Is the Activity instance really destroyed after onDestroy() method is called?

Question

I'm confused while learning savedInstanceState Bundle. I found that the Activity instance is not destroyed after onDestroy() method is called, or it can save the data without the help of savedInstanceState Bundle.

This is my test code:

package com.example.hellotest;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;

public class MainActivity extends Activity {
    private static final String TAG = MainActivity.class.getSimpleName();
    private static int testNum = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d(TAG, "onCreate");
        setContentView(R.layout.activity_main);
        testNum++;
        Log.d(TAG, "testNum: " + testNum);
        if(savedInstanceState == null)
            Log.d(TAG, "savedInstanceState is null");
        else
            Log.d(TAG, "savedInstanceState is NOT null");
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.d(TAG, "onDestroy");
    }
}

And here is the log info:

11-17 22:10:14.433: D/MainActivity(23303): onCreate
11-17 22:10:14.463: D/MainActivity(23303): testNum: 1
11-17 22:10:14.463: D/MainActivity(23303): savedInstanceState is null
11-17 22:10:17.527: D/MainActivity(23303): onDestroy
11-17 22:10:18.278: D/MainActivity(23303): onCreate
11-17 22:10:18.298: D/MainActivity(23303): testNum: 2
11-17 22:10:18.298: D/MainActivity(23303): savedInstanceState is null
11-17 22:10:19.569: D/MainActivity(23303): onDestroy
11-17 22:10:20.200: D/MainActivity(23303): onCreate
11-17 22:10:20.220: D/MainActivity(23303): testNum: 3
11-17 22:10:20.220: D/MainActivity(23303): savedInstanceState is null

The testNum never returns to 1 unless I totally kill the process on the phone, which seems to me that the Activity instance is not destroyed really, so we don't need to override the onSaveInstanceState(Bundle outState) method to save the data.

Is there somewhere I'm misunderstanding?

Answer 1

testNum is declared as static, and is therefore associated with the class MainActivity, not any particular instance of it. Unless you need to access testNum statically, you should remove the static identifier:

private int testNum = 0;

Doing this will result in testNum being associated with an instance of the MainActivity class, and will indeed be destroyed.

You can, if you choose, override onSaveInstanceState to maintain instance variables across rotations or if MainActivity is destroyed and recreated for any other configuration changes:

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt("testNum", testNum);
}

Answer 2

The confusion is between static and instance variables.

Your counter is static, so it has the lifetime of your process rather than any particular instance.

Since process lifetime is not guaranteed, you generally don't want to store anything which cannot be trivially recreated there.