选择导航抽屉项时更改TextView

Question

我试图使导航抽屉中的三个不同选项使用相同的片段，但在该片段的TextView中使用不同的值。

我使用Android Studio创建了一个标准的Navigation Drawer Activity，并尝试在更改标题的相同方法中使用setText，如下所示：

public void onSectionAttached(int number) {
        switch (number) {
            case 1:
                mTitle = getString(R.string.title_section1);
                Namn.setText("Item 1");
                break;
            case 2:
                mTitle = getString(R.string.title_section2);
                Namn.setText("Item 2");
                break;
            case 3:
                mTitle = getString(R.string.title_section3);
                Namn.setText("Item 3");
                break;
        }
    }

当我尝试运行应用程序时，它会立即关闭，并且出现以下错误：

Process: com.test.je.test, PID: 22605
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.test.je.test/com.test.je.test.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2187)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2236)
            at android.app.ActivityThread.access$800(ActivityThread.java:138)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1199)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5034)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:611)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.test.je.test.MainActivity.onSectionAttached(MainActivity.java:68)
            at com.test.je.test.MainActivity$PlaceholderFragment.onAttach(MainActivity.java:153)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:903)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1126)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1489)
            at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:548)
            at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
            at android.app.Activity.performStart(Activity.java:5251)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2160)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2236)
            at android.app.ActivityThread.access$800(ActivityThread.java:138)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1199)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5034)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:611)
            at dalvik.system.NativeStart.main(Native Method)

我已经用TextView Namn声明了TextView； 和Namn =（TextView）findViewById（R.id.twnamn）;

我在这里做错了什么？

Answer 1

因此，用户@feresr帮助我了解了他的评论出了什么问题。

我在onCreateView（）方法中创建了一个if语句，它解决了问题。

在创建片段之后，我还必须声明textview。

这是代码：

public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView;
            Bundle args = getArguments();
            int currentView = getArguments().getInt(ARG_SECTION_NUMBER);

            if(currentView == 1){
                rootView = inflater.inflate(R.layout.fragment_main, container, false);
                TextView Namn = (TextView) rootView.findViewById(R.id.twnamn);
                Namn.setText("Val 1");
            }else if(currentView == 2){
                rootView = inflater.inflate(R.layout.fragment_main, container, false);
                TextView Namn = (TextView) rootView.findViewById(R.id.twnamn);
                Namn.setText("Val 2");
            }else if(currentView == 3){
                rootView = inflater.inflate(R.layout.fragment_main, container, false);
                TextView Namn = (TextView) rootView.findViewById(R.id.twnamn);
                Namn.setText("Val 3");
            }else {
                rootView = inflater.inflate(R.layout.fragment_main, container, false);
            }


            return rootView;
        }