getting random value from Firebase Database

I have set up an activity for my users to input messages into Firebase database and another activity to recall the messages at random. However I couldn't seem to get the random method going... the recall method just call the same messages over and over again on click.

I am using and editing the method that username nothingness posted here get random value android firebase java so I don't actually understand how it works...

Here's the code to write to the database:

public class PostActivity extends AppCompatActivity {

    private EditText PostField;
    private Button postSubmit;
    private DatabaseReference mDatabasePosts;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_post);

        postSubmit = (Button)findViewById(R.id.postSubmit);
        postField = (EditText)findViewById(R.id.postField);
        mDatabasePosts = FirebaseDatabase.getInstance().getReference().child("posts");

        postSubmit.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startPosting();
            }

            private void startPosting() {
                final String postVal = postField.getText().toString().trim();
                if (!TextUtils.isEmpty(postVal)) {

                    //post creation
                    DatabaseReference newPost = mDatabasePosts.push();
                    newPost.setValue(postVal);
                    Toast.makeText(PostActivity.this, "Post registered", Toast.LENGTH_SHORT).show();

                    startActivity(new Intent(PostActivity.this, HomeActivity.class));
                }


            }
        });
    }
}

nothing fancy there...

on the retrieving activity, I have a button that will display a toast randomly (well... at least the goal was to) of the texts written to the database... here's the code for that activity:

DatabaseReference mPost;

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


button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                mPost.addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        long allNum = dataSnapshot.getChildrenCount();
                        int maxNum = (int)allNum;
                        int randomNum = new Random().nextInt(maxNum);

                        int count = 0;
                        Iterable<DataSnapshot> ds = dataSnapshot.getChildren();
                        Iterator<DataSnapshot> ids = ds.iterator();
                        String newPost = (String) ids.next().getValue();
                        while(ids.hasNext() && count < randomNum) {
                            ids.next();
                            count ++; // used as positioning.
                        }

                        Toast.makeText(HomeActivity.this, newPost,Toast.LENGTH_SHORT).show();
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });

            }
        });

The Database looks like this:

posts
  L random key: "text 1"
  L random key: "text 2"

So at the moment, everytime i click on the button... it keeps on calling "text 1" I want it to call text 1 and 2 alternately and randomly...

Can anyone help me with this?