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(), "再按一下退出应用",

                    Toast.LENGTH_SHORT).show();

            exitTime = System.currentTimeMillis();

        } else {

            finish();

            System.exit(0);

        }

        return true;

    }

    return super.onKeyDown(keyCode, event);

}

Android 再按一次退出应用的代码