为什么显示TextView对象为null？

Question

我正在制作一个应用程序，其中我正在使用片段。我以纵向和横向模式制作了它。

在纵向模式下，我所做的是将显示一个ListView，在选择下一个活动的项目时，将显示描述

在横向模式下，我在同一活动中使用了两个片段，其中一个片段具有ListView，第二个片段具有描述

但是当我点击列表视图中的项目时，它显示错误

cannot invoke method on null TextView object 

但是我已经在Fragment2.java中初始化了onActivityCreated（）方法，那为什么会出现这个错误呢？

Main.xml（肖像）

<LinearLayout

    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.grepthor.fragment4.MainActivity">

    <fragment
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/fragment1"
        android:name="com.grepthor.fragment4.Fragment1"
        />

</LinearLayout>

Main.xml（格局）

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal" android:layout_width="match_parent"
    android:layout_height="match_parent">


    <fragment
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:id="@+id/fragment1"
        android:name="com.grepthor.fragment4.Fragment1"/>


    <fragment
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="1"
        android:id="@+id/fragment2"
        android:name="com.grepthor.fragment4.Fragment2"/>
</LinearLayout>

Main.java

import android.app.FragmentManager;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity implements Fragment1.Communicator {


    Fragment1 f1;
    Fragment2 f2;
    FragmentManager fm;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        fm=getFragmentManager();
        f1= (Fragment1) fm.findFragmentById(R.id.fragment1);
        f1.setCommunicator(this);
    }

    @Override
    public void respond(int position) {

       f2= (Fragment2) fm.findFragmentById(R.id.fragment2);

        if(f2!=null && f2.isVisible())
        {
f2.changeData(position);
        }
        else
        {
            Intent i=new Intent(this,SecondActivity.class);
            i.putExtra("index",position);
            startActivity(i);
        }
    }
}

Fragment1.xml

<RelativeLayout

    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.grepthor.fragment4.Fragment1">

    <ListView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/list"/>

</RelativeLayout>

Fragment1.java

import android.app.Fragment;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

public class Fragment1 extends Fragment implements AdapterView.OnItemClickListener {

    ListView l;
    Communicator comm;
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.activity_fragment1,container,false);
    }


    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);



        l= (ListView) getActivity().findViewById(R.id.list);
        ArrayAdapter ad=ArrayAdapter.createFromResource(getActivity(),R.array.topic,android.R.layout.simple_list_item_1);
        l.setAdapter(ad);
       // Toast.makeText(getActivity(),"helo",Toast.LENGTH_LONG).show();
        l.setOnItemClickListener(this);
    }

    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
comm.respond(position);
    }


    public void setCommunicator(Communicator c)
    {
        comm=c;
    }

    interface Communicator
    {
        void respond(int data);
    }
}

Fragment2.xml

<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.grepthor.fragment4.Fragment2">



    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/text"/>

    </RelativeLayout>

Fragment2.java

public class Fragment2 extends Fragment {

    TextView t;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.activity_fragment2,container,false);
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        // Toast.makeText(getActivity(),"helo",Toast.LENGTH_LONG).show();
        t=(TextView)getActivity().findViewById(R.id.text);
    }


    public void changeData(int position)
    {
        String des[]= getResources().getStringArray(R.array.des);
        t.setText(des[position]);
    }
}

SecondActivity.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.grepthor.fragment4.SecondActivity">


    <fragment
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/fragment2"
        android:name="com.grepthor.fragment4.Fragment2"/>
</LinearLayout>

SecondActivity.java

import android.content.Intent;
import android.app.FragmentManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class SecondActivity extends AppCompatActivity {

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

        Intent i=getIntent();
        int position=i.getExtras().getInt("index",0);

        FragmentManager fm=getFragmentManager();
        Fragment2 f2= (Fragment2) fm.findFragmentById(R.id.fragment2);

    f2.changeData(position);
       }
}

的strings.xml

<resources>
    <string name="app_name">Fragment4</string>

    <string-array name="topic">
        <item>Heading</item>
        <item>Stories</item>
        <item>Settings</item>
        <item>Music</item>
    </string-array>

    <string-array name="des">
        <item>ldwkflkwdflkdqdvmqmdvmqdvmqlkrvqkneroqknre;qoiengqrknqrqvqfvaa</item>
        <item>vqdlkvnqldvnqdnvqdnvqepojpqojjoergjelgefbfmsmfv</item>
        <item>dwkfmnqdkwnfqkjwdnflwndfqkwheflqierqieurpqeirqrkmqwrgfqnqnmdqd</item>
        <item>skjsbdkafsknaslnaslfnaljfhadfkjgjhgasjdvsadablvasfgasfkgnjlkrglrgqmqbvmadbsv</item>
    </string-array>

</resources>

logcat的

FATAL EXCEPTION: main
                                                                        Process: com.grepthor.fragment4, PID: 31014
                                                                        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.grepthor.fragment4/com.grepthor.fragment4.SecondActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                                                                            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2430)
                                                                            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)
                                                                            at android.app.ActivityThread.access$900(ActivityThread.java:157)
                                                                            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351)
                                                                            at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                            at android.os.Looper.loop(Looper.java:148)
                                                                            at android.app.ActivityThread.main(ActivityThread.java:5437)
                                                                            at java.lang.reflect.Method.invoke(Native Method)
                                                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                                         Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                                                                            at com.grepthor.fragment4.Fragment2.changeData(Fragment2.java:35)
                                                                            at com.grepthor.fragment4.SecondActivity.onCreate(SecondActivity.java:21)
                                                                            at android.app.Activity.performCreate(Activity.java:6532)
                                                                            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
                                                                            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2383)
                                                                            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490) 
                                                                            at android.app.ActivityThread.access$900(ActivityThread.java:157) 
                                                                            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1351) 
                                                                            at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                            at android.os.Looper.loop(Looper.java:148) 
                                                                            at android.app.ActivityThread.main(ActivityThread.java:5437) 
                                                                            at java.lang.reflect.Method.invoke(Native Method) 
                                                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

Answer 1

您的Fragment2拥有TextView，并且您正试图在Activity中找到它。您需要在Fragment2中执行此操作，并将其从onActivityCreated()：

中删除

@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.activity_fragment2,container,false);
    TextView textView = (TextView) rootView.findViewById(R.id.text);
}

同样在将来，只发布代码的相关部分与发布所有部分。

Answer 2

将textView初始化放到onViewCreated：

//Fragment2.java
@Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    t = (TextView) view.findViewById(R.id.text);
}