?> kind)
{
/*
* This knows that the last returned object from getSpans()
* will be the most recently added.
*/
final Object[] objs=text.getSpans(0,text.length(),kind);
if(objs.length==0)
return null;
return objs[objs.length-1];
}
private static class Ul
{
}
private static class Ol
{
}
}

I had the problem, that I always got an empty line after a list with #Kuitsis solution. I added a few lines in handleTag() and now the empty lines are gone:
#Override
public void handleTag(final boolean opening, final String tag, final Editable output, final XMLReader xmlReader) {
if (UL_TAG.equalsIgnoreCase(tag)) {
if (opening) { // handle <ul>
lists.push(new Ul());
} else { // handle </ul>
lists.pop();
if (output.length() > 0 && output.charAt(output.length() - 1) == '\n') {
output.delete(output.length() - 1, output.length());
}
}
} else if (OL_TAG.equalsIgnoreCase(tag)) {
if (opening) { // handle <ol>
lists.push(new Ol()); // use default start index of 1
} else { // handle </ol>
lists.pop();
if (output.length() > 0 && output.charAt(output.length() - 1) == '\n') {
output.delete(output.length() - 1, output.length());
}
}
} else if (LI_TAG.equalsIgnoreCase(tag)) {
if (opening) { // handle <li>
lists.peek().openItem(output);
} else { // handle </li>
lists.peek().closeItem(output, lists.size());
}
} else {
Log.d("TagHandler", "Found an unsupported tag " + tag);
}
}

this is a confirmation to what kassim has stated. there is fragmentation. i found how to resolve this. i have to rename <li> and ul to a custom tag. so:
myHTML.replaceAll("</ul>","</customTag>").replaceAll("<ul>","<customTag>");
//likewise for li
then in my handler i can look for that customTag (which does nothing) and make it do something.
//now my handler can handle the customtags. it was ignoring them after nougat.
public class UlTagHandler implements Html.TagHandler {
//for ul in nougat and up this tagHandler is completely ignored
#Override
public void handleTag(boolean opening, String tag, Editable output,
XMLReader xmlReader) {
if (tag.equals("customtag2") && opening)
output.append("\n\t\u25CF\t");
if (tag.equals("customtag2") && !opening)
output.append("\n");
}
}
this should make it work for all versions of android.