Java 片段通信问题（尝试调用虚拟方法）

我有一个带有2个片段的选项卡式活动。我试图将一个字符串从第一个片段传输到第二个片段，但得到了NULL指针异常。 以下是MainActivity.java：

package com.example.myapplication;

import android.os.Bundle;
import com.google.android.material.tabs.TabLayout;

import androidx.fragment.app.FragmentTransaction;
import androidx.viewpager.widget.ViewPager;
import androidx.appcompat.app.AppCompatActivity;
import com.example.myapplication.ui.main.SectionsPagerAdapter;

public class MainActivity extends AppCompatActivity implements Frag1.Exchange {

    private static final String TAG = "MainActivity";

    private SectionsPagerAdapter sectionsPagerAdapter;

    private ViewPager viewPager;

    private TabLayout tabLayout;

    private void setupViewPager(ViewPager viewPager){
        SectionsPagerAdapter sectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
        sectionsPagerAdapter.addFragment(new Frag1(), "FirstTab");
        sectionsPagerAdapter.addFragment(new Frag2(), "SecondTab");
        viewPager.setAdapter(sectionsPagerAdapter);
    }

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

        sectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        viewPager = (ViewPager) findViewById(R.id.view_pager);
        setupViewPager(viewPager);

        tabLayout = (TabLayout) findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(viewPager);

    }

    @Override
    public void stringExchange(String s) {
        Frag2 frag2 = (Frag2) getSupportFragmentManager().findFragmentByTag("SecondTab");
        frag2.displayExchange(s);
    }
}

下面是SectionsPagerAdapter.java

package com.example.myapplication.ui.main;

import android.content.Context;

import androidx.annotation.Nullable;
import androidx.annotation.StringRes;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentPagerAdapter;

import com.example.myapplication.R;

import java.util.ArrayList;
import java.util.List;

/**
 * A [FragmentPagerAdapter] that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */
public class SectionsPagerAdapter extends FragmentPagerAdapter {

    private final List<Fragment> fragmentList = new ArrayList<>();
    private final List<String> stringList = new ArrayList<>();

    public void addFragment(Fragment fragment, String title){
        fragmentList.add(fragment);
        stringList.add(title);
    }

    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a  (defined as a static inner class below).
        return fragmentList.get(position);
    }

    @Nullable
    @Override
    public CharSequence getPageTitle(int position) {
        return stringList.get(position);
    }

    @Override
    public int getCount() {
        // Show 2 total pages.
        return fragmentList.size();
    }
}

其xlm tab1_fragment.xlm：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp"
    android:id="@+id/tab1"
    >
    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/transfer"
        android:layout_centerInParent="true"/>

</RelativeLayout>

最后是Frag2.java：

package com.example.myapplication;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

import java.util.ArrayList;

public class Frag2 extends Fragment {
    private static final String TAG = "SecondTab";
    private ListView listView;
    private ArrayList<String> arrayList = new ArrayList<>();

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.tab2_fragment, container, false);
        listView = (ListView) view.findViewById(R.id.list_view);
        ArrayAdapter arrayAdapter = new ArrayAdapter(getContext(),android.R.layout.simple_list_item_1,arrayList);

        listView.setAdapter(arrayAdapter);

        return view;
    }

    public void displayExchange(String s){
        arrayList.add(s);
    }


}

包com.example.myapplication；
导入android.os.Bundle；
导入android.view.LayoutInflater；
导入android.view.view；
导入android.view.ViewGroup；
导入android.widget.ArrayAdapter；
导入android.widget.ListView；
导入android.widget.Toast；
导入androidx.annotation.NonNull；
导入androidx.annotation.Nullable；
导入androidx.fragment.app.fragment；
导入java.util.ArrayList；
公共类Frag2扩展了片段{
私有静态最终字符串TAG=“SecondTab”；
私有列表视图列表视图；
私有ArrayList ArrayList=新ArrayList（）；
@可空
@凌驾
创建视图时的公共视图（@NonNull LayoutInflater inflater、@Nullable ViewGroup container、@Nullable Bundle savedInstanceState）{
视图=充气机。充气（R.layout.tab2_碎片，容器，假）；
listView=（listView）view.findViewById（R.id.list\u视图）；
ArrayAdapter ArrayAdapter=新的ArrayAdapter（getContext（），android.R.layout.simple_list_item_1，arrayList）；
setAdapter（arrayAdapter）；
返回视图；
}
公共无效显示交换（字符串s）{
arrayList.add（s）；
}
}

当我调用findFragmentByTag（“SecondTab”）时，我认为问题在于MainActivity，因为它的标记没有设置为SecondTab，但我不知道如何设置它。

---

任何帮助都将不胜感激

使用ViewModel在片段之间共享数据。参考：

package com.example.myapplication;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

import java.util.ArrayList;

public class Frag2 extends Fragment {
    private static final String TAG = "SecondTab";
    private ListView listView;
    private ArrayList<String> arrayList = new ArrayList<>();

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.tab2_fragment, container, false);
        listView = (ListView) view.findViewById(R.id.list_view);
        ArrayAdapter arrayAdapter = new ArrayAdapter(getContext(),android.R.layout.simple_list_item_1,arrayList);

        listView.setAdapter(arrayAdapter);

        return view;
    }

    public void displayExchange(String s){
        arrayList.add(s);
    }


}