AndroidStudio3.5   Android X   Android Q

               首先上效果图                                1.可视化界面添加 BottomNavigationView控件  

2.1MainActivity的xml文件具体代码

<?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"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottomNavigationView"
        android:layout_width="0dp"
        android:layout_height="60dp"
        android:background="#FFFFFF"
        app:itemTextColor="#03A9F4"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:menu="@menu/bottom_navigation_tab" />

    <fragment
        android:id="@+id/fragment_empty"
        android:name="com.example.shiwu.Fragment.EmptyFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintBottom_toTopOf="@+id/bottomNavigationView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

2.2 导航栏菜单Menu文件

<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <item
        android:id="@+id/tab_one"
        android:icon="@drawable/home"
        android:title="首页"/>

    <item
        android:id="@+id/tab_two"
        android:icon="@drawable/kind"
        android:title="分类"/>

    <item
        android:id="@+id/tab_three"
        android:icon="@drawable/mes"
        android:title="消息"/>

    <item
        android:id="@+id/tab_four"
        android:icon="@drawable/my"
        android:title="我的"/>
</menu>

3.创建四个待添加的Fragment和一个布局为空的Fragment做为初始Fragment,并且为BottomNavigationView的四个子菜单添加点击切换Fragment的方法

package com.example.shiwu;

import androidx.annotation.NonNull;
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.util.Log;
import android.view.MenuItem;
import android.widget.Toast;

import com.example.shiwu.Fragment.EmptyFragment;
import com.example.shiwu.Fragment.HomeFragment;
import com.example.shiwu.Fragment.KindFragment;
import com.example.shiwu.Fragment.MesFragment;
import com.example.shiwu.Fragment.MyFragment;
import com.google.android.material.bottomnavigation.BottomNavigationMenu;
import com.google.android.material.bottomnavigation.BottomNavigationView;

import java.util.Random;

public class MainActivity extends AppCompatActivity implements BottomNavigationView.OnNavigationItemSelectedListener {
    private BottomNavigationView bottomNavigationView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();
        replaceFragment(new HomeFragment()); //启动时用HomeFragment替换掉空Fragment
    }

    public void initView() {
        bottomNavigationView = findViewById(R.id.bottomNavigationView);
        bottomNavigationView.setOnNavigationItemSelectedListener(this);//设置导航栏监听器
        bottomNavigationView.setSelectedItemId(R.id.tab_one);//设置默认选择的导航栏子项tab_one即首页
        bottomNavigationView.setItemIconTintList(null);//取消导航栏子项图片的颜色覆盖
    }

    @Override
    //处理导航栏子项的点击事件
    public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
        int itemId = menuItem.getItemId();//获取点击的位置以及对应的id
        switch (itemId) {
            case R.id.tab_one:
                replaceFragment(new HomeFragment());//id为tab_one则第一项被点击，遂用HomeFragment替换空Fragment
                menuItem.setChecked(true);
                break;
            case R.id.tab_two:
                replaceFragment(new KindFragment());
                menuItem.setChecked(true);
                break;
            case R.id.tab_three:
                replaceFragment(new MesFragment());
                menuItem.setChecked(true);
                break;
            case R.id.tab_four:
                replaceFragment(new MyFragment());
                menuItem.setChecked(true);
                break;
        }
        return false;
    }

//替换Fragment的方法
    public void replaceFragment(Fragment fragment) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction transaction = fragmentManager.beginTransaction();
        transaction.replace(R.id.fragment_empty, fragment);
        transaction.commit();
    }
}

github地址：https://github.com/yukaida/shiwu/tree/BottomNavigationView