retrofit2_example_MainActivity(initInstance).java

private LinearLayout layoutForm;
private LinearLayout layoutProgress;
private LinearLayout layoutResult;
private EditText edUsername;
private Button btSend;
private TextView tvResult;

@Override
protected void onCreate(Bundle savedInstanceState) {
  setContentView();
  initInstance()
}

private void initInstance() {  
  //Layout Form
  layoutForm = (LinearLayout) findViewById(R.id.layoutForm);
  edUsername = (EditText) findViewById(R.id.edUsername);
  btSend= (Button) findViewById(R.id.btSend);
  //Layout Progress
  layoutProgress = (LinearLayout) findViewById(R.id.layoutProgress);
  //Layout Result
  layoutResult = (LinearLayout) findViewById(R.id.layoutResult);
  tvResult = (TextView) findViewById(R.id.tvResult);
  //setOnClick
  btOk.setOnClickListener(onBtSendClickListener);
}