攝即是空

<1>

Button btn1 = (Button) findViewById(R.id.button1);
btn1.setOnClickListener(btn1Listener);

private Button.OnClickListener btn1Listener = new Button.OnClickListener() {
  @Override
  public void onClick(View v) {
  // TODO Auto-generated method stub  
  }
};

<2>

public void btnTest(){
Button btn2 = (Button) findViewById(R.id.btn2);
         btn2.setOnClickListener(new OnClickListener(){
 
    @Override
    public void onClick(View v) {
     // TODO Auto-generated method stub
      
    }
           
         });
}

<3>

@Override
 
public class MainActivity extends Activity implements OnClickListener {
 
@Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
 
  ((Button) findViewById(R.id.button1)).setOnClickListener(this);
  ((Button) findViewById(R.id.button2)).setOnClickListener(this);
  ((Button) findViewById(R.id.button3)).setOnClickListener(this);
 
 }
 
 @Override
 public void onClick(View v) {
  switch (v.getId()) {
   
  case R.id.button1:
   Intent intent1 = new Intent(MainActivity.this, Activity1.class);
   startActivity(intent1);
   break;
    
  case R.id.button2:
   Intent intent2 = new Intent(MainActivity.this, Activity2.class);
   startActivity(intent2);
   break;
    
  case R.id.button3:
   Toast.makeText(MainActivity.this, "Hi", Toast.LENGTH_LONG).show();
   break;
 
  }
 }
 
}