PostActivity.java

import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

public class PostActivity extends AppCompatActivity {

    private static final String TAG = "PostActivity";
    EditText contentText;
    Button postButton;
    ProgressBar progressBar;

    FirebaseDatabase mDatabase;
    FirebaseAuth mAuth;
    DatabaseReference mRootRef, mPostRef, mUserRef;

    String mUserId;
    String mFullName;

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

        contentText = findViewById(R.id.postContent);
        postButton = findViewById(R.id.postButton);
        progressBar = findViewById(R.id.progressBar);

        postButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                savePost();
            }
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        mDatabase = FirebaseDatabase.getInstance();
        mRootRef = mDatabase.getReference();
        mAuth = FirebaseAuth.getInstance();
        getFullName();
    }

    private void savePost() {
        mRootRef = mDatabase.getReference();
        mPostRef = mRootRef.child("post");

        progressBar.setVisibility(View.VISIBLE);

        String content = contentText.getText().toString();
        Post post = new Post(mFullName, content);
        mPostRef.push().setValue(post).addOnCompleteListener(new OnCompleteListener<Void>() {
            @Override
            public void onComplete(@NonNull Task<Void> task) {
                if (task.isSuccessful()) {
                    finish();
                    progressBar.setVisibility(View.INVISIBLE);
                } else {
                    Log.d(TAG, task.getException().getMessage());
                    progressBar.setVisibility(View.INVISIBLE);
                }
            }
        });

    }

    private String getFullName() {
        mUserId = mAuth.getUid();
        mUserRef = mRootRef.child("user");
        mUserRef.child(mUserId).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                User user = dataSnapshot.getValue(User.class);
                mFullName = user.name;
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
        return mFullName;
    }
}