为什么我的fg老是显示出现错误，提示我androidx.fragment.app.Fragment无法转换为android.app.Fragmen

package com.example.wechatinterfacechange;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;

import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.view.View;
import android.widget.ImageView;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ImageView imageView1=(ImageView)findViewById(R.id.image1);
    ImageView imageView2=(ImageView)findViewById(R.id.image2);
    ImageView imageView3=(ImageView)findViewById(R.id.image3);
    ImageView imageView4=(ImageView)findViewById(R.id.image4);
    imageView1.setOnClickListener(l);
    imageView2.setOnClickListener(l);
    imageView3.setOnClickListener(l);
    imageView4.setOnClickListener(l);
}
View.OnClickListener l=new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        FragmentManager fm=getFragmentManager();
        FragmentTransaction ft=fm.beginTransaction();
        Fragment fg=null;
        switch (v.getId()){
            case R.id.image1:
                fg=new WeChat_Fragment();
                break;
            case R.id.image2:
                fg=new Message_Fragment();
                break;
            case R.id.image3:
                fg=new Find_Fragment();
                break;
            case R.id.image4:
                fg=new Me_Fragment();
                break;
            default:
                break;
        }
        ft.replace(R.id.fragment,fg);
        ft.commit();
    }
};
}

为什么我的fg老是显示出现错误，提示我androidx.fragment.app.Fragment无法转换为android.app.Fragment

因为你是Androidx包下的，而你要转换的却不是