Why is it necessary to check savedInstanceState inside of OnCreate?

Question

What is the purpose of the if block in the onCreate() method? Why is it necessary to check if savedInstanceState is null?

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
}

@njzk2 depending on how new someone is, they may not know where to find the documentation. In this case, Abhijit, you can use [this link](http://developer.android.com/reference/android/app/Activity.html#onCreate(android.os.Bundle)) for more info. To quote the docs regarding the parameter: "If the activity is being re-initialized after previously being shut down then this Bundle contains the data it most recently supplied in onSaveInstanceState(Bundle). Note: Otherwise it is null." — AdamMc331, Jul 07 '15 at 20:10
@McAdam331 This is android, the documentation is in the IDE. It is in google. It is right next to where you download the SDK. It is in stackoverflow. It is everywhere! (Don't take my word for it. Just Google `onCreate`.) — njzk2, Jul 07 '15 at 20:54

score 4 · Accepted Answer · answered Jul 07 '15 at 19:49

4

When your activity is recreated, such as after a screen rotation or other configuration change, fragments are automatically reattached. By checking if savedInstanceState == null, you ensure that you are not re-adding a fragment that has already been added for you.

answered Jul 07 '15 at 19:49

ianhanniballake

191,609
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So every time I re-start the app, savedInstanceState should be null. Only then a new fragment will be added, right? – Jul 07 '15 at 19:58
Right, it is `null` only the first time the activity is created. – ianhanniballake Jul 07 '15 at 19:59

score 0 · Answer 2 · answered Jul 07 '15 at 19:49

0

Well that's so you can remember where someone was when they last left your app. So for instance chrome remembers your last visited tabs.

answered Jul 07 '15 at 19:49

trinityalps

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4
19

Why is it necessary to check savedInstanceState inside of OnCreate?

2 Answers2

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