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In Java:

int count = (Integer) null;

throws a java.lang.NullPointerException.

Why doesn't this throw a Class Cast Exception for ease in programmer understanding?

Why was this exception chosen over any other exception?

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3 Answers 3

up vote 34 down vote accepted

When executing your code, the Java runtime does the following:

  1. Cast null to an object of class Integer.
  2. Try to unbox the Integer object to an int by calling the method intValue()
  3. Calling a method on a null object throws a NullPointerException.

In other words, null can be cast to Integer without a problem, but a null integer object cannot be converted to a value of type int.

EDIT

I had a related question a while ago at Stack Overflow, see here.

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Java successfully casts null to an Integer reference that references no object.

That's OK because being unistantiated is a valid state for a reference.

It's the calling of a method of a non existing object that can't be performed.

Performing the cast (Integer)null is the same as declaring an Integer variable and then failing to assign it an new (or already existing) Integer object instance.

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To unbox a Integer into a int i.e. in int i = new Integer(15);, i actually equals new Integer(15).intValue() i = (Integer) o; where Object o = 15 is the same as o = Integer.valueOf(15); but i = null; throws an NullPointerException because i then equals null.intValue() which throws a NullPointerException.

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2  
The answer that was accepted almost two years ago provides almost the same explanation, but is more clear. This answer is also not correct for Java 5+: it now uses the valueOf() factory methods rather than creating new instances for boxing. –  Snowman May 5 at 19:50

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