//记录用户首次点击返回键的时间
 private long firstTime = 0;

 /**
  * 第一种解决办法 通过监听keyUp
  * @param keyCode
  * @param event
  * @return
  */
 @Override
 public boolean onKeyUp(int keyCode, KeyEvent event) {
  if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_UP) {
   long secondTime = System.currentTimeMillis();
   if (secondTime - firstTime > 2000) {
    Toast.makeText(MainActivity.this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
    firstTime = secondTime;
    return true;
   } else {
    System.exit(0);
   }
  }

  return super.onKeyUp(keyCode, event);
 }

 /**
  * 第二种办法
  * @param keyCode
  * @param event
  * @return
  */
 @Override
 public boolean onKeyDown(int keyCode, KeyEvent event) {
  if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {
   long secondTime = System.currentTimeMillis();
   if (secondTime - firstTime > 2000) {
    Toast.makeText(MainActivity.this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
    firstTime = secondTime;
    return true;
   } else {
    System.exit(0);
   }
  }
  return super.onKeyDown(keyCode, event);
 }

 /**
  * 第三种方法
  */
 @Override
 public void onBackPressed() {
  long secondTime = System.currentTimeMillis();
  if (secondTime - firstTime > 2000) {
   Toast.makeText(MainActivity.this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
   firstTime = secondTime;
  } else {
   System.exit(0);
  }

/**
  * 第四种方法
  */
 @Override
 public void onBackPressed() {
  if (!mBackKeyPressed) {
   Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
   mBackKeyPressed = true;
   new Timer().schedule(new TimerTask() {
    @Override
    public void run() {
     mBackKeyPressed = false;
    }
   }, 2000);
  } else {
   this.finish();
   System.exit(0);
  }
 }

  /**
  * 第五种办法
  * @param keyCode
  * @param event
  * @return
  */
 @Override
 public boolean onKeyUp(int keyCode, KeyEvent event) {
  if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_UP) {
   if (!mBackKeyPressed) {
    Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
    mBackKeyPressed = true;
    new Timer().schedule(new TimerTask() {
     @Override
     public void run() {
      mBackKeyPressed = false;
     }
    }, 2000);
    return true;
   } else {
    this.finish();
    System.exit(0);
   }
  }
  return super.onKeyUp(keyCode, event);
 }

 /**
  * 第六种办法
  * @param keyCode
  * @param event
  * @return
  */
 @Override
 public boolean onKeyDown(int keyCode, KeyEvent event) {
  if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {
   if (!mBackKeyPressed) {
    Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
    mBackKeyPressed = true;
    new Timer().schedule(new TimerTask() {
     @Override
     public void run() {
      mBackKeyPressed = false;
     }
    }, 2000);
    return true;
   } else {
    this.finish();
    System.exit(0);
   }
  }
  return super.onKeyDown(keyCode, event);
 }