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I am trying to make StatusBar transparent. Here's setTransparentStatusbar():

public void setTransparentStatusbar() 
 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) 
 getWindow().setStatusBarColor(Color.TRANSPARENT);
 AppBarLayout appBarLayout = (AppBarLayout) findViewById(R.id.main_appbar);
 Toolbar toolbar = findViewById(R.id.main_toolbar);
 appBarLayout.setFitsSystemWindows(true);
 toolbar.setFitsSystemWindows(true);
 toolbar.getLayoutParams().height = (int)getResources().getDimension(R.dimen.toolbar_height) + SizeUtil.getStatusBarHeight(this);
 

I use class BaseActivity to set up toolbars for all child activities. In child activities' onCreate() I use super.setContentViewAndSetUpToolBar() instead of setContentView(), Here's parts of setContentViewAndSetUpToolBar(int layoutResID):

setContentView(layoutResID);
// Init Toolbar
 androidx.appcompat.widget.Toolbar toolbar = (androidx.appcompat.widget.Toolbar) findViewById(R.id.main_toolbar);
 androidx.appcompat.widget.Toolbar toolbar_lower = (androidx.appcompat.widget.Toolbar) findViewById(R.id.main_toolbar_lower);
 setSupportActionBar(toolbar);
 getSupportActionBar().setDisplayShowTitleEnabled(false);

 // Set ic icon
 ActionBar actionBar = getSupportActionBar();
 if (actionBar != null) 
 actionBar.setDisplayHomeAsUpEnabled(true);
 actionBar.setHomeAsUpIndicator(R.mipmap.twotone_event_note_white_24);
 
setTransparentToolbar();

Here's main_toolbar.xml file:

<?xml version="1.0" encoding="utf-8"?>

<com.google.android.material.appbar.AppBarLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/main_appbar"
android:layout_height="wrap_content"
android:layout_width="match_parent"
>

<androidx.appcompat.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/main_toolbar"
android:layout_width="match_parent"
android:layout_height="@dimen/toolbar_height"
android:background="@color/colorPrimary"
android:theme="@style/ThemeOverlay.AppCompat.ActionBar"
app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
 android:visibility="visible"
 >

 <LinearLayout
 android:id="@+id/title_div"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:orientation="horizontal"
 >
 <TextView
 android:id="@+id/toolbar_title"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:text="@string/app_name"
 android:textColor="#fff"
 android:textSize="@dimen/toolbar_title"
 android:visibility="visible"
 android:layout_marginRight="16dp"
 />

 </LinearLayout>

</androidx.appcompat.widget.Toolbar>


</com.google.android.material.appbar.AppBarLayout>

Here's activity's xml file:

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/drawerlayout"
android:layout_width="match_parent"
android:layout_height="match_parent">

 <androidx.coordinatorlayout.widget.CoordinatorLayout
 android:id="@+id/root_coordinatorlayout"
 android:layout_height="match_parent"
 android:layout_width="match_parent"
 android:background="#fff"
 android:fitsSystemWindows="true">

 <include
 android:id="@+id/circle_one_appbar"
 layout="@layout/main_toolbar"/>



 <!--Main layout-->
 ...
 <!--Main layout END-->

 </androidx.coordinatorlayout.widget.CoordinatorLayout>

<include
 layout="@layout/circle_one_drawer"/>

</androidx.drawerlayout.widget.DrawerLayout>

When I call setTransparentStatusbar(), program throws NullPointerException, here's error info:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.appbar.AppBarLayout.setFitsSystemWindows(boolean)' on a null object reference
 at com.example.elmliu.uniport.activity.BaseActivity.setTransparentToolbar(BaseActivity.java:532)
 at com.example.elmliu.uniport.activity.BaseActivity.setContentViewAndSetUpToolBar(BaseActivity.java:101)
 at com.example.elmliu.uniport.activity.visual.CircleThree.onCreate(CircleThree.java:34)
 at android.app.Activity.performCreate(Activity.java:6662)

Problem solved.
In main_toolbar.xml, id main_appbar was given. However,

<include
 android:id="@+id/circle_one_appbar"
 layout="@layout/main_toolbar"/>

the new id circle_one_appbar will overwrite the old id. Thus, I shouldn't use id main_appbar to find the AppBarLayout.