Android Fragment Lifecycle and savedInstanceState Debugging

Background

In high school, I developed an app called “Dongcheon Alimi” for fellow students. It was a convenience app providing meal info, timetables, announcements, etc.

One day, users started reporting a bug. The app would crash at seemingly random times. Log analysis showed NullPointerException, but I couldn’t figure out where it came from.

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Debugging Process

Spent 2 weeks on this. During that time, I learned a lot about Android Fragment lifecycle.

What I Discovered:

The app was using ViewPager + Fragment structure. When the user changed tabs, previous Fragments were being destroyed and recreated.

// Original code - the problem
@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    // Accessing data initialized in onCreate
    textView.setText(data.getTitle());  // data is null!
}

When the Fragment was recreated, onCreate was called again but data was not recovered.

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Root Cause

When Android destroys and recreates a Fragment (due to memory pressure, screen rotation, etc.), you must save state using onSaveInstanceState and restore it from savedInstanceState.

@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putString("title", data.getTitle());
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    if (savedInstanceState != null) {
        String title = savedInstanceState.getString("title");
        // Restore data
    }
}

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Solution

Saved necessary data in onSaveInstanceState and restored it from savedInstanceState when Fragment is recreated.

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Lessons Learned

• Understanding Fragment lifecycle is crucial for Android development
• Fragments can be destroyed and recreated anytime
• Must use savedInstanceState to preserve state
• 2 weeks on one bug was rough, but learned deep debugging

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From developing “Dongcheon Alimi” app in high school. (Published on Play Store, promoted through student council, acquired real users)

• DongcheonAlimi