what is the use of overriding hashCode() in java?

http://stackoverflow.com/questions/27581/overriding-equals-and-hashcode-in-java

The idea is so that it behaves properly in the Map classes. Each difference object should produce not only a unique hashcode, but one that makes sense with its purpose and internal structure

http://www.technofundo.com/tech/java/equalhash.html

Theres a contract defined in the javadoc (as explained in the link posted by a_b)
If you change the equals() method, you need to ensure that the hashCode() method still meets that contract.

Hi All,

Please refer the below code..here i am comparing two stud objects, but it is executes only equals() but not hashCode(), so how can i write in my second app, to execute the hashCode() also

package app;

public class Student {
	int studId;
	String studName;
	int studMarks;

	public Student(int studId, String studName, int studMarks) {
		this.studId = studId;
		this.studName = studName;
		this.studMarks = studMarks;
	}

	public boolean equals(Object o) {
		boolean result = false;
		Student s = (Student) o;
		if (this == o) {
			return true;
		} else {
			result = this.studId == s.studId;
			return result;
		}

	}

	public int hashCode() {
		System.out.println("Hash Code is:" + this.hashCode());
		return this.hashCode();
	}

}

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package app;

public class Two 
{
	public static void main(String[] args) {
		
		Student stud1=new Student(100,"Haneef",70);
		Student stud2=new Student(100,"Haneef",80);
		
		
		
		if(stud1.equals(stud2))
			System.out.println("Both are Same");
		else
			System.out.println("Not Same");
			
	}
}

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Based on the above two application, by using the equals() method it self, we could compare the two objects, then why we need to call the hashCode() again.

hashCode() is not used for comparing objects, thats what equals() is for. In fact two objects that are equals can have the same hash code.

hashCode() is used for storing Objects in collections like Map's.

http://en.wikipedia.org/wiki/Java_hashCode()
http://www.javapractices.com/topic/TopicAction.do?Id=28

Hi Objects,

Thanks..

u r telling it is not for comparing two objects, then why we need to override hashCode() while comparing the two objects.(i think it is mandatory to override)

Your class should be something like the following, based on your intentions:

public class Student {
    int studId;
    String studName;
    int studMarks;

    public Student(int studId, String studName, int studMarks) {
        this.studId = studId;
        this.studName = studName;
        this.studMarks = studMarks;
    }

    public boolean equals(Object o) {
        boolean result = false;
        Student s = (Student) o;

        if (this == o) {
            result = true;
        } else {
            result = (this.studId == s.studId);
        }

        return result;
    }

    public int hashCode() {
        return studId;
    }
}

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It is so help full.