Android中點擊物理返回按鍵（再按一次退出程序）

private long exitTime = 0;

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
 if(keyCode == 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.onKeyDown(keyCode, event);
}