Question

class ThrowNull {
    public static void main(String[] args) {
        throw null;
    }
}

We know that rule for throw is throw ThrowableInstance;, where ThrowableInstance must be an object of type Throwable or a subclass of Throwable.

Simple types, such as int or char, as well as non-Throwable classes, such as String and Object, cannot be used as exceptions. null is a special Java literal which represents a null value.

So why would throw null; compile in this code?

Answer 1

According to the language specification, a throw statement is defined as:

throw Expression

And if the Expression evaluates to null, then a NullPointerException is thrown. Specifically,

If evaluation of the Expression completes normally, producing a null value, then an instance V’ of class NullPointerException is created and thrown instead of null.

Since NullPointerException extends RuntimeException, it is an unchecked exception. This could explain why there’s no compile-time error reported from this construct.