Prohibit HashSet to allow duplicate object in JAVA

Question

In Main

Student st=new Student(1,"snack");
Student st1=new Student(2,"jack");
Student st2=new Student(2,"jack");
Set<Student> hs=new HashSet<Student>();
hs.add(st);
hs.add(st1);
hs.add(st2);    
System.out.println(hs);

O/P:[1 snack, 2 jack, 2 jack]

In Student

@Override
public int hashCode() {
    // TODO Auto-generated method stub
    return this.id;
}

@Override
public boolean equals(Student obj) {

    if(this.id==obj.id)
        return false;
    else
        return true;

My objective is not to allow student who has the same id..name can be the same. Please provide details how HashSet checks which element is a duplicate? All I know that HashSet returns true or false based on hashcode() and equals() method. What works in back end?

`public boolean equals(Student obj)` does not override `Object.equals`. It should rather be `public boolean equals(Object obj)` — ernest_k, Sep 29 '18 at 05:32
Do I need to override both equals and hashcode? or only hashcode is enough? — SUVAM ROY, Sep 29 '18 at 05:34
If you use your IDE to generate your hashCode and equals it will be right or a good starting point. You can also use your IDE for getters, setters and toString. — Peter Lawrey, Sep 29 '18 at 06:19

Thiyagu · Accepted Answer · 2018-09-29T06:09:44.970

4

You got the condition reversed. You have to return true if two students have the same id.

if(this.id == obj.id)
    return true;
else
    return false;

You are not overriding the equals in the Object class(Object.equals). The equals takes an Object as a parameter and not Student.

Here's a way you can do it.

@Override
public boolean equals(Object obj) {
    if (!(obj instanceof Student))
        return false;
    if (obj == this)
        return true;
    return this.id == ((Student) obj).id;
}

edited Sep 29 '18 at 06:09

answered Sep 29 '18 at 05:33

Thiyagu

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Your null check is redundant; `instanceof` takes care of that already. – chrylis -cautiouslyoptimistic- Sep 29 '18 at 05:55
@chrylis You are right. Changed it. Thanks – Thiyagu Sep 29 '18 at 06:10
this.id == ((Student) obj).id please explain this line? which id is compared to which id? – SUVAM ROY Sep 29 '18 at 06:16
It is same as your code. It compares the id of the object on which equals is invoked on with the id of the object that we are comparing against for equals. – Thiyagu Sep 29 '18 at 06:38

Prohibit HashSet to allow duplicate object in JAVA

1 Answers1