java - Android Fragment 导致 NullPointerException

Question

首先要说的是，我知道有很多类似的帖子，并且我看过它们，但是我对 Android 开发很陌生，还不够熟悉它，无法修改其他答案来修复我的确切问题问题。

我有一个默认使用包含按钮和 TextView 的新项目创建的 fragment ，并且在我的 MainActivity.java 中我为该按钮创建了一个 onClick 函数。该函数抛出 NPE。这是 LogCat:

03-10 19:26:52.620: D/AndroidRuntime(878): Shutting down VM
03-10 19:26:52.620: W/dalvikvm(878): threadid=1: thread exiting with uncaught exception (group=0xb2af2ba8)
03-10 19:26:52.630: E/AndroidRuntime(878): FATAL EXCEPTION: main
03-10 19:26:52.630: E/AndroidRuntime(878): Process: com.example.main, PID: 878 
03-10 19:26:52.630: E/AndroidRuntime(878): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.main/com.example.main.MainActivity}: java.lang.NullPointerException
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.app.ActivityThread.access$800(ActivityThread.java:135)
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.os.Handler.dispatchMessage(Handler.java:102)
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.os.Looper.loop(Looper.java:136)
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.app.ActivityThread.main(ActivityThread.java:5017)
03-10 19:26:52.630: E/AndroidRuntime(878):  at java.lang.reflect.Method.invokeNative(Native Method)
03-10 19:26:52.630: E/AndroidRuntime(878):  at java.lang.reflect.Method.invoke(Method.java:515)
03-10 19:26:52.630: E/AndroidRuntime(878):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
03-10 19:26:52.630: E/AndroidRuntime(878):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
03-10 19:26:52.630: E/AndroidRuntime(878):  at dalvik.system.NativeStart.main(Native Method)
03-10 19:26:52.630: E/AndroidRuntime(878): Caused by: java.lang.NullPointerException
03-10 19:26:52.630: E/AndroidRuntime(878):  at com.example.main.MainActivity.onCreate(MainActivity.java:31)
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.app.Activity.performCreate(Activity.java:5231)
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
03-10 19:26:52.630: E/AndroidRuntime(878):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
03-10 19:26:52.630: E/AndroidRuntime(878):  ... 11 more
03-10 19:31:54.292: I/Process(878): Sending signal. PID: 878 SIG: 9

MainActivity.java:

package com.example.main;

import android.support.v7.app.ActionBarActivity;
...

public class MainActivity extends ActionBarActivity {

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

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new     PlaceholderFragment()).commit();
        }

        final TextView myText = (TextView) findViewById(R.id.text1);
        Button myButton = (Button) findViewById(R.id.button1);

        myButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                myText.setText("Yes!");
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            return rootView;
        }
    }

}  

Answer 1

如果TextView和Button位于布局的fragment中，则需要在fragment的onCreatedView方法中找到它们的id。然后，你应该这样做:

public static class PlaceholderFragment extends Fragment {

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        final TextView myText = (TextView) rootView.findViewById(R.id.text1);
        Button myButton = (Button) rootView.findViewById(R.id.button1);
        myButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                myText.setText("Yes!");
            }
        });
        return rootView;
    }
}