override .equals() and .hashcode()

[Steve Bradley]

Hi,

This question comes from Sierra / Bates chapter 7, question 4.

Which class does not override the equals() and hashcode() methods, inheriting them directly from class object?

A. java.lang.String
B. java.lang.Double
C. java.lang.StringBuffer
D. java.lang.Character
E. java.util.ArrayList


The correct answer is c. I want to understand the logic of this question so that, if necessary, I can answer a similar question from a different list of classes??

[Murtuza Akhtari]

All Wrapper classes, the String Class and the Collections classes override the equals() and the hashcode() methods.

I dont know wht you mean by logic...do u mean why the classes that override, override them?

[Peter den Haan]

The #1 thing to remember here - because it can freak the unwary - is that the StringBuffer class does not implement equals() and hashCode(). Probably because it was conceived simply for string manipulation rather than representation (we've got the String class for that). Classes that represent or contain data generally have hashCode() and equals() implementations.

- Peter