equals() and hashCode()

[Harikrishna Gorrepati]

Hi, Why do I hear this sentence quite often "When you override equals() method, override hashCode() method also". From the following code, without hashCode() method, I am able to compare and print true. Also, I am thinking that hashCode() is useful only if we use either HashSet or HashMap. Am I correct/ wrong? Please advice

 public class P552_equals_hashCode_classes00 {

    private int age;

    public P552_equals_hashCode_classes00(int age) {
	this.age = age;
    }

    public boolean equals(Object obj) {

	boolean flag = false;

	if (obj instanceof P552_equals_hashCode_classes00) {
	    P552_equals_hashCode_classes00 objectRef1 = (P552_equals_hashCode_classes00) obj;
	    if (objectRef1.age == age)
		flag = true;
	}
	return flag;
    }

    public static void main(String args[]) {
	P552_equals_hashCode_classes00 emp1 = new P552_equals_hashCode_classes00(23);
	P552_equals_hashCode_classes00 emp2 = new P552_equals_hashCode_classes00(23);
	System.out.println("emp1.equals(emp2)--->>>  " + emp1.equals(emp2)); // prints "true"
    }
}   

[Christophe Verré]

This answer is in the API of the Object#equals method :

API wrote:
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

[Himanshu Gupta]

It is good to follow the contract because there may be other guys in the team who wants to put the Objects in the Hash Collections<HashMap, HashSet>.

This can result in giving some unusual behavior and unexpected results.

[Rikesh Desai]

Oh ok.
So does this mean, (as Harikrishna pointed out), that we need to use HashCode implementation just for HashSet, HashTable and HashMap?
Wont we need to implement it for TreeMap too?

[Harikrishna Gorrepati]

Rikesh Desai wrote:Oh ok.
So does this mean, (as Harikrishna pointed out), that we need to use HashCode implementation just for HashSet, HashTable and HashMap?
Wont we need to implement it for TreeMap too?

Hi Rikesh, Christophe and Himanshu are saying something like this. Add this code to the program.

	System.out.println(emp1.hashCode()); //11394033
	System.out.println(emp2.hashCode());//4384790

Do you see, we are getting 2 different outputs (may differ in your system), which it should NOT be as per contract. We need to implement hashCode() wherever you write equals() method. That's why String and wrapper classes implements these 2 methods.

[Trivikram Kamat]

Harikrishna Gorrepati wrote:
I am thinking that hashCode() is useful only if we use either HashSet or HashMap.

Rikesh Desai wrote:
So does this mean, (as Harikrishna pointed out), that we need to use HashCode implementation just for HashSet, HashTable and HashMap?

hashCode() is method in java.lang.Object
As all classes inherit java.lang.Object, hanshCode() is also inherited.
It can be used in any class, only thing is that equals() should maintain general contract with hashCode() as told by Christophe Verré