底部导航栏不会切换碎片

Question

我（初学者）刚刚在Android Studio（API19）的这个论坛的帮助下编写了一个底部导航栏，它在模拟器中编译启动，但我无法弄清楚为什么它不会在你点击时切换片段栏中的项目。对于Java中方法中的每个Case，我创建了一个没有任何特殊代码的片段。尽管我在开始时在gradle中实现了设计并添加了一个浮动按钮，但这几乎就是我所做的。

非常感谢您的帮助！

这是我的Java代码

public class MainActivity extends AppCompatActivity {

    //Bottom Navigation Bar
    Fragment currentFragment = null;
    FragmentTransaction ft;

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

        //Bottom Navigation Bar
        ft = getSupportFragmentManager().beginTransaction();
        currentFragment = new HomeFragment();
        ft.replace(R.id.main_frame_layout, currentFragment);
        ft.commit();

        //Listener Bottom Navigation Bar
        BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation_bar);
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
    }

    // Method for Nav. Bar Listener
    private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener = new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {

            switch (item.getItemId()) {

                case R.id.nav_home:
                    currentFragment = new HomeFragment();
                    ft = getSupportFragmentManager().beginTransaction();
                    ft.replace(R.id.main_frame_layout, currentFragment);
                    ft.commit();
                    return true;

                case R.id.nav_overview:
                    currentFragment = new OverviewFragment();
                    ft = getSupportFragmentManager().beginTransaction();
                    ft.replace(R.id.main_frame_layout, currentFragment);
                    ft.commit();
                    return true;

                case R.id.nav_charts:
                    currentFragment = new ChartsFragment();
                    ft = getSupportFragmentManager().beginTransaction();
                    ft.replace(R.id.main_frame_layout, currentFragment);
                    ft.commit();
                    return true;

                case R.id.nav_targets:
                    currentFragment = new TargetsFragment();
                    ft = getSupportFragmentManager().beginTransaction();
                    ft.replace(R.id.main_frame_layout, currentFragment);
                    ft.commit();
                    return true;
            }
            return true;
        }
    };
}

这是我的XML代码

<?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/main_frame_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab_add"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:clickable="true"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="72dp"
        android:src="@drawable/ic_add_white"
        android:layout_gravity="bottom|end"
        android:focusable="true"
        app:backgroundTint="@color/colorAccent"
        app:fabSize="normal"/>

    </FrameLayout>

    <android.support.design.widget.BottomNavigationView

        android:id="@+id/navigation_bar"
        android:layout_width="match_parent"
        android:layout_height="56dp"
        android:layout_alignParentBottom="true"
        app:itemBackground="@color/colorPrimary"
        app:itemIconTint="@color/colorAccent"
        app:itemTextColor="@color/colorAccent"
        app:menu="@menu/navigation_items"/>

</RelativeLayout>

这是我生成导航项的菜单文件，但我认为它不重要

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <item
        android:id="@+id/nav_home"
        android:icon="@drawable/ic_home_white_24dp"
        android:title="@string/nav_home"
        android:enabled="true"/>
    <item
        android:id="@+id/nav_overview"
        android:icon="@drawable/ic_format_list_bulleted_white_24dp"
        android:title="@string/nav_overview"
        android:enabled="true"/>
    <item
        android:id="@+id/nav_charts"
        android:icon="@drawable/ic_insert_chart_white_24dp"
        android:title="@string/nav_charts"
        android:enabled="true"/>
    <item
        android:id="@+id/nav_targets"
        android:icon="@drawable/ic_monetization_on_white_24dp"
        android:title="@string/nav_targets"
        android:enabled="true"/>

</menu>