Autoboxing and the NullPointerException Mystery

Autoboxing and the NullPointerException Mystery

During one of the Java classes I teach, a student came up to me with a peculiar bug. Below is a simplified version:

HashMap<String,Boolean> map = new HashMap<String,Boolean>();    if (map.get("a") == true) // -> NullPointerException      // do something  

The NullPointerException here seems odd. While the == true seemed superfluous, I initially thought this would work. Even if it didn't, I couldn't understand why this would give a NullPointerException.

A NullPointerException occurs when you try to use a reference that hasn't been initialized yet (its value is null), but the only reference used here is map, which was definitely initialized!

I tried a few things:

System.out.println( map.get("a") );  // -> null  

As I expected, this gave a null.

Then I tried:

System.out.println( null == true );  // -> CompileError: Incomparable types: <null> and boolean  

Ok, so I had forgotten that in Java, you can't compare two objects of different types. In some other languages, null is considered "falsy".

But when I try:

System.out.println( map.get("a") == true )  // -> NullPointerException  

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