触发时我的界面得到 NullPointerException

Question

我尝试了一个 Toast Message 界面。如果应用程序没有连接互联网，我想显示一个 Toast 消息并且我想要 java 接口。

这是MotherActivity.java。该文件实现ToastMessagges.ToastMessaggeCallback

   public class MotherActivity extends ActionBarActivity implements ToastMessagges.ToastMessaggeCallback {

        ToastMessagges toastMessagges;

        @Override
        protected void onStart() {
            super.onStart();

        }

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

            toastMessagges      = new ToastMessagges();

            AppStarter();
        }

        private void AppStarter(){

            boolean checkinternet   = InternetControl.checkInternetConnection( getApplicationContext() );
            if( checkinternet ) {
                toastMessagges.show_toast_messagge();
            }
            else {

            }

        }

        @Override
        public void LongToastMessagge() {
            Toast.makeText(getApplicationContext(), "Hello World", Toast.LENGTH_LONG).show();


}
}

这是我的 ToastMessagges.java 文件。

public class ToastMessagges {

    ToastMessaggeCallback toastMessaggeCallback;
    public void show_toast_messagge(){
        toastMessaggeCallback.LongToastMessagge();
    }


    public static interface ToastMessaggeCallback {
        public void LongToastMessagge();
    }
}

当启动这个应用程序。我收到 NullPointerException 错误。

Caused by: java.lang.NullPointerException
            at com.medyasef.bulenttirasnewapp.bulenttiras.functions.ToastMessagges.show_toast_messagge(ToastMessagges.java:22)
            at com.medyasef.bulenttirasnewapp.bulenttiras.MotherActivity.AppStarter(MotherActivity.java:36)
            at com.medyasef.bulenttirasnewapp.bulenttiras.MotherActivity.onCreate(MotherActivity.java:29)

ToastMessagges.java:22

toastMessaggeCallback.LongToastMessagge();

抱歉英语不好。 请帮忙。 谢谢。

Answer 1

你还没有初始化你ToastMessaggeCallback toastMessaggeCallback。

要做到这一点，写

ToastMessaggeCallback toastMessaggeCallback = new ToastMessaggeCallback(){

    public void LongToastMessagge(){
        // add some toasting code here
    }

};

这将使一个对象实现您的接口（称为“匿名类”）。当然，你的ToastMessaggeCallback 应该在方法LongToastMessagge 中做一些事情，所以在那里添加所需的代码。

Answer 2

您的ToastMessagges 类需要提供注册回调的方法。然后，您的Activity 需要在您构造ToastMessages 对象后立即调用此方法将自己注册为回调。

Answer 3

我会建议你创建一个Util 类而不是Interface。我在这里给你一个Util类的例子。

public class Util {

    public static void showToast(Context context, String text) {

        Toast.makeText(context, text, Toast.LENGTH_SHORT).show();

    }

}

然后从您的活动中调用showToast() 方法，如下所示...

Util.showToast(YourActivity.this, "text");

更新：

将您的Interface 声明为个人，而不是在下面的类中...

public interface ToastMessaggeCallback {

    public void showLongToastMessagge(String text);

}

然后实现Interface如下...

public class MotherActivity extends ActionBarActivity implements ToastMessaggeCallback {

    @Override
    protected void onStart() {
        super.onStart();

    }

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

        AppStarter();
    }

    private void AppStarter(){

        boolean checkinternet   = InternetControl.checkInternetConnection( getApplicationContext() );
        if( checkinternet ) {

            showLongToastMessagge("Hello World");

        }
        else {

        }

    }

    @Override
    public void showLongToastMessagge(String text) {

        Toast.makeText(getApplicationContext(), text, Toast.LENGTH_LONG).show();

   }
}