HashSet unique values

[Aakash Chandel]

I have a question about Set. I know that sets do not allow duplicates. I tried the following sample but didn't quite understand how did the JVM automatically understood that I have added d3 object more than once? I have not added any equals() method which would say that two objects are equal based on some instance variable. Please guide.

package collections;

import java.util.HashSet;
import java.util.Set;

class Dept {
    public int id;
    public String name;
    
    Dept(int id, String name) {
        this.id = id;
        this.name = name;
    }
    
    public String toString() {
        return id + "-" + name;
    }
}

public class SetDemo3 {
 public static void main(String[] args) {
        Dept d1 = new Dept(1, "Accounts");
        Dept d2 = new Dept(2, "Finance");
        Dept d3 = new Dept(3, "Admin");
        Dept d4 = new Dept(4, "HR");
        Dept d5 = new Dept(5, "Purchase");
        
        Set s = new HashSet();
        s.add(d1);
        s.add(d2);
        s.add(d3);
        s.add(d4);
        s.add(d5);
        s.add(d3); //adding again
        s.add(d3); //adding again
        System.out.println("s=" + s); //output: s=[4-HR, 1-Accounts, 2-Finance, 3-Admin, 5-Purchase]
    }   
}

[Paul Clapham]

True, you haven't implemented an equals() method. That means that your Dept class inherits its equals() method from Object, and therefore that two Dept objects are "equal" if and only if they are the same object.

But I don't understand your question. Or your code. You're asking about whether the add() method of Set notices when you add an object which is already in the set, but your code doesn't bother to find out. Which it could very easily. Read the documentation for Set's add() method and you will see.

[Aakash Chandel]

Paul Clapham wrote:True, you haven't implemented an equals() method. That means that your Dept class inherits its equals() method from Object, and therefore that two Dept objects are "equal" if and only if they are the same object.

But I don't understand your question. Or your code. You're asking about whether the add() method of Set notices when you add an object which is already in the set, but your code doesn't bother to find out. Which it could very easily. Read the documentation for Set's add() method and you will see.

Let me try to make this clear.
The output of the above program is :

s=[4-HR, 1-Accounts, 2-Finance, 3-Admin, 5-Purchase]

I was hoping that the output of the program will have 3-Admin department displayed 3 times since I added it 3 times using set.add(). But that didn't happen.

So, if I try to add two String objects with same values Set won't allow because it uses the String's equals() method to determine if they are same. How does Set figure out if two objects (Dept) are equal and does not add them to Set even though we have not provided equals() method for Dept object?

Hope this makes sense.

[Satya Maheshwari]

Even though you have not provided equals, its inherited from Object.

[Amu Hlongwane]

Hi try the following you will understand how equals() work...

create another object as follows...

Dept d6 = new Dept(3, "Admin");

add it into the set

s.add(d6); //adding again

now compiling and running the code you will see this output...
s=[5-Purchase, 2-Finance, 3-Admin, 1-Accounts, 4-HR, 3-Admin]

now override equals() and hashcode() methods...

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Dept other = (Dept) obj;
        if (this.id != other.id) {
            return false;
        }
        if ((this.name == null) ? (other.name != null) : !this.name.equals(other.name)) {
            return false;
        }
        return true;
    }

    @Override
    public int hashCode() {
        int hash = 7;
        return hash;
    }

now compiling and running the code you will see this output...
s=[5-Purchase, 4-HR, 3-Admin, 2-Finance, 1-Accounts]

conclusion...
d3 will always refers to the same object whether you add it several times or not, the compiler will see the same object, this was achieved by this...

 Dept d3 = new Dept(3, "Admin"); 

but every time you say

 new Dept(3, "Admin"); 

and giving it a different reference, e.g d6, and Dept does not override equals(), then everytime it's a complete new object, which will never be equal to any other

 new Dept(3, "Admin"); 

with different reference, until equals() is overriden.

Dept d3 = new Dept(3, "Admin");
Dept d6 = new Dept(3, "Admin");
d3 will always be equal to d3, but d3 will only be equal to d6 if equals() is overriden.

Hope this helps.

[Aakash Chandel]

Much better and thanks for the very good explanation.