CODEWITHSUNDEEP
javaexceptionnull
Reuben
Reuben

Reputation: 5736

Why is "throw null" not creating a compilation error in Java?

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?

Upvotes: 35

Views: 6995

Answers (4)

GargantuChet
GargantuChet

Reputation: 5789

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.

Upvotes: 56

Peter Lawrey
Peter Lawrey

Reputation: 533670

There are many things a compiler doesn't check, it assumes you do things for a good reason which it might not know about. What it does try to prevent is the common mistakes developers make.

It is possible some one thinks this is a good short hand for

throw new NullPointerException();

Integer i = null;
try {
    i.intValue();
} catch (NullPointerException npe) {
    System.err.println("Caught NPE");
    npe.printStackTrace();
}

and

try {
    throw null;
} catch (NullPointerException npe) {
    System.err.println("Caught NPE");
    npe.printStackTrace();
}

prints in Java 6 update 38

Caught NPE
java.lang.NullPointerException
    at Main.main(Main.java:9)

Upvotes: 5

sampathpremarathna
sampathpremarathna

Reputation: 4064

I think because Null can be cast in to any type of reference.so in compile time its nothing wrong if you are throwing null instead of throwable.

Upvotes: 3

Esben Skov Pedersen
Esben Skov Pedersen

Reputation: 4517

In generel, and not just throw. Any object variable can be assigned null. So we can see that throw is not a special case. Should it be? maybe. Is it consistent? Yes.

Upvotes: 2

Related Questions