Example3.java

public class Foo {
    
    @Override 
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        } 
        else if (obj == null || obj.getClass() != getClass()) {
            return false;
        } 
        else {
            Foo foo = (Foo) obj;
            return Objects.equals(foo.firstName, firstName);
        }
    }
}

Foo foo = new Foo("John");
Foo fooIdentical = new Foo("John");
Bar bar = new Bar("John");
Set < Foo > foos = new HashSet < > ();
foos.add(foo);
foos.add(fooIdentical);
System.out.println(foos); // [Foo@23514a] 
foos.add(bar); 
System.out.println(foos); // [Foo@23514a, Bar@23514a]