Create a new project in Android Studio and name it StateSaver. We need only a single activity, so the auto-generated main activity is sufficient. However, we will need a few widgets, including EditText, Button, and TextView. Their layout (in activity_main.xml) will be as follows:

<EditText
android:id="@+id/editText"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_alignParentStart="true"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintBottom_toTopOf="@+id/button"/>

<Button
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:text="Count"
android:onClick="onClickCounter"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintBottom_toBottomOf="parent"/>

<TextView
android:id="@+id/textViewCounter"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@id/button"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toBottomOf="@id/button"
app:layout_constraintBottom_toBottomOf="parent"/>

Perform the following set of steps:

static final String KEY_COUNTER = "COUNTER"; 
private int mCounter=0; 

public void onClickCounter(View view) {
mCounter++;
((TextView)findViewById(R.id.textViewCounter))
            .setText("Counter: " + Integer.toString(mCounter));
}

@Override 
protected void onSaveInstanceState(Bundle outState) { 
    super.onSaveInstanceState(outState); 
    outState.putInt(KEY_COUNTER,mCounter); 
} 
 
@Override 
protected void onRestoreInstanceState(Bundle savedInstanceState) { 
    super.onRestoreInstanceState(savedInstanceState); 
    mCounter=savedInstanceState.getInt(KEY_COUNTER); 
} 

All activities go through multiple states during their lifetime. By setting up callbacks to handle the events, we can have our code save important information before the activity is destroyed.

Step 3 is where the actual saving and restoring occurs. The OS sends a Bundle (a data object that also uses name/value pairs) to the methods. We use the onSaveInstanceState() callback to save the data and pull it out in the onRestoreInstanceState() callback.

But wait! Did you try typing text in the EditText view before rotating the device? If so, you'd have noticed that the text was also restored, but we don't have any code to handle that view. By default, the system will automatically save the state, provided it has a unique ID.

The onRestoreInstanceState() callback is not the only place where the state can be restored. Look at the signature of onCreate():

onCreate(Bundle savedInstanceState) 

Both methods receive the same Bundle instance named savedInstanceState. You could move the restore code to the onCreate() method and it would work the same. But one catch is that the savedInstanceState bundle will be null if there is no data, such as during the initial creation of the activity. If you want to move the code from the onRestoreInstanceState() callback, just check to make sure that the data is not null. Here's how that code would look:

if (savedInstanceState!=null) { 
    mCounter = savedInstanceState.getInt(KEY_COUNTER); 
}