Dan's mock exam: Hashcodes

[Aaron Anders]

A class C has legal implementations of the equals and hashCode methods. On Monday, an instance of the class is created and the hashCode method is invoked. On Tuesday the program is loaded again and an instance of class C is created containing the same data that was loaded on Monday. If the hashCode method is invoked after restarting the program on Tuesday then the hashCode method must return the same integer value that was returned on Monday. (answer is false)

but why? is it
- because C may have members that store date/time related information?
or
- because the question didn't state that the 2 objects are equal?

[Jasper Vader]

The purpose of hashCode() is to represent an object via a single int. If two objects that are equal as regards the equals() method, then they should return the same hascode.
Was there any more info in the question to indicate anything else about the situation?
or maybe the instances were instances of different objects?

[Rahul Gupta]

Dear Aaron,
The java contract for hashCode() says that if equals() method returns true for comparison on two objects then the hashCode() for both should return the same value. However it does not ensure that for different runs of the same program the hascode returned should be same.
For example suppose hascode returned x on Monday and equals returns true. Then on Tuesday if the same program runs then if the equals method returns true then they both objects will should return the same hashcode but not necessarily x.
This is from javadoc of java.lang.Object
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.

For more information please go through the javadoc of Object class

enjoy!!!
rahul

[Jasper Vader]

so maybe the key is in the use of the word 'must' in the question ... where in fact the hashcode method only 'must' return the same hashcode for that object in the same session.

[Dan Chisholm]

Thank you Rahul. Your explanation of my exam question is exactly correct.
Yes Jasper, the key is the word "must". Although the hashCode method can return the same value on Tuesday that was returned on Monday, there is no requirement that it "must" return the same value after the program has been shut down and restarted.