按二次back键退出程序应用的实现

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package com.loaderman.twoexitdemo;   import android.os.Bundle; import android.os.Handler; import android.os.Message; import android.support.v7.app.AppCompatActivity; import android.view.KeyEvent; import android.widget.Toast;   public class MainActivity extends AppCompatActivity {     private Handler handler = new Handler() {         @Override         public void handleMessage(Message msg) {             isExit = false;         }     };       @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);     }       @Override     public boolean onKeyDown(int keyCode, KeyEvent event) {         exit();         return event.equals(KeyEvent.KEYCODE_BACK );     }       private static boolean isExit = false;       private void exit() {         if (!isExit) {             isExit = true;             Toast.makeText(this, "再次点击退出程序", Toast.LENGTH_SHORT).show();             handler.sendEmptyMessageDelayed(0, 2000);         } else {             finish();             System.exit(0);         }     } }