Why does HashSet<Integer> store the integer value itself, but HashSet<Demo> stores the reference of the Demo object?

Question

Hi All,

class Demo{
    private int a;
    Demo(int a){
        this.a = a;
    }
}

public class Test{
    public static void main(String[] args){
        
        Demo d1 = new Demo(10);
        Demo d2 = new Demo(20);
        
        Set<Demo> set = new HashSet<Demo>();
        
        set.add(d1);
        set.add(d2);
        
        System.out.println(set);
    }
}

Output: [javaSE_8.Demo@15db9742, javaSE_8.Demo@6d06d69c]

I understand that the above code is printing the object reference.

public class Test{
    public static void main(String[] args){
        
        Integer i1 = 10;
        Integer i2 = 20;
        
        Set<Integer> set = new HashSet<Integer>();
        
        set.add(i1);
        set.add(i2);
        
        System.out.println(set);
    }
}

Output: [20, 10]

Where as this code prints the value itself in-spite of Integer being a class(Wrapper class) and i1, i2 being its objects.

I want to know why is this happening, or why does this difference exist.

Thank you, Suthan

Answer 1

Both HashSet<Demo> and HashSet<Integer> store references to object.

System.out.println(set) prints the String returned by set.toString() method. This method calls object.toString() for each object in the set and concatenates them with a space.

Now because Integer.toString() returns the actual value as a String, you are seeing the value being printed for Integer whereas in the case of Demo, Demo.toString() returns the object reference as a String since it is the default method java.lang.Object.toString(). You can override this method in Demo to return the actual value as a String.