android点击一个按钮退出整个app

 protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ExitApplication.getInstance().addActivity(this);
        btnLogin=findViewById(R.id.btn_login);
        etAccount=findViewById(R.id.account_input);
        etPassword=findViewById(R.id.password_input);

/**完成代码***/
    }

    public void exit() {

        for (Activity act : activityList) {
            act.finish();
        }

        System.exit(0);

    }
        @Override
    public void onClick(View v) {
        switch(v.getId()){
            case R.id.btn_login:
                ExitApplication.getInstance().exit();
                break;
        }
    }
}

ExitApplication冒红有错误

ExitApplication 是否自己注册并再清单文件中声明，另外冒红的时候，鼠标放上去有提示