Android 片段空指针异常

Question

我正在学习android开发（新手）。我想用textview向mainActivity添加一个片段并更改文本。此代码完美运行：

public class Fragment1 extends Fragment {
//@Nullable
//@Override
TextView textt;
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view =  inflater.inflate(R.layout.fragment_fragment1, container, false);
    ((TextView)view.findViewById(R.id.textview)).setText("some text");

    return view;
}

}

但是这段代码调用了空指针异常：

public class Fragment1 extends Fragment {
//@Nullable
//@Override
TextView textt;
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View view =  inflater.inflate(R.layout.fragment_fragment1, container, false);

    textt =  (TextView)view.findViewById(R.id.textview);
    return view;
}

public void updateText() {
    textt.setText("some text");
}
}

和：

public class Fragment1 extends Fragment {
//@Nullable
//@Override
View view;
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    view =  inflater.inflate(R.layout.fragment_fragment1, container, false);

    return view;
}

public void updateText() {

    ((TextView)view.findViewById(R.id.textview)).setText("some text");
}
}

主要活动：

public class MainActivity extends AppCompatActivity {

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


    FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
    Fragment1 frg1 = new Fragment1();
    ft.replace(R.id.fragment_container, frg1);
    ft.addToBackStack(null);
    ft.commit();
    frg1.updateText();
}
}

为什么这些代码提供不同的结果？

my project

Answer 1

这里调用updateText函数的时候并没有创建textview

在您的片段中执行此类调用。不在包含片段的 activity 上。

Answer 2

frg1.updateText(); 在片段的 onCreateView 生命周期方法被调用之前调用。您应该在设置

之后调用该方法

textt = (TextView)view.findViewById(R.id.textview);

 View view =  inflater.inflate(R.layout.fragment_fragment1, container, false);

    textt =  (TextView)view.findViewById(R.id.textview);
    this.updateText();//here
    return view;

查看有关 onCreateView

的 fragment lifecycle 文档

The system calls this when it's time for the fragment to draw its user interface for the first time.

加载片段时会发生这种情况，而不是在 activity 中创建片段时发生。

Android 片段空指针异常

Android Fragment Null pointer exception

android

nullpointerexception

android-fragments

android-fragmentactivity