再按一次推出

private long exitTime = 0;
@Override
 public boolean dispatchKeyEvent(KeyEvent event) {  
  
  if(event.getKeyCode() == KeyEvent.KEYCODE_BACK
    && event.getAction() == KeyEvent.ACTION_DOWN){  
      if((System.currentTimeMillis()-exitTime) > 2000){ 
          Toast.makeText(getApplicationContext(), "再按一次退出程序", Toast.LENGTH_SHORT).show();                               
          exitTime = System.currentTimeMillis();  
   } else {
    finish();
    System.exit(0);
   }
      return true;  
     }
  return super.dispatchKeyEvent(event);
 }