现在在一些app中没用menu菜单来实现退出程序，很简化地使用点击两次返回键来实现退出程序，这个方法简单实用，软件退出不在赖在后台。

代码实现，重写onKeyDown来监听：

public class MainActivity extends AppCompatActivity {

    private long waitTime = 2000;

    private long touchTime = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        //2秒之类连续点击2次返回键则退出程序
        if (keyCode == KeyEvent.KEYCODE_BACK && event.getAction() == KeyEvent.ACTION_DOWN) {
            long currentTime = System.currentTimeMillis();
            if ((currentTime - touchTime) > waitTime) {
                Toast.makeText(this, "请再按一次退出", Toast.LENGTH_LONG).show();
                touchTime = currentTime;
            } else {
                finish();
                System.exit(0);
            }
            return true;
        }
        return super.onKeyDown(keyCode, event);
    }
}