How to Set selected item in BottomNavigationView

Question

I am trying to set default item on activity created but it isn't working? This is my code:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_userhome);
    mTextMessage = (TextView) findViewById(R.id.message);
    profile = (FrameLayout) findViewById(R.id.profile);
    mall = (FrameLayout) findViewById(R.id.mall);
    dietplan =(FrameLayout) findViewById(R.id.dietplan);
    BottomNavigationView navigation = (BottomNavigationView) 
    findViewById(R.id.navigation);
    navigation.setSelectedItemId(R.id.dietplan);
     navigation.setOnNavigationItemSelectedListener 
    (mOnNavigationItemSelectedListener);
}

But it seems that navigation.setSelectedItemId(R.id.dietplan); is not working. Please help me to set default item of bottom navigation bar:

This is my stack trace(logcat):

FATAL EXCEPTION: main
   Process: gym.android.ommsoftware.gym, PID: 1915
   java.lang.RuntimeException: Unable to start activity ComponentInfo{gym.android.ommsoftware.gym/gym.android.ommsoftware.gym.Userhome}: java.lang.NullPointerException
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2404)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2464)
       at android.app.ActivityThread.access$900(ActivityThread.java:172)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:146)
       at android.app.ActivityThread.main(ActivityThread.java:5653)
       at java.lang.reflect.Method.invokeNative(Native Method)
       at java.lang.reflect.Method.invoke(Method.java:515)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
       at dalvik.system.NativeStart.main(Native Method)
    Caused by: java.lang.NullPointerException
       at gym.android.ommsoftware.gym.Userhome.onCreate(Userhome.java:57)
       at android.app.Activity.performCreate(Activity.java:5541)
       at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1093)
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2368)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2464) 
       at android.app.ActivityThread.access$900(ActivityThread.java:172) 
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308) 
       at android.os.Handler.dispatchMessage(Handler.java:102) 
       at android.os.Looper.loop(Looper.java:146) 
       at android.app.ActivityThread.main(ActivityThread.java:5653) 
       at java.lang.reflect.Method.invokeNative(Native Method) 
       at java.lang.reflect.Method.invoke(Method.java:515) 
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291) 
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107) 
       at dalvik.system.NativeStart.main(Native Method) 

Answer 1

Just share my working source code

In Xml,

 <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <android.support.design.widget.BottomNavigationView
        android:background="@color/colorWhite"
        android:id="@+id/gfPrlBnvBtmView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="start"
        android:layout_alignParentBottom="true"
        app:menu="@menu/bottom_navigation_main" />
</LinearLayout>

In Java,

  public class TestActivity extends AppCompatActivity implements BottomNavigationView.OnNavigationItemSelectedListener
{
    private BottomNavigationView mBtmView;
    private int mMenuId;
    @Override
    public void onCreate(@Nullable Bundle savedInstanceState, @Nullable PersistableBundle persistentState) {
        super.onCreate(savedInstanceState, persistentState);
        setContentView(R.layout.test);
        mBtmView = (BottomNavigationView) findViewById(R.id.gfPrlBnvBtmView);
        mBtmView.setOnNavigationItemSelectedListener(this);
        mBtmView.getMenu().findItem(R.id.action_yoga).setChecked(true);
    }

    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
        // uncheck the other items.
        mMenuId = item.getItemId();
        for (int i = 0; i < mBtmView.getMenu().size(); i++) {
            MenuItem menuItem = mBtmView.getMenu().getItem(i);
            boolean isChecked = menuItem.getItemId() == item.getItemId();
            menuItem.setChecked(isChecked);
        }

        switch (item.getItemId()) {
            case R.id.action_food: {
            }
            break;
            case R.id.action_medical: {
            }
            break;
            case R.id.action_yoga: {
            }
            break;
            case R.id.action_postures: {
            }
            break;
        }
        return true;
    }
}