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

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?

Although this is not flagged as a problem by the Java Compiler itself at the present time (unless the language specification changes at some future time), it clearly is a candidate for static checkers like "Sonar" to root out and point out to the developer. Maybe the developer wanted to write "return null"... — David Tonhofer
– David Tonhofer, Commented Feb 22, 2013 at 10:36
We know that the rule is that the result of calling String a() {return null} must be an object of (sub)class or String. Simple classes, like Throwable or BigInteger cannot be used as Strings. Null is a special Java literal which represents a null value. Then why return null is not creating any compile time error?????!!!! I am exposing your double standards here. Why do you complain about throw null but not about return null? I cannot upvote your question therefore. It is too localized by means of double standards. — Val
– Val, Commented Jul 11, 2013 at 18:20

Pang · Accepted Answer · 2013-07-21 05:56:17Z

56

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.

edited Jul 21, 2013 at 5:56

Pang

10.2k146 gold badges87 silver badges126 bronze badges

answered Jul 6, 2011 at 5:39

GargantuChet

5,8091 gold badge34 silver badges41 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Sumit Sagar Over a year ago

so null somehow converts to a NullPointer Exception, but what if I throw a String or a number? I gives runtime exception and says No exception of type String can be thrown; an exception type must be a subclass of Throwable why is it so?

GargantuChet Over a year ago

It's not that null "somehow converts" to a NPE. The language specification says that when throw gets a null value, it needs to create and throw a NullPointerException. So it does. The spec says that throw must be given "1) a variable or value of a reference type which is assignable (§5.2) to the type Throwable, or 2) the null reference". Anything else will result in a compiler error.

GargantuChet Over a year ago

It's also not clear what you would expect the language to do. A NullPointerException is commonly thrown when a value is expected but null is received (see java.io.File's constructor, for example). There's no obviously right behavior, though, for throwing an arbitrary object. (Would it generate an exception? Which one? Checked or unchecked? Or would you expect to catch an arbitrary object too? Would you catch(String s), or catch("oops")?) Letting the compiler reject unclear things is often better than having potentially surprising behavior at runtime.

Peter Lawrey · Accepted Answer · 2013-01-31 19:07:01Z

5

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)

edited Jan 31, 2013 at 19:07

answered Jul 6, 2011 at 5:29

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

1 Comment

Peter Lawrey Over a year ago

@DanielWerner All exceptions and Errors and catchable even OutOfmemoryError and ThreadDeath. ;) See my example.

sampathpremarathna · Accepted Answer · 2011-07-06 05:43:29Z

3

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.

answered Jul 6, 2011 at 5:43

sampathpremarathna

4,0645 gold badges28 silver badges37 bronze badges

2 Comments

Val Over a year ago

Exactly. Your method, that returns a String, may return null. Why string variable can be null but Throwable variable cannot? OP definitely is either short-sighted (x)or exercises the double standards.

Vlasec Over a year ago

This might sound nicely, but it isn't true. See the accepted answer.

Esben Skov Pedersen · Accepted Answer · 2011-07-06 05:34:24Z

2

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.

answered Jul 6, 2011 at 5:34

Esben Skov Pedersen

4,5072 gold badges36 silver badges49 bronze badges

Collectives™ on Stack Overflow

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

4 Answers 4

3 Comments

1 Comment

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

1 Comment

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related