MainActivity.java

@Override protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  final TextView text = (TextView)findViewById(R.id.helloText);
  final Button button = (Button)findViewById(R.id.helloButton);
  button.setOnClickListener(new View.OnClickListener() { 
    @Override
    public void onClick(View view) { 
      text.setText("Hello World!"); 
    }
  });
}