Hey Guys, I received NullPointerException in logs ...

# android

[Deepti M]

Hey Guys,
I received NullPointerException in logs even after putting a check for 'null'. Can anyone tell me how and when can I get NullPointerException for checking "isPreview" in the below statement...

if (lastWallpaperEngine != null && !lastWallpaperEngine!!.isPreview) {
}

Logs received in play-store...

java.lang.NullPointerException: 

  at android.service.wallpaper.WallpaperService$Engine.isPreview (WallpaperService.java:390)

  at VideoWallpaperEngine.onSurfaceDestroyed (VideoWallpaperService.java:158)

[Valentin Moscone]

Hello! The issue there is that you are evaluating a null value (isPreview is null). If (null) is going to fail

[Joost Klitsie]

@Deepti M always think about the worst case what can happen 🙂 there are 2 things that could go wrong: lastWallpaperEnging is not null, but nullified just before you call !lastWallpaperEnging... This could for example happen with async code, or perhaps by getting the value it will nullify it (I know that it is weird but in theory you can do that with kotlin)
Then you have indeed the case that lastWallpaperEnging is not null, but the isPreview variable. the best way to check this, is to use Kotlin as kotlin and not as Java. Meaning: you never write the old Java null check if (stuff != null) but you write it as kotlin:

if (lastWallpaperEngine?.isPreview == false) {}

This will only return true if your value is not null and also false. It has a double null check inside, because if either lastWallpaperEngine or isPreview is null you are safe.

[Deepti M]

oh wow, I was so wrong. I mean, I didn't realise that I am writing Java code to be safe. This is just because I am new to Kotlin. Thanks @Joost Klitsie for explaining the issue so well! Will never forget this in future.


Also Thanks to @Valentin Moscone for confirming the case!