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我已经了解 Fragment ，当我尝试Fragment通过点击进行更改Button并且应用程序被强制停止时，它会给出这样的错误

中的错误消息Logcat：

  E/AndroidRuntime: FATAL EXCEPTION: main
    Process: mb.ganesh.fragment_practices, PID: 10451
    java.lang.IllegalStateException: Could not execute method for android:onClick
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:402)
        at android.view.View.performClick(View.java:6597)
        at android.view.View.performClickInternal(View.java:6574)
        at android.view.View.access$3100(View.java:778)
        at android.view.View$PerformClick.run(View.java:25885)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Method.invoke(Native Method)
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:397)
        at android.view.View.performClick(View.java:6597) 
        at android.view.View.performClickInternal(View.java:6574) 
        at android.view.View.access$3100(View.java:778) 
        at android.view.View$PerformClick.run(View.java:25885) 
        at android.os.Handler.handleCallback(Handler.java:873) 
        at android.os.Handler.dispatchMessage(Handler.java:99) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'androidx.fragment.app.FragmentTransaction androidx.fragment.app.FragmentManager.beginTransaction()' on a null object reference
        at mb.ganesh.fragment_practices.MainActivity.changeFragemnet(MainActivity.java:29)
        at java.lang.reflect.Method.invoke(Native Method) 
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:397) 
        at android.view.View.performClick(View.java:6597) 
        at android.view.View.performClickInternal(View.java:6574) 
        at android.view.View.access$3100(View.java:778) 
        at android.view.View$PerformClick.run(View.java:25885) 
        at android.os.Handler.handleCallback(Handler.java:873) 
        at android.os.Handler.dispatchMessage(Handler.java:99) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 

实际上我想通过单击按钮更改片段，任何开发人员请解决此问题。

主要活动：

package mb.ganesh.fragment_practices;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;
import android.os.Bundle;
import android.view.View;
public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    public void changeFragemnet(View view){
        Fragment fragment;
        if (view == findViewById(R.id.b1)){
            fragment = new FragmentOne();
            FragmentManager fm = fragment.getFragmentManager();
            FragmentTransaction ft = fm.beginTransaction();
             ft.replace(R.id.firstFM , fragment);
            ft.commit();
        }
    }
}

上面的代码有一个警告FragmentTransaction ft = fm.beginTransaction();

活动_xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">
    <Button
        android:id="@+id/b1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:onClick="changeFragemnet"
        android:padding="20dp"
        android:text="@string/fragment_1" />
    <fragment
        android:id="@+id/firstFM"
        android:name="mb.ganesh.fragment_practices.FragmentOne"
        android:layout_width="match_parent"
        android:layout_height="0px"
        android:layout_weight="1" />
    <Button
        android:id="@+id/b2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="20dp"
        android:onClick="changeFragemnet"
        android:text="@string/fragment_2" />
</LinearLayout>
4

1 回答 1

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在 MainActivity 中：

FragmentManager fm = fragment.getFragmentManager();

应该：

FragmentManager fm = getSupportFragmentManager();

返回 FragmentManager 以与与此活动关联的片段进行交互。

正如您在文档中所读到的

返回 FragmentManager 以与与此片段的活动关联的片段进行交互。getActivity()请注意，在从片段被放置到 aFragmentTransaction直到它被提交并附加到其活动的这段时间之前，这将是非空的。

并且为刚刚创建的 Fragment 返回 null。

于 2020-04-30T06:17:27.723 回答