Android OnCreate 输入包始终 null

Question

I am undertaking project but I have been having some issues with activities and saving state. So I have gone back to basics to try and understand what I was missing. However, even with a very simple set up I am having the same issues.

So I started with an empty project and placed 2 textViews and 1 button on the activity_main.xml and 1 button on the activity_2.xml. The user will enter some text in a text box, press the button to advance to the Activity2 Activity and then press the button (in the Activity2 layout) to return back to the MainActivity.

My hope is that the text entered into the text box has been saved by the overriden onSaveInstanceState method and then the bundle argument in the OnCreate will hold this value. However, when MainActivity is hit, the bundle is alywas null despite onSaveInstanceState being called when button to start Activity2 is pressed.

Am I missing something? The onSaveInstanceState method is definitely initiated and savedInstanceState is populated within it, so why when the MainActivity is started again and onCreate is called is the Bundle savedInstanceState null? Is it the way I am setting up starting other Activities?

Thanks for your help!

For reference, here is my setup:

My MainActivity:

public class MainActivity extends AppCompatActivity {

    Button forwardButton;
    TextView editText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        forwardButton = findViewById(R.id.buttonForward);
        editText = findViewById(R.id.editText);
        forwardButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                // Code here executes on main thread after user presses button
                Intent activity2 = new Intent(MainActivity.this,Activity2.class);
                startActivity(activity2);
            }
        });
    }

    @Override
    public void onSaveInstanceState(Bundle savedInstanceState) {
        String textToSave = editText.getText().toString();
        savedInstanceState.putString("editText",textToSave);
        super.onSaveInstanceState(savedInstanceState);
    }
}``

My Activity2 class:

public class Activity2 extends AppCompatActivity {

    Button goBack;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_2);

        goBack = findViewById(R.id.goBack);
        goBack.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                // Code here executes on main thread after user presses button
                Intent activity2 = new Intent(Activity2.this,MainActivity.class);
                startActivity(activity2);
            }
        });
    }
}

Activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/buttonForward"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="162dp"
        android:layout_marginTop="232dp"
        android:layout_marginEnd="162dp"
        android:layout_marginBottom="76dp"
        android:text="Forward"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/editText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="96dp"
        android:layout_marginTop="43dp"
        android:layout_marginEnd="102dp"
        android:layout_marginBottom="268dp"
        android:ems="10"
        android:inputType="textPersonName"
        android:text="Enter"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView" />

</androidx.constraintlayout.widget.ConstraintLayout>

activity_2.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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=".Activity2">

    <Button
        android:id="@+id/goBack"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="162dp"
        android:layout_marginTop="232dp"
        android:layout_marginEnd="162dp"
        android:layout_marginBottom="76dp"
        android:text="Go back"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

In your Activity2

    goBack.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Activity2.this.finish();
        }
    });

And get your saved instance in MainActivity from onRestoreInstanceState

//this will trigger when the same instance of activity is restored
//by the back press that finishes the Activity2
@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
  super.onRestoreInstanceState(savedInstanceState);

  String data = savedInstanceState.getString("editText");

}