How does unboxing work in short-circuited boolean expressions?

Question

I recently tried to run the following two snippets of code and was surprised at the output.

First:

// ...
System.out.println( (Boolean)null || true );
// ...

Second:

// ...
System.out.println( (Boolean)null || false );
// ...

The first example results in the following output:
true

The second example results in the following output:
Exception in thread "main" java.lang.NullPointerException
   at com.blah.main(SanityCheck.java:26)

I would have thought both examples should result in a null pointer exception, as any short-circuiting is applied left-to-right. The attempt to unbox the boolean from the Boolean should have failed before the other side of the logical or was given consideration.

Can anyone explain this inconsistent behaviour?

Answer 1

I ran the class file through JAD to see what the optimised code looked like. For the true case:

// ...
System.out.println(true);
// ...

For the false case:

// ...
System.out.println(null.booleanValue());
// ...