Java 替换片段时出现NullpointerException

我有两个片段（MainFragment和另一个片段）和MainActivity，如下所述

在MainActivity的“onCreate”中，我添加了MainFragment。单击后，我将添加另一个片段，该片段在两个editText字段中获取数据，并将其传递给显示在textView字段中的mainfragment

MainFragment.java

Button a;
TextView t1,t2;
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_main, container, false);
    return rootView;
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    a = (Button) getActivity().findViewById(R.id.button);
    t1 = (TextView) getActivity().findViewById(R.id.textView);
    t2 = (TextView) getActivity().findViewById(R.id.textView2);
    //a.setOnClickListener(this);
}

public void showData(String a, String b) {
    t1.setText(a);
    t2.setText(b);
}

另一个片段.java MainActivity.java 片段替换导致nullpointerexception

java.lang.NullPointerException
        at com.exa.researchanddev.app.MainFragment.showData(MainFragment.java:31)
        at com.exa.researchanddev.app.MainActivity.sendData(MainActivity.java:57)
        at com.exa.researchanddev.app.AnotherFragment.onClick(AnotherFragment.java:42)
        at android.view.View.performClick(View.java:4084)
        at android.view.View$PerformClick.run(View.java:16966)
        at android.os.Handler.handleCallback(Handler.java:615)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:4745)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
        at dalvik.system.NativeStart.main(Native Method)

而且

@Override
public void sendData(String a, String b) {
    Toast.makeText(this, a + " = "+ b, Toast.LENGTH_SHORT).show();
    MainFragment mainFragment = new MainFragment();
    getSupportFragmentManager().beginTransaction().replace(R.id.container,mainFragment).commit();
    //mainFragment.showData(a,b);
}

当我对showData函数进行注释时，代码运行良好。碎片正在被替换

---

未注释时，将抛出NullPointerException。

您的问题在于：

a = (Button) getActivity().findViewById(R.id.button);
...

在此之前被称为

return inflater.inflate(R.layout.fragment_another, container, false);

您获得的NPE是因为您试图从尚未膨胀的布局中引用视图的ID[

R.ID.button

]

在视图膨胀后，尝试将

a=…

和以下变量赋值移动到

覆盖

onLayout

方法，该方法将在视图膨胀后调用。在这种情况下，您应该能够从布局中进行变量赋值

protected void onLayout(boolean changed, int l, int t, int r, int b){
    super.onLayout(changed, l, t, r, b);
    ...
}

你的问题是：

@Override
public void sendData(String a, String b) {
    MainFragment mainFragment = new MainFragment();
    getSupportFragmentManager().beginTransaction().replace(R.id.container,mainFragment).commit();
    mainFragment.showData(a,b); 
}

片段的事务是异步的。因此，片段中的文本视图为空。更好的方法是将字符串作为参数传递给片段

@Override
public void sendData(String a, String b) {
    MainFragment mainFragment = new MainFragment();
    Bundle bundle = new Bundle();
    bundle.putString("A", a);
    bundle.putString("B", b);
    mainFragment.setArguments(bundle);
    getSupportFragmentManager().beginTransaction().replace(R.id.container,mainFragment).commit();

}

现在，您可以在Fragments onCreateView中读取参数的数据

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    a = (Button) getActivity().findViewById(R.id.button);
    t1 = (TextView) getActivity().findViewById(R.id.textView);
    t2 = (TextView) getActivity().findViewById(R.id.textView2);

    if (getArguments() != null) {
          t1.setText(getArguments().getString("A"));
          t2.setText(getArguments().getString("B"));
    }
}

@Override
public void sendData(String a, String b) {
    MainFragment mainFragment = new MainFragment();
    Bundle bundle = new Bundle();
    bundle.putString("A", a);
    bundle.putString("B", b);
    mainFragment.setArguments(bundle);
    getSupportFragmentManager().beginTransaction().replace(R.id.container,mainFragment).commit();

}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    a = (Button) getActivity().findViewById(R.id.button);
    t1 = (TextView) getActivity().findViewById(R.id.textView);
    t2 = (TextView) getActivity().findViewById(R.id.textView2);

    if (getArguments() != null) {
          t1.setText(getArguments().getString("A"));
          t2.setText(getArguments().getString("B"));
    }
}