ToolBar usage and exception handling (ToolBar does not take effect)

1. Exception handling

Attempt to invoke virtual method ‘ActionBar.setHomeButtonEnabled(boolean)’ on a null object reference

java.lang.RuntimeException: Unable to start activity ComponentInfo{
    
    }: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setHomeButtonEnabled(boolean)' on a null object reference
// ......
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.setHomeButtonEnabled(boolean)' on a null object reference
// ......

2. The reason

The ToolBar layout file is not referenced in the Activity layout, but only dynamically set, so that when you use or get the ActionBar, you will get an error because the ToolBar is not set successfully.

3. Solve

3.1 Layout file

// 主布局文件
<LinearLayout android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">
    <include layout="@layout/toobar"/>
</LinearLayout>

// toolbar.xml文件
<?xml version="1.0" encoding="utf-8"?>
<androidx.appcompat.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/toolbar_test"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:contentInsetStart="0dp">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="Toolbar"
        android:textSize="20dp" />
</androidx.appcompat.widget.Toolbar>

3.2 Java files

    private void initToolBar() {
    
    
        Toolbar toolbar_test = (Toolbar)findViewById(R.id.toolbar_test);
        setSupportActionBar(toolbar_test);
    }

4. Results

5. Matters needing attention

Java introduces ToolBar as follows:

import androidx.appcompat.widget.Toolbar;