Decoding variable information when debugging Java

Question

I am using IntelliJ IDEA 8 to debug some Java, but this question is probably applicable to all Java debuggers. In the list of variables they are displayed as:

myVariable = {some.package.SomeClass@12345}

I'm curious about the number that appears after the class name. What is the exact number? Do two variables have the same numbers if they refer to the same base object?

Thanks in advance.

Answer 1

This is the objectId reported by the JVM, see the JDWP specification for details.

Uniquely identifies an object in the target virtual machine. A specific object will be identified by exactly one objectID in the JDWP Command and responses for its entire lifetime (or until the objectID is explicitly placed). An object identifier is not reused to identify another object if it was not explicitly, regardless of whether the referenced object was garbage collected. Object identifier 0 is a null object. Note that the existence of an object identifier does not prevent the collection of object garbage. Any attempt to access the garbage collected object with the object identifier will result in an INVALID_OBJECT error code. Garbage collection can be disabled using the DisableCollection command, but usually it is not necessary to do this.

Answer 2

It looks like this is just the direct result of toString generated when the class does not override toString with its own implementation.

If this is the case (which you can verify by overriding toString and looking to see if you have a different output), then according to the Java documentation for Object ( http://java.sun.com/j2se/1.5.0/docs/api/java /lang/Object.html ) the default for toString is:

getClass().getName() + '@' + Integer.toHexString(hashCode()) 

Hashcodes - what you are talking about is quite complicated. Again, from the Java documentation for the hashcode object, it is implemented with the following contract:

 The general contract of hashCode is: * Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application. * If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. * It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables. 

So, the short answer, you number does not mean that it is the same object reference. They can have the same values, but even these are not set.