昨天介紹了BottomNavigationView的使用方法，今天我要分享使用BottomNavigationView切換不同的頁面。
首先設定activity_main.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">

<FrameLayout
    android:id="@+id/frame"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintBottom_toTopOf="@+id/bottomNavigation"/>
<com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/bottomNavigation"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:background="#A9FFFFFF"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintBottom_toBottomOf="parent"
    app:menu="@menu/menu"/>
</androidx.constraintlayout.widget.ConstraintLayout>

新增Fragment1.Activity,Fragment2.Activity,Fragment3.Activity，並各別設計UI
fragment_1.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"
    android:background="#FFB700"
tools:context=".Fragment1">

<!-- TODO: Update blank fragment layout -->
<TextView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:text="This is Fragment1"
    android:textSize="40sp"
    android:textColor="#2196F3"/>

</FrameLayout>

fragment_2.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"
    android:background="#3044F6"
    tools:context=".Fragment2">

    <!-- TODO: Update blank fragment layout -->
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="500dp"
        android:layout_marginLeft="30dp"
        android:text="This is Fragment2"
        android:textSize="40sp"
        android:textColor="#21F34E" />

</FrameLayout>

fragment_3.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"
    android:background="#CDB6D3"
    tools:context=".Fragment3">

    <!-- TODO: Update blank fragment layout -->
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="150dp"
        android:layout_marginLeft="80dp"
        android:text="This is Fragment3"
        android:textSize="40sp"
        android:textColor="#F35C21" />

</FrameLayout>

MainActivity程式設計

package com.example.newtest;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AlertDialog;
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.view.MenuItem;


import com.google.android.material.bottomnavigation.BottomNavigationView;

public class MainActivity extends AppCompatActivity {
    //宣告FragmentManager跟FragmentTransaction
    private FragmentManager fragmentManager;
    private FragmentTransaction fragmentTransaction;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //顯示第一個進來的頁面
        bottomnavugationset();
        
        BottomNavigationView bottomNavigation=(BottomNavigationView)findViewById(R.id.bottomNavigation);
        //監聽BottomNavigationView事件
        bottomNavigation.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                //fragmentManager跟fragmentTransaction切換畫面
                fragmentManager=getSupportFragmentManager();
                fragmentTransaction=fragmentManager.beginTransaction();
                switch (item.getItemId()){
                    case R.id.setting:
                        fragmentTransaction.replace(R.id.frame,new Fragment1());
                        fragmentTransaction.commit();
                        return  true;
                    case R.id.comment:
                        fragmentTransaction.replace(R.id.frame,new Fragment2());
                        fragmentTransaction.commit();
                        return  true;
                    case R.id.account:
                        fragmentTransaction.replace(R.id.frame,new Fragment3());
                        fragmentTransaction.commit();
                        return  true;
                }
                return false;
            }
        });
    }
    private void bottomnavugationset() {
        fragmentManager=getSupportFragmentManager();
        fragmentTransaction=fragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.frame,new Fragment1());
        fragmentTransaction.commit();
    }
}

成果圖