Understanding hashcode()

[Sonali Sehgal]

This question is from enthuware software:-

 
Consider the following class:

public class X
{
	private int a;
	private int b;
	public void setA(int i){ this.a = i; }
	public int getA(){ return this.a; }
	public void setB(int i){ this.b = i; }
	public int getB(int b){ return b; }
	public boolean equals(Object obj)
	{
	   return ( obj instanceof X && this.a == ((X) obj).a );
	}
	public int hashCode()
	{
	  //1
	}
}

   

Which of the following options would satisfy the equals and hashCode contract at //1?

Option 1: return 0;
Option 2: return a;;
Option 3: return a+b;
Option 4:return a*a;
Option 5: return a/2;

Please help me understanding the hashcode as in this problem it is not clear to me what will be the output and why?

[Sebastian Janisch]

The equals method must be Reflexive, Symmetric, Transitive and Consistent. That means:

Reflexive: x.equals(x) must be true
Symmetric: if x.equals(y) is true , then y.equals(x) must be true
Transitive: if x.equals(y) is true and x.equals(z) is true, then y.equals(z) must be true
Consistent: multiple calls to equals must return in the same result

Now as for hashCode(), there is one major rule:
If two objects are equal, the hashcode needs to be equal as well. The same however does not apply in reverse.

Given the information above you should be able to solve the question.

[Sonali Sehgal]

Hi Sebastian ,

I could not understand what are you trying to say on the hashcode???

[Sebastian Janisch]

If two objects are equal (x.equals(y) == true), then x.hashCode() and y.hashCode() must return the same value.

However, if x.equals(y) == false, then x.hashCode() and y.hashCode() may return the same value.

It's as simple as that.

[Narcisa Pantilimon]

Hello!

A very good explanation on hashcodes can be found in "SCJP Sun Certified Programmer for Java 6 Study Guide (Exam 310-065)" by Katherine Sierra, Bert Bates.
Here is a direct link on the subject: http://books.google.com/books?id=Eh5NcvegzMkC&pg=RA1-PA550&lpg=RA1-PA550&dq=hashCode+buckets&source=bl&ots=ESMaHJV2Lo&sig=-WqKNWtannh0sBwugUtirYXJXp8&hl=en&ei=xBWBSve8HdeG_Aa7lYy5Cw&sa=X&oi=book_result&ct=result&resnum=6#v=onepage&q=hashCode%20buckets&f=false

[Seetharaman Venkatasamy]

Welcome to javaranch Narcisa

[Stephen Davies]

Is the answer offered Multiple choice?

[Sonali Sehgal]

@ Narcisa
I am already studying from Kathy Sierra

@Stephen yes the question is answered multiple choice and I already know the answer

[Nitish Bangera]

Leaving option 3 all others are appropriate. only the contract that needed to be checked is equals = true(definitely same hashcodes) and hashcode != true(definitely not equal).