Androidstudio底部导航栏实现

1. 编写activity_main.xml 作者：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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/content"
        android:layout_above="@id/main_bottom_nv"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>


    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/main_bottom_nv"
        android:layout_alignParentBottom="true"
        android:layout_width="match_parent"
        app:labelVisibilityMode="labeled"
        android:background="@color/white"
        android:layout_height="wrap_content"
        app:menu="@menu/navigation" />


</RelativeLayout> 

2. 在res下新建menu文件夹，然后在menu下新建navigation.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/home"
        android:icon="@drawable/baseline_home_24"
        android:title="首页" />


    <item
        android:id="@+id/car"
        android:icon="@drawable/baseline_shopping_cart_24"
        android:title="购物车" />


    <item
        android:id="@+id/order"
        android:icon="@drawable/baseline_reorder_24"
        android:title="订单" />


    <item
        android:id="@+id/mine"
        android:icon="@drawable/ic_baseline_supervisor_account_24"
        android:title="我的" />

</menu> 作者：浩宇软件开发 https://www.bilibili.com/read/cv26765516/ 出处：bilibili

3. 在MainActivity中实现代码

public class UserMainActivity extends MainActivity {
    
    

    //创建4个 Fragment
    private HomeFragment mHomeFragment;
    private CardFragment mCardFragment;
    private OrderFragment mOrderFragment;
    private MineFragment mMineFragment;

    private BottomNavigationView main_bottom_nv


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


      //初始化控件
      main_bottom_nv = findViewById(R.id.main_bottom_nv)

      //main_bottom_nv tab点击切换
      mainBottomNv.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
    
    
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
    
    

                if (item.getItemId() == R.id.home) {
    
    
                    selectedFragment(0);
                } else if (item.getItemId() == R.id.car) {
    
    
                    selectedFragment(1);
                } else if (item.getItemId() == R.id.order) {
    
    
                    selectedFragment(2);
                } else if (item.getItemId() == R.id.mine) {
    
    
                    selectedFragment(3);
                }

                return true;
            }
        });


      //默认首页选中
      selectedFragment（0）；

    }



    public void selectedFragment(int position) {
    
    
         //获取fragmentManager管理器
        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        hideFragment(transaction);
        if (position == 0) {
    
    
            if (mHomeFragment == null) {
    
    
                mHomeFragment = new HomeFragment();
                transaction.add(R.id.content, mHomeFragment);
            } else {
    
    
                transaction.show(mHomeFragment);
            }
        } else if (position == 1) {
    
    
            if (mCardFragment == null) {
    
    
                mCardFragment = new CardFragment();
                transaction.add(R.id.content, mCardFragment);
            } else {
    
    
                transaction.show(mCardFragment);

            }
        } else if (position == 2) {
    
    
            if (mOrderFragment == null) {
    
    
                mOrderFragment = new OrderFragment();
                transaction.add(R.id.content, mOrderFragment);
            } else {
    
    
                transaction.show(mOrderFragment);

            }
        } else {
    
    
            if (mMineFragment == null) {
    
    
                mMineFragment = new MineFragment();
                transaction.add(R.id.content, mMineFragment);
            } else {
    
    
                transaction.show(mMineFragment);
            }
        }

        //最后设置提交     注意：这句话一定不能少！！！！！！
        transaction.commit();

    }


    private void hideFragment(FragmentTransaction transaction) {
    
    
        if (mHomeFragment != null) {
    
    
            transaction.hide(mHomeFragment);
        }
        if (mMineFragment != null) {
    
    
            transaction.hide(mMineFragment);
        }
        if (mCardFragment != null) {
    
    
            transaction.hide(mCardFragment);
        }
        if (mOrderFragment != null) {
    
    
            transaction.hide(mOrderFragment);
        }

    }

} 

效果图