原生方案: Bundle
Java語言中類與類自己通訊常用方案:介面

新建IFragmentCallback介面 

package com.example.flagmentmanagers2;

public interface IFragmentCallback {

    void sendMsgToActivity(String msg);

    String getMsgToActivity(String msg);

}
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BlankFragment.java

package com.example.flagmentmanagers2;

import android.os.Bundle;

import androidx.fragment.app.Fragment;

import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.Toast;


public class BlankFragment extends Fragment {
    private static final String TAG = "BlankFragment";
    private View rootview;

    public BlankFragment() {
        // Required empty public constructor
    }

    //新程式碼
    private IFragmentCallback iFragmentCallback;
    public void setiFragmentCallback(IFragmentCallback callback){
        iFragmentCallback=callback;
    }


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Bundle bundle=getArguments();//返回的是activity裡面bf.setArguments(bundle)的值
        String string = bundle.getString("message");
        Log.d(TAG,"onCreate"+string);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        //新程式碼
        if(rootview==null){
            rootview=inflater.inflate(R.layout.fragment_blank, container, false);
        }
        Button button=rootview.findViewById(R.id.btn_3);
        button.setOnClickListener(new View.OnClickListener() {//匿名內部類方式實現
            @Override
            public void onClick(View view) {
//                iFragmentCallback.sendMsgToActivity("hello i love you");傳送
                String msg=iFragmentCallback.getMsgToActivity("null");//獲取
                Toast.makeText(BlankFragment.this.getContext(),msg,Toast.LENGTH_SHORT).show();;


            }
        });
        return rootview;
    }

    @Override
    public void onResume() {
        super.onResume();
    }
}

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MainActivity.java

package com.example.flagmentmanagers2;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;
import androidx.fragment.app.ListFragment;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

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

        Button button1=findViewById(R.id.btn_1);
        button1.setOnClickListener(this);
        Button button2=findViewById(R.id.btn_2);
        button2.setOnClickListener(this);
    }

    @Override
    public void onClick(View view) {
        switch (view.getId()){
            case R.id.btn_1:
                Bundle bundle=new Bundle();
                bundle.putString("message","我喜歡XXKT");
                BlankFragment bf=new BlankFragment();
                bf.setArguments(bundle);
                //新程式碼
                bf.setiFragmentCallback(new IFragmentCallback() {
                    @Override
                    public void sendMsgToActivity(String msg) {
                        Toast.makeText(MainActivity.this,msg,Toast.LENGTH_SHORT).show();
                    }

                    @Override
                    public String getMsgToActivity(String msg) {
                        return "i hate u i love u";
                    }
                });
            replaceFragment(bf);
            break;
            case R.id.btn_2:
                replaceFragment(new ItemFragment());
                break;
        }
    }

    private void replaceFragment(Fragment fragment) {
        FragmentManager manager=getSupportFragmentManager();//獲取
        FragmentTransaction transaction = manager.beginTransaction();//開啟事務
        transaction.addToBackStack(null);//新增棧
        transaction.replace(R.id.FL_1,fragment);//替換
        transaction.commit();//提交
    }

}

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fragment_blank.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout 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"
    tools:context=".BlankFragment">

    <!-- TODO: Update blank fragment layout -->
    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="@string/hello_blank_fragment" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="資料傳遞"
        android:layout_gravity="center"
        android:id="@+id/btn_3"/>

</FrameLayout>

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activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<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"
    android:orientation="vertical"
    tools:context=".MainActivity">

<Button
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/btn_1"
    android:text="chage">
</Button>

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/btn_2"
        android:text="replace">
    </Button>

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#ff3366"
        android:id="@+id/FL_1"/>

</LinearLayout>

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新建ItemFragment.class

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 其他方案:eventBus,LiveData

//包含了一個設計模式叫觀察者模式或者叫釋出訂閱模式
//可以觀察activity的變化，當fragment發現activity的變化是它所需要的資訊的時候，就會收到這條資訊