How to override equals method of student class properly so that HashSet can differentiate duplicare objects of Student class

Question

this is my Student Class, I want to differentiate Student Object on the basis of Name field. But When main class executed i am getting wrong output.

public class Student {

int id;
String name;

public Student(String name, int id)
{
    this.name=name;
    this.id= id;
}


public int hashCode()
{
    return this.id;
}

public String toString()
{
    return "Student: "+this.name+"@"+Integer.toHexString(hashCode());
}



public boolean equals(Object o)
{
    if(o instanceof Student)
    {
    String name=this.name;
    Student s=(Student)o;

    if(s.name.equals(name))
    return true;
    else 
    return false;
    }
    else
    return false;

}}

//this is my main class

public class HashSett {

public static void main (String[] args)
{

    HashSet<Student> h=new HashSet<>();

    h.add(new Student("Nimit",1));
    h.add(new Student("Rahul",3));
    h.add(new Student("Nimit",2));

    System.out.println(h); 
 }
}

//this is wrong output i got

[Student: Nimit@1, Student: Nimit@2, Student: Rahul@3]

why two times "Nimit" objects are added???

score 3 · Answer 1 · answered Jan 16 '19 at 08:05

Your hashCode doesn't match your equals implementation. If a.equals(b) is true, a.hashCode == b.hashCode() must also be true.

Since equals only requires that the names are equal, hashCode should return name.hashCode().

public int hashCode()
{
    return name.hashCode();
}

score 0 · Answer 2 · answered Jan 16 '19 at 08:10

0

Generate toString, hashCode and equals from Eclipse generate option or similar option from any IDE

answered Jan 16 '19 at 08:10

prabhanelamalli

23
2

that would not be ideal, since it would not be in alignment with his requirements. – Stultuske Jan 16 '19 at 08:30

How to override equals method of student class properly so that HashSet can differentiate duplicare objects of Student class

2 Answers2