How to get the list of all children in Firebase android?

Question

I have this model below in Firebase

I am trying to get all the values from child node posts using below code

  databasePostsReference = 
  FirebaseDatabase.getInstance().getReference().child("posts");

 final List<UserPostPOJO> list = Collections.EMPTY_LIST;

  databasePostsReference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for(DataSnapshot snapshot : dataSnapshot.getChildren()){
                Log.d(TAG, "onDataChange: entered list adding");
                UserPostPOJO post = snapshot.getValue(UserPostPOJO.class);
                list.add(post);
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

but I am getting error java.lang.UnsupportedOperationException.

in the above scenario i know that error is because list is declared final and i am trying to assign value in onDataChangeMethod so is throwing error but if i am not declaring the list variable final then i cant access list variable from within the onDataChange() method how to solve this

  public class UserPostPOJO {
    String uid;
    Double elevation;
    String uri;
    String feel;

    public UserPostPOJO() {
    }

    public UserPostPOJO(String uid, Double elevation, String uri, String feel) {
        this.uid = uid;
        this.elevation = elevation;
        this.uri = uri;
        this.feel = feel;
    }

    public String getUid() {
        return uid;
    }

    public void setUid(String uid) {
        this.uid = uid;
    }

    public Double getElevation() {
        return elevation;
    }

    public void setElevation(Double elevation) {
        this.elevation = elevation;
    }

    public String getUri() {
        return uri;
    }

    public void setUri(String uri) {
        this.uri = uri;
    }

    public String getFeel() {
        return feel;
    }

    public void setFeel(String feel) {
        this.feel = feel;
    }
    } 

Error Log:

  05-08 22:19:49.706 6421-6421/redeyes17.com.abhi.android.iamat                      
    D/recentfragment: onDataChange: entered list adding
   05-08 22:19:49.726 6421-6421/redeyes17.com.abhi.android.iamat 
    E/AndroidRuntime: FATAL EXCEPTION: main

   Process: redeyes17.com.abhi.android.iamat, PID: 6421

   java.lang.UnsupportedOperationException

   at java.util.AbstractList.add(AbstractList.java:404)

   at java.util.AbstractList.add(AbstractList.java:425)

   at redeyes17.com.abhi.android.iamat.UI.tabs
  .RecentFragment$1.onDataChange(RecentFragment.java:78)

  at com.google.android.gms.internal.zzbpx.zza(Unknown Source)

  at com.google.android.gms.internal.zzbqx.zzZT(Unknown Source)

  at com.google.android.gms.internal.zzbra$1.run(Unknown Source)

   at android.os.Handler.handleCallback(Handler.java:739)

  at android.os.Handler.dispatchMessage(Handler.java:95)

  at android.os.Looper.loop(Looper.java:158)

  at android.app.ActivityThread.main(ActivityThread.java:7230)

  at java.lang.reflect.Method.invoke(Native Method)

 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller
 .run(ZygoteInit.java:1230)

 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)

Answer 1

To get all those values please use this code:

    databasePostsReference = FirebaseDatabase.getInstance().getReference().child("posts").child(postId);
    ValueEventListener eventListener = new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
           List<UserPostPOJO> list = new ArrayList<>();

           String elevation = (String) dataSnapshot.child("elevation").getValue();
           String feel = (String) dataSnapshot.child("feel").getValue();
           String uid = (String) dataSnapshot.child("uid").getValue();
           String uri = (String) dataSnapshot.child("uri").getValue();

          UserPostPOJO userPostPOJO = new UserPostPOJO();
          userPostPOJO.setElevation(elevation);
          userPostPOJO.setFeel(feel);
          userPostPOJO.setUid(uid);
          userPostPOJO.setUri(uri);

          list.add(userPostPOJO);
    }

          @Override
          public void onCancelled(DatabaseError databaseError) {}
};
databasePostsReference.addListenerForSingleValueEvent(eventListener);

In which postId is the unique id generated by the push() method. And notice, that the declaration of your list must be inside the onDataChange() method, otherwise, you'll get null.

Hope it helps.