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?
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.
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)
我认为，因为Null可以投入到任何类型的引用中，所以在编译时它没有错，如果你抛出null而不是throwable。
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