Question

在处理从主活动到片段的通信时，我遇到了这个问题。监视器一直说：

   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.mick.myApplication.FragmentCal.readfiles(java.util.LinkedList)' on a null object reference
                                                                                 at com.example.mick.myApplication.main.onCreate(main.java:101)
                                                                                 at android.app.Activity.performCreate(Activity.java:6289)
                                                                                 at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
                                                                                 at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
                                                                                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2758) 
                                                                                 at android.app.ActivityThread.access$900(ActivityThread.java:177) 
                                                                                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1448) 
                                                                                 at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                                 at android.os.Looper.loop(Looper.java:145) 
                                                                                 at android.app.ActivityThread.main(ActivityThread.java:5942) 
                                                                                 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:1399) 
                                                                                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

这是片段事务和发送消息的代码：

    fragmentinfo = new FragmentInfo();
    fragmentcal = new FragmentCal();
    fragmentrep = new FragmentRep();
    fragmentdicn = new FragmentDicn();
    fragmentManager = getSupportFragmentManager();

    if (!fragmentcal.isAdded()) {
        if (currentFragment == null) {
            fragmentManager.beginTransaction().add(R.id.fragmentlayout, fragmentcal).commit();
        } else {
            fragmentManager.beginTransaction().hide(currentFragment).add(R.id.fragmentlayout, fragmentcal).commit();
        }
        FragmentCal fragmentCal = (FragmentCal) getSupportFragmentManager().findFragmentById(R.id.fragmentlayout);
        fragmentCal.readfiles(nameList);
    } else {
        fragmentManager.beginTransaction().hide(currentFragment).show(fragmentcal).commit();
    }
    currentFragment = fragmentcal;

为此，我为所有活动和片段导入android.support.v4.app.Fragment。

Answer 1

在将片段添加到活动之前，您无法执行findFragmentById。当您创建setContentView时，Activity会使布局膨胀并将其视图添加到当前窗口。在此布局中具有Fragment的情况下，将其与xml中提供的给定Id或Tag一起添加到FragmentManager中。只有在这一刻你才能在那里找到它。所以你要做的就是将findFragmentById移到setContentView下面。

来源：NullpointerException when findFragmentById

希望它有所帮助！祝你好运！

Android findFragmentById返回null

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