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Why the .whereEqualTo isn't working? [closed]

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Why this working
Query firstQuery1 = firebaseFirestore.collection("Test").whereEqualTo("test", "1");
And this isn't working
Query firstQuery1 = firebaseFirestore.collection("Test");
firstQuery1.whereEqualTo("test", "1");

firestore.collection("Test") returns an object of type CollectionReference while ref.whereEqualTo(..) returns a Query
So instead of
Query firstQuery1 = firestore.collection("Test");
firstQuery1.whereEqualTo("test", "1");
it needs to be
CollectionReference ref = firestore.collection("Test");
Query query = ref.whereEqualTo("test","1");

Why I am not getting the correct output for array.set? [closed]

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Firstly, I am storing the data in ArrayList.Now based on my index(current position) I want to replace that data with new data,but what I am getting is
old data,old data,old data,true,true,new data,new data....
Any suggestions
ArrayList<String> arr = new ArrayList<String>();
if (arr.isEmpty()) {
for(int i = 0;i<=mcq.size();i++) {
arr.add(s);
}
} else {
arr.set(currentPosition, String.valueOf(arr.add(s)));
}
"s" is a String value I am getting from somewhere else.

This looks really weird:
arr.set(currentPosition, String.valueOf(arr.add(s)));
ArrayList.set changes the elements stored at currentPosition. You want it to change it to the string value of what arr.add returns.
ArrayList.add returns a boolean, so there you get your true values from.
I think you want to do arr.set(currentPosition, s);

How to sort a string in Android [closed]

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I have a string like "RSEBAK" and I'd like to sort that string to "ABEKRS"
Is it possible in android?
If it is, then please guide me through.

Your spec is a bit fuzzy, but I think you mean to sort the charters in a string? If so…
String s1 = "RSEBAK";
char[] arrC = s1.toCharArray();
Arrays.sort(arrC);
String strSorted = new String(arrC);
strSorted should have "ABEKRS"

adding string as integer [closed]

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in my application i have got 0.00 in edit text.next when i enter 2or3 or any digit its getting concat with 0.00 and getting 0.003.
if(item.equals("0.00")){
item=item+string;}
i changed the code.but this time too its getting an error
if(item.equals("0.00")){
int a=Integer.parseInt(item.toString());
int b=Integer.parseInt(string.toString());
int c=a+b;
item=String.valueOf(c);}

If you are using decimal places then you will want to use a data type that supports those, such as double. Try this:
double d = Double.parseDouble(item.toString());
NOTE: If both your variables are of string type, then you won't need to use toString()

You want O.OO but still using Integer. Try using FLOAT.
Also, FLOAT is NEVER a perfect value. So you might have .XX added.

You have an error, because "0.00" has decimal places, so it is parsed as a floating point number, not integer.
Parse it as a double:
String item = "0.00";
double result = Double.parseDouble(item) + 0.03; // result is 0.03

Regular Expression in numbers and string length Android [closed]

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I need to point out the users that a number can only start from 9 and and its length is to be exactly equal to 10 digits. How do I manipulate this with a regex ? Please don't mind the question, I am relatively new.

To validate that the input is exactly 10 digits, starting with 9, use:
/^9\d{9}$/
The ^9 part matches a starting 9, {d}9 matches 9 digits

Here you are. A simple solution (not with regex):
String number = "12345";
...
if (number.length() != 10 || number.startsWith("9") == false)
{
Toast.makeText(getApplicationContext(), "Please check your input.", Toast.LENGTH_SHORT).show();
}
Hope this helps. Write if you have any problems.