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hashCode() Question on Dan's Mock Exam.

George Jin

Joined: Oct 18, 2002
Posts: 4
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.
a. false
b. true

should the hashCode() return the same value constistantly for objects that have same member data?
Jamal Hasanov
Ranch Hand

Joined: Jan 08, 2002
Posts: 411
Hi, George
I think it will help you.

class Object
public int hashCode()
Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable.
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.
As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

Jamal Hasanov
Dan Chisholm
Ranch Hand

Joined: Jul 02, 2002
Posts: 1865
Your question has been popular recently. Here's another thread where this topic was recently covered.

Dan Chisholm<br />SCJP 1.4<br /> <br /><a href="" target="_blank" rel="nofollow">Try my mock exam.</a>
I agree. Here's the link:
subject: hashCode() Question on Dan's Mock Exam.
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