Java 如何扩展navHostFragment？

我想要一个碎片。但我不能

首先，我创建了新片段（CustomNavFragment）。这是一块碎片

public abstract class CustomNavFragment extends NavHostFragment {
     public CustomNavFragment(){}

     public abstract boolean customAction();
}

我创建了登录、连接、结果等片段。 他们还执行了自定义操作。 当发生某些事件时，MainActivity调用当前片段的CustomAction

我的主要活动是下一个

public class MainActivity extends AppCompatActivity implements SomeEventListener{
    NavController navController = null;
    CustomNavFragment currentFragment = null;

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

        currentFragment = (CustomNavFragment) getSupportFragmentManager().findFragmentById(R.id.nav_host_fragment);
        navController = currentFragment.getNavController();

    }

    @Override
    public void eventListener(){
        if(currentFragment.customAction()) // do something;
    }

这是主要活动的xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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=".MainActivity">

    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="com.alticast.framework.ui.CustomNavFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/nav_graph"/>

</androidx.constraintlayout.widget.ConstraintLayout>

它说“确保类名存在，是公共的，并且有一个空的公共构造函数。”但是它们有空的构造函数

谁来帮这个

谢谢

这一行是错误的：

    android:name="com.alticast.framework.ui.CustomNavFragment"

无法创建抽象类的实例。您应该将其替换为

LoginFragment

或

CustomNavFragment

的另一个实现

Caused by: android.view.InflateException: Binary XML file line #9 in com.example.mypkg:layout/activity_main: Binary XML file line #9 in com.example.mypkg:layout/activity_main: Error inflating class fragment
     Caused by: android.view.InflateException: Binary XML file line #9 in com.example.mypkg:layout/activity_main: Error inflating class fragment
     Caused by: androidx.fragment.app.Fragment$InstantiationException: Unable to instantiate fragment com.example.mypkg.ui.CustomNavFragment: make sure class name exists, is public, and has an empty constructor that is public
        at androidx.fragment.app.Fragment.instantiate(Fragment.java:555)
        at androidx.fragment.app.FragmentContainer.instantiate(FragmentContainer.java:57)
        at androidx.fragment.app.FragmentManager$3.instantiate(FragmentManager.java:390)
        at androidx.fragment.app.FragmentLayoutInflaterFactory.onCreateView(FragmentLayoutInflaterFactory.java:97)
        at androidx.fragment.app.FragmentController.onCreateView(FragmentController.java:135)
        at androidx.fragment.app.FragmentActivity.dispatchFragmentsOnCreateView(FragmentActivity.java:356)
        at androidx.fragment.app.FragmentActivity.onCreateView(FragmentActivity.java:335)
        at android.view.LayoutInflater.tryCreateView(LayoutInflater.java:1069)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:997)
        at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:961)
        at android.view.LayoutInflater.rInflate(LayoutInflater.java:1123)
        at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:1084)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:682)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:534)
        at android.view.LayoutInflater.inflate(LayoutInflater.java:481)
        at androidx.appcompat.app.AppCompatDelegateImpl.setContentView(AppCompatDelegateImpl.java:696)
        at androidx.appcompat.app.AppCompatActivity.setContentView(AppCompatActivity.java:170)
        at com.example.mypkg.MainActivity.onCreate(MainActivity.java:33)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)

    android:name="com.alticast.framework.ui.CustomNavFragment"