Android + java.lang.NullPointerException：尝试调用接口方法

Android + java.lang.NullPointerException: Attempt to invoke interface method

提问by BlueHatCoder

When compiling my code i get this error ("java.lang.NullPointerException: Attempt to invoke interface method") and I still can't figure why.

编译我的代码时出现此错误（“java.lang.NullPointerException：尝试调用接口方法”），但我仍然无法弄清楚原因。

Here is my code:

这是我的代码：

    public class FragmentDetails extends Fragment {

        private TextView text1, text2, text3, text4, text5 = null;
        private Button button1 = null;


        OnDetailsDefinedListener mCallback;

        public interface OnDetailsDefinedListener {
            void executeOrder(String a, String b, String c, String d, String e);
        }


        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

            rootView = inflater.inflate(R.layout.fragment_details, container, false);

            button1 = (Button) rootView.findViewById(...);

            button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                text1 = getActivity().findViewById(...);
                text2 = getActivity().findViewById(...);
                text3 = getActivity().findViewById(...);
                text4 = getActivity().findViewById(...);
                text5 = getActivity().findViewById(...);

                String a = text1.getText().toString();
                String b = text2.getText().toString();
                String c= text3.getText().toString();
                String d= text4.getText().toString();
                String e= text5.getText().toString();

                //this is where my error appears
                mCallback.executeOrder(a, b, c, d, e);
            }
            return rootView;
        }
    }

(here is my error message)

（这是我的错误信息）

    05-26 09:39:20.868    2916-2916/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
        Process: com.example.blueHatCoder.myapplication, PID: 2916
        java.lang.NullPointerException: Attempt to invoke interface method 'void com.example.blueHatCoder.myapplication.FragmentDetails$OnDetailsDefinedListener.executeOrder(java.lang.String, java.lang.String, java.lang.String, java.lang.String, java.lang.String)' on a null object reference
        at com.example.blueHatCoder.myapplication.FragmentDetails.onClick(FragmentDetails.java:131)
        at android.view.View.performClick(View.java:4756)
        at android.view.View$PerformClick.run(View.java:19749)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5221)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

Thank you in advance.

先感谢您。

回答by Jens

Your variable mCallbackis defined but not initialized. You have to initialize it before you can use it.

您的变量mCallback已定义但未初始化。您必须先初始化它，然后才能使用它。

回答by Stefan Beike

mCallbackis never initiated. you have to do something like that:

mCallback从未启动。你必须做这样的事情：

mCallback = new OnDetailsDefinedListenerImpl(); 
mCallback.executeOrder(a, b, c, d, e);

or

或者

mCallback = new OnDetailsDefinedListener(){
   @Override
   void executeOrder(String a, String b, String c, String d, String e){
     //doSomething
   };
}

回答by Sethbrin

You should add a public method to the FragmentDetails class.

您应该向 FragmentDetails 类添加一个公共方法。

public setOnDetailsDefinedListener(OnDetailsDefinedListener listener) {
    mCallback = listener;
}

And just implement it in the caller class. just like how you do when add button listener.

只需在调用者类中实现它。就像添加按钮侦听器时所做的一样。

mFragmentDetails.setOnDetailsDefinedListener(new OnDetailsDefinedListener() {
// You code
})

回答by Miguel Gonzalez

Maybe you need to use onAttach(Activity activity) fragments method

也许你需要使用 onAttach(Activity activity) 片段方法

public void onAttach(Activity activity){
    super.onAttach(activity);
    if(activity instanceof onSomeListener){
        mListener=(onSomeListener) activity;
    }
}