I have to return a list of lists with LiveData.
The background is, that I load some data out of my Room database.
The database call depends on a counter. So I tried the following approach:
MediatorLiveData<List<List<Foo>> forView = new MediatorLiveData<>();
forView.addSource(counter, (trigger) -> {
List<List<Foo>> output = new LinkedList<>();
for(int i = 0; i < counter; i++) {
output.add(new ArrayList<>());
forView.addSource(repository.getDataForCounter(i), (data) -> {
//With some workaround I thought I've access to the position
output.remove("i");
output.add("i", data);
});
}
return output;
});
I think the workaround for the index didn't work, because the variables in lambdas have to be implicit final.
Is there any other approach to "merge" a variable number of lists got out of a database in one list of such lists?
Related
How would I load multiple Firestore query results into the same array? I am using a for-loop but since Firestore queries are async, the array is set to 0. Here is my code:
private void loadMealplans() {
for (int i = 0; i < restaurantArrayList.size(); i++) {
Query planQuery = firebaseFirestore.collection("Meal_Plans").whereEqualTo("restaurantId", restaurantArrayList.get(i).getRestaurantId());
planQuery.get().addOnSuccessListener(queryDocumentSnapshots -> {
for (QueryDocumentSnapshot allPlans : queryDocumentSnapshots) {
Mealplan mealplan = allPlans.toObject(Mealplan.class);
mealplan.setMealplanId(allPlans.getId());
myPlans.add(mealplan);
}
});
Log.d(TAG, "mealplan size:"+ myPlans.size()); // this is always zero
}
// Loading data into recycler view
}
When you call planQuery.get() it returns a Task<QuerySnapshot>. To wait for multiple tasks to be completed, gather all those tasks and pass them to Tasks.whenAll.
For more on this, I recommend reading Become a Firebase task master.
I'm adding three different objects to an ArrayList, but the list contains three copies of the last object I added.
For example:
for (Foo f : list) {
System.out.println(f.getValue());
}
Expected:
0
1
2
Actual:
2
2
2
What mistake have I made?
Note: this is designed to be a canonical Q&A for the numerous similar issues that arise on this site.
This problem has two typical causes:
Static fields used by the objects you stored in the list
Accidentally adding the same object to the list
Static Fields
If the objects in your list store data in static fields, each object in your list will appear to be the same because they hold the same values. Consider the class below:
public class Foo {
private static int value;
// ^^^^^^------------ - Here's the problem!
public Foo(int value) {
this.value = value;
}
public int getValue() {
return value;
}
}
In that example, there is only one int value which is shared between all instances of Foo because it is declared static. (See "Understanding Class Members" tutorial.)
If you add multiple Foo objects to a list using the code below, each instance will return 3 from a call to getValue():
for (int i = 0; i < 4; i++) {
list.add(new Foo(i));
}
The solution is simple - don't use the static keywords for fields in your class unless you actually want the values shared between every instance of that class.
Adding the Same Object
If you add a temporary variable to a list, you must create a new instance of the object you are adding, each time you loop. Consider the following erroneous code snippet:
List<Foo> list = new ArrayList<Foo>();
Foo tmp = new Foo();
for (int i = 0; i < 3; i++) {
tmp.setValue(i);
list.add(tmp);
}
Here, the tmp object was constructed outside the loop. As a result, the same object instance is being added to the list three times. The instance will hold the value 2, because that was the value passed during the last call to setValue().
To fix this, just move the object construction inside the loop:
List<Foo> list = new ArrayList<Foo>();
for (int i = 0; i < 3; i++) {
Foo tmp = new Foo(); // <-- fresh instance!
tmp.setValue(i);
list.add(tmp);
}
Your problem is with the type static which requires a new initialization every time a loop is iterated. If you are in a loop it is better to keep the concrete initialization inside the loop.
List<Object> objects = new ArrayList<>();
for (int i = 0; i < length_you_want; i++) {
SomeStaticClass myStaticObject = new SomeStaticClass();
myStaticObject.tag = i;
// Do stuff with myStaticObject
objects.add(myStaticClass);
}
Instead of:
List<Object> objects = new ArrayList<>();
SomeStaticClass myStaticObject = new SomeStaticClass();
for (int i = 0; i < length; i++) {
myStaticObject.tag = i;
// Do stuff with myStaticObject
objects.add(myStaticClass);
// This will duplicate the last item "length" times
}
Here tag is a variable in SomeStaticClass to check the validity of the above snippet; you can have some other implementation based on your use case.
Had the same trouble with the calendar instance.
Wrong code:
Calendar myCalendar = Calendar.getInstance();
for (int days = 0; days < daysPerWeek; days++) {
myCalendar.add(Calendar.DAY_OF_YEAR, 1);
// In the next line lies the error
Calendar newCal = myCalendar;
calendarList.add(newCal);
}
You have to create a NEW object of the calendar, which can be done with calendar.clone();
Calendar myCalendar = Calendar.getInstance();
for (int days = 0; days < daysPerWeek; days++) {
myCalendar.add(Calendar.DAY_OF_YEAR, 1);
// RIGHT WAY
Calendar newCal = (Calendar) myCalendar.clone();
calendarList.add(newCal);
}
Every time you add an object to an ArrayList, make sure you add a new object and not already used object. What is happening is that when you add the same 1 copy of object, that same object is added to different positions in an ArrayList. And when you make change to one, because the same copy is added over and over again, all the copies get affected.
For example,
Say you have an ArrayList like this:
ArrayList<Card> list = new ArrayList<Card>();
Card c = new Card();
Now if you add this Card c to list, it will be added no problem. It will be saved at location 0. But, when you save the same Card c in the list, it will be saved at location 1. So remember that you added same 1 object to two different locations in a list. Now if you make a change that Card object c, the objects in a list at location 0 and 1 will also reflect that change, because they are the same object.
One solution would be to make a constructor in Card class, that accepts another Card object. Then in that constructor, you can set the properties like this:
public Card(Card c){
this.property1 = c.getProperty1();
this.property2 = c.getProperty2();
... //add all the properties that you have in this class Card this way
}
And lets say you have the same 1 copy of Card, so at the time of adding a new object, you can do this:
list.add(new Card(nameOfTheCardObjectThatYouWantADifferentCopyOf));
It can also consequence of using the same reference instead of using a new one.
List<Foo> list = new ArrayList<Foo>();
setdata();
......
public void setdata(int i) {
Foo temp = new Foo();
tmp.setValue(i);
list.add(tmp);
}
Instead of:
List<Foo> list = new ArrayList<Foo>();
Foo temp = new Foo();
setdata();
......
public void setdata(int i) {
tmp.setValue(i);
list.add(tmp);
}
I created a loop that will get the data from my cursor, however I noticed that even though it is looping(with the correct count) it only shows the first value.
int vv = 0;
if ((CR3.moveToFirst()) || CR3.getCount() !=0){
while (CR3.isAfterLast() == false) {
vendoName[vv] = CR3.getString(0);
vendoEsch[vv] = CR3.getString(1);
vendoAsch[vv] = CR3.getString(2);
vendoTag[vv] = CR3.getString(3);
vv++;
CR3.moveToNext();
}}
and when I call all my data( I only need the first three records)
ArrayList<SearchResults2> results2 = new ArrayList<SearchResults2>();
SearchResults2 sr2 = new SearchResults2();
for(int j = 0;j < 3;j++)
{
sr2.setName(vendoName[j]);
sr2.setEsch(vendoEsch[j]);
sr2.setAsch(vendoAsch[j]);
sr2.setTag(vendoTag[j]);
results2.add(sr2);
}
I am putting inside a listview, when I check, it is showing always the first data.
This is an example I used as a reference to my code(It's almost the same except that I used an array to put my data from)
http://geekswithblogs.net/bosuch/archive/2011/01/31/android---create-a-custom-multi-line-listview-bound-to-an.aspx
Am I doing something wrong which is why it is only getting the first piece of data?
Is it not easier to do something like this (if you don't need more than 3 results):
ArrayList<SearchResults2> results2 = new ArrayList<SearchResults2>();
CR3.moveToFirst();
for (i = 0; i < 3; i++) {
SearchResults2 sr2 = new SearchResults2();
sr2.setName(CR3.getString(0));
sr2.setEsch(CR3.getString(1));
sr2.setAsch(CR3.getString(2));
sr2.setTag(CR3.getString(3));
results2.add(sr2);
CR3.moveToNext();
}
I think that maybe the cursor doesn't iterate properly through your results in your while-loop and that's why you become one and the same result for the three items
I have two lists of Default and Chrome browsers history.
I want to merge these two lists into one list.
I need to update item if I find it duplicate (is common between two lists).
So, my "BrowserRecord" class is like this:
public class BrowserRecord {
private long id;
private int bookmark;
private long created;
private long date;
private String title;
private String url;
private long visits;
#Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
BrowserRecord record = (BrowserRecord) o;
return url.equals(record.url);
}
#Override
public int hashCode() {
return url.hashCode();
}
// other getter setter methods
...
}
and finally, I have a method that gets browsers histories and does merging:
public List<BrowserRecord> getHistory() {
List<BrowserRecord> browserList = new ArrayList<BrowserRecord>();
// get history of default and chrome browsers
List<BrowserRecord> defaultList = getDefaultBrowserHistory();
List<BrowserRecord> chromeList = getChromeBrowserHistory();
Log.e(TAG, "=> size of Default browser:" + defaultList.size());
Log.e(TAG, "=> size of Chrome browser:" + chromeList.size());
// compare list A with B, update A item if equal item found in B and push it to tempList
for(int i=0; i<chromeList.size(); i++) {
BrowserRecord chromeBrowser = chromeList.get(i);
for(int j=0; j<defaultList.size(); j++) {
BrowserRecord defaultBrowser = defaultList.get(j);
if(chromeBrowser.equals(defaultBrowser)) {
if(chromeBrowser.getBookmark() != defaultBrowser.getBookmark())
chromeBrowser.setBookmark(1);
chromeBrowser.setVisits(chromeBrowser.getVisits() + defaultBrowser.getVisits());
}
}
browserList.add(chromeBrowser);
}
// compare list B with A, jump if equal item found in A, push to tempList if item not found
for(int i=0; i<defaultList.size(); i++) {
BrowserRecord defaultBrowser = defaultList.get(i);
boolean found = false;
for(int j=0; j<chromeList.size(); j++) {
BrowserRecord chromeBrowser = chromeList.get(j);
if(defaultBrowser.equals(chromeBrowser)) {
found = true;
break;
}
}
if(!found)
browserList.add(defaultBrowser);
}
Log.e(TAG, "=> size of final browser:" + browserList.size());
return browserList;
}
I have tested this method and is working fine. Since my history records on mobile device after 3 years didn't exceed more than 200 records on one list and 150 for others, I assume something similar is happening for other users. But I'm sure is not optimum way.
What do you recommend?
any suggestion would be appreciated. Thanks.
Not sure I understand correctly, but it seems like what you're trying to do is, given both lists, create a final list which will contain all of the elements from both lists, removing any duplicates.
If this is the case, then take a look at Java's TreeSet class. If you iterate over all of the elements from both your lists and insert them into a TreeSet, you will basically get the result you're looking for. You can then use an Iterator to create an ArrayList containing all of the non-duplicate items from both your lists. As a side-effect of using a TreeSet, they will ordered (you can also use either a HashSet if you don't care about the order or a LinkedHashSet if you want to preserve the order of insertion).
I'm iterating through a cursor and populating a SparseArray with ArrayList's containing bundles of information from the cursor:
// An ArrayList to hold all of our components per section
ArrayList<ObjectKanjiLookupChar> al = new ArrayList<ObjectKanjiLookupChar>();
// We'll hold on to all of the above ArrayLists and process them at once
SparseArray<ArrayList<ObjectKanjiLookupChar>> compArray = new SparseArray<ArrayList<ObjectKanjiLookupChar>>();
do
{
// Read values from the cursor
int id = cursor.getInt(cursor.getColumnIndex("_id"));
String component = cursor.getString(cursor.getColumnIndex("component"));
int compStrokes = cursor.getInt(cursor.getColumnIndex("strokes"));
// Create a new object for this component so we can display it in the GridView via an adapter
ObjectKanjiLookupChar oklc = new ObjectKanjiLookupChar();
oklc.setCharacterID(id);
oklc.setCharacter(component);
oklc.setStrokeCount(compStrokes);
al.add(oklc);
// Add headers whenever we change stroke groups
if(compStrokes != strokesSection)
{
compArray.put(strokesSection, al);
al.clear();
strokesSection = compStrokes;
}
}
while(cursor.moveToNext());
// Add the final group of components to the array
compArray.put(strokesSection, al);
Immediately afterwards, I iterate through the SparseArray:
for(int i = 0; i < compArray.size(); i++)
{
Integer strokes = compArray.keyAt(i);
ArrayList<ObjectKanjiLookupChar> alComp = compArray.get(strokes);
// DEBUG
Log.i("DialogKanjiLookup", "Components in Section " + strokes + ": " + alComp.size());
ll.addView(createNewSection(String.valueOf(strokes), alComp));
}
For some unknown reason, the Log() call above reports that alComp has zero entries. I verified that ArrayList.size() was returning numbers greater than 0 when I put() them into the SparseArray, so I must be doing something incorrect when iterating through the SparseArray. What is going on?
I suspect that the problem comes from this piece of code:
if(compStrokes != strokesSection)
{
compArray.put(strokesSection, al);
al.clear(); // Here
strokesSection = compStrokes;
}
You cleared the array list after you added to the SparseArray. You might think that after you have added the list to the SparseArray, SparseArray would keep a copy of the ArrayList. However, they actually share the same reference. Since you cleared the ArrayList, you cleared out the one inside SparseArray too.
The following code should fix the problem.
// We'll hold on to all of the above ArrayLists and process them at once
SparseArray<ArrayList<ObjectKanjiLookupChar>> compArray = new SparseArray<ArrayList<ObjectKanjiLookupChar>>();
do
{
// Read values from the cursor
int id = cursor.getInt(cursor.getColumnIndex("_id"));
String component = cursor.getString(cursor.getColumnIndex("component"));
int compStrokes = cursor.getInt(cursor.getColumnIndex("strokes"));
// Create a new object for this component so we can display it in the GridView via an adapter
ObjectKanjiLookupChar oklc = new ObjectKanjiLookupChar();
oklc.setCharacterID(id);
oklc.setCharacter(component);
oklc.setStrokeCount(compStrokes);
ArrayList<ObjectKanjiLookupChar> al = compArray.get(comStrokes);
if(al == null) {
al = new ArrayList<ObjectKanjiLookupChar>();
compArray.put(comStrokes, al);
}
al.add(oklc);
}
while(cursor.moveToNext());