替换片段时出现 NullpointerException

Question

我有两个 Fragment（MainFragment 和 AnotherFragment）和 MainActivity，如下所述。

在 MainActivity 的“onCreate”中，我添加了 MainFragment。单击后，我添加了 AnotherFragment，它在两个 editText 字段中获取数据并将它们传递给显示在 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);
}

AnotherFragment.java

Button b;
EditText e1,e2;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_another, container, false);
    return rootView;
}

Comm comm;
@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    comm = (Comm) activity;
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    b = (Button) getActivity().findViewById(R.id.buttonA);
    e1 = (EditText) getActivity().findViewById(R.id.editText);
    e2 = (EditText) getActivity().findViewById(R.id.editText2);
    b.setOnClickListener(this);
}

@Override
public void onClick(View view) {
    comm.sendData(e1.getText().toString(), e2.getText().toString());
}

public interface Comm {
    public void sendData(String a, String b);
}

MainActivity.java

FragmentManager fragmentManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    fragmentManager = getSupportFragmentManager();
    if (savedInstanceState == null) {
        fragmentManager.beginTransaction()
                .add(R.id.container, new MainFragment())
                .commit();
    }
}

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

public void newone(View view) {
    AnotherFragment anotherFragment = new AnotherFragment();
    getSupportFragmentManager().beginTransaction().replace(R.id.container,anotherFragment).commit();
}

片段替换导致空指针异常。

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)

还有……

@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);
}

当我评论 showData 函数时，代码运行良好。碎片正在被替换。

如果没有注释，则抛出 NullPointerException。

Answer 1

你的问题在这里：

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

Fragment 的事务是异步的。因此，您的 Fragment 中的 TextViews 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"));
    }
}

Answer 2

你的问题是这样的：

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

在此之前调用

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 [R.id.button] 的 ID。

尝试在视图膨胀后将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。在此，您应该能够从布局中进行变量分配。

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