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

Where is the NullPointerException coming from?

The Culprit: Autoboxing and Unboxing

Introduced in Java 1.5 (later called Java 5), autoboxing and unboxing is the automatic conversion of primitives and their wrapper classes.

Before Java 1.5, you had to manually convert the wrapper class to its primitive form by calling a specific function. For example, if you created an ArrayList of Integers, you would need to call the intValue() function on the Integer object to get the primitive value

int x = arr.get(0); // Wrong, will give you an error
int x = arr.get(0).intValue(); // Correct!

With autoboxing/unboxing, Java will implicitly call these functions (such as intValue) from the wrapper classes when it notices you are trying to use the primitive value.

Normally, map.get(x) will return a Boolean object, but will also return null if x is not found in the HashMap. Note that null is a valid value for a Boolean object (or any Java object)

However, Java sees that I'm comparing a Boolean object with a boolean primitive, so it implicitly calls the Boolean object's booleanValue() function.

Because map.get(x) returns null, Java is actually implicitly calling booleanValue() on the null object, hence the NullPointerException!

Solution

The solution is to check if the object is null before performing a comparison

if( map.get("a") != null && map.get("a") == true )
    // do something

or, more simply,

if( map.get("a") != null && map.get("a") )
    // do something