inserting into set class in java

Question

import java.util.*;
class emp{
    String city;
    String name;
    emp(String a, String b)
    {
        city=a;
        name=b;
    }
}
public class HelloWorld{

     public static void main(String []args){
         Set<emp> s = new HashSet<emp>();
         emp s1=new emp("bangalore","mukesh");
         emp s2= new emp("bangalore","mukesh");
         s.add(s1);
         s.add(s2);
         System.out.println(s1.equals(s2));
         System.out.println("finsih");
         System.out.println(s);

     }
}

i am creating two objects, which are same but they get entered in set and why s1.equals(s2) return false?

You need to write your own equals function by overriding the equals function of object class ,if you want these two objects to be equal. — bit-shashank, Jun 18 '18 at 04:02
If it was your intention that the set only have one `emp` object (because you consider them equal), then you need to implement both `equals()` and `hashCode()`. See [Understanding the workings of equals and hashCode in a HashMap](https://stackoverflow.com/q/1894377/5221149). — Andreas, Jun 18 '18 at 04:06
*FYI:* Java naming convention is for class names to start with uppercase letter, i.e. `Emp`. Also, naming the parameters `a` and `b` is not very helpful to people calling the method. — Andreas, Jun 18 '18 at 04:07

Rudy · Accepted Answer · 2018-06-18T04:13:58.087

It has nothing to do with inserting to set. You need to override equals, for example below is the basic equal method :

@Override
public boolean equals(Object o) {

    if (o == this) return true;
    if (!(o instanceof Emp)) {
        return false;
    }
    Emp emp = (Emp) o;
    return  Objects.equals(city, emp.city) &&
            Objects.equals(name, emp.name);
}

Follow details , follow the sample here: https://www.mkyong.com/java/java-how-to-overrides-equals-and-hashcode/

score -1 · Answer 2 · answered Jun 18 '18 at 04:08

Since you have not overridden equals(), Object#equals is used which by default uses == operator. Since you have created 2 objects with new operator, their refernces are different and hence call to equals() returns false. You can override both equals and hashcode to provide equality:

class emp{
    String city;
    String name;
    emp(String a, String b) {
        city=a;
        name=b;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof emp)) return false;
        emp test = (emp) o; 
        return Objects.equals(city, emp.city) &&
                Objects.equals(name, emp.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(city, name);

}

Now attempting to store the objects will store only one object in the set.

inserting into set class in java

2 Answers2