Equals and HasCode

[Abhi vijay]

Source: MasterExam

  If a class Dog follows equals() and hashCode() contracts then which statements are true?

A. x.equals(y) is true then x==y must be true.
B. x.equals(y) is false x==y may be true.
C. x.equals(y) is true x==y may be false.
D.x.equals(y) is true, x.hashCode()==y.hashCode() may be false. 
E. x.equals(y) is false, x.hashCode()==y.hashCode() may be true.
F.x==y is false,x.equals(y) must be false.

The answer given is C, E.
But A, B are also correct?

[Ruben Soto]

No, they are not correct Abhi. The way to think about the three relationships (hashCode() values, equals() return, and == return) is:
The most strict (specific) relationship is ==. If == is true, you can be assured that if the implementations of equals and hashCode are correct, equals will return true, and the hashCode values will be the same.
The next relationship in order of strictness is equals. If equals is true, you must have that hashCode values are the same.
Finally, you have hashCode. This is the least strict relationship. Even for completely unrelated objects of the same class, hashCode may return the same values.
Let me know if that makes sense, and try to reason why A and B are false.

[Punit Singh]

public class Dog {
private String name;
Dog(String name){
	this.name=name;
}
@Override
public boolean equals(Object obj) {
	if(obj instanceof Dog){
		Dog d=(Dog)obj;
		return name.equals(d.name);
	}
	return false;
}
@Override
public int hashCode() {
	
	return name.length(); 
}


public static void main(String[] args) {
	// A. x.equals(y) is true then x==y must be true.
	Dog d1=new Dog("Tommy");
	Dog d2=new Dog("Tommy");
	System.out.println(d1.equals(d2));
	System.out.println(d1==d2);
	
	//B. x.equals(y) is false x==y may be true.
	Dog d3=new Dog("Tommy");
	Dog d4=new Dog("diano");
	System.out.println(d3.equals(d4));
	System.out.println(d3==d4);
	
}
}    

[James Tharakan]

Hope Ruben cleared your doubt, Just an example here

 class A{
     int i;
     A(int i){
         this.i=i;
     }
 }  
 
 A a1= new A(1);
 A a2= new A(1);

For option A: Even though the objects are meaningfully equal, there memory location is not same.
For option B: it says "x==y may be true.". If it is true then it MUST be equal.

Hope i am clear.

[Balagopal Kannampallil]

"==" refers to the memory where the objects are located. If both the objects lies on the same location "==" will return true.

    
Dog dog= new Dog ("Dido");
Dog dog1= new Dog ("Dido");
System.out.println(dog.equals(dog1));//will be true because both has same names
System.out.println(dog==dog1); //will return false because both objects are 
                               //located on different memory locations

[Abhi vijay]

That makes perfect sense.
But while adding objects to a hash-based collection we say that if you override the equals() method and dont override the hashCode() then the 2 objects will be considered unequal or different.
So I was trying to figure out the answer that way.

[Punit Singh]

Abhi vijay wrote:That makes perfect sense.
But while adding objects to a hash-based collection we say that if you override the equals() method and dont override the hashCode() then the 2 objects will be considered unequal or different.
So I was trying to figure out the answer that way.

Good, Hash based collections first matches hashCode of both objects , if both objects found in same bucket than only it will call equals(Object) method.