Why does set.size() show different different values for String and Java Object?

Question

Why does set.size() show different values for String and Java Object when they are for Duplicate elements?

If I insert duplicate values in String and run set.size() it only counts the unique elements, but in Object, when I insert duplicate values it counts the total, including the duplicate itself. Why is this?

FIRST PROGRAM
==============
/** Insert Duplicate in String **/
public class SetDemo {

    public static void main(String[] args) {

        Set<String> set = new HashSet<>();
        // Insert duplicate values
        set.add("Rai");
        set.add("Rai");
        set.add("Tama");

        // show set size
        System.out.println(set.size());
    }

}
Output: 2  
=======================================================================


SECOND PROGRAM 
====================
   /* Insert Duplicate in Object */



class Book {  
int id;  
String name,author,publisher;  
int quantity;


public Book(int id,String name,String author,String publisher,int quantity) 
    {  
         this.id = id;  
         this.name = name;  
         this.author = author;  
         this.publisher = publisher;  
         this.quantity = quantity;  
     }  
    }    

public class HashSetExample {  
    public static void main(String[] args) {  
        HashSet<Book> set=new HashSet<Book>();  
        //Creating Books  
        Book b1=new Book(101,"Let us C","Yashwant Kanetkar","BPB",8);  
        Book b2=new Book(102,"Data Communications & 
 Networking","Forouzan","Mc Graw Hill",4); 
       // Insert duplicate values 
        Book b3=new Book(103,"Operating System","Galvin","Wiley",6);
        Book b4=new Book(103,"Operating System","Galvin","Wiley",6);
        //Adding Books to HashSet  
        set.add(b1);  
        set.add(b2);  
        set.add(b3);  
        set.add(b4);
        //Traversing HashSet  
        for(Book b:set){  
        System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" 
  "+b.quantity);  


    }           


     // show set size
    System.out.println(set.size());   

}  
}  

Output: 4

If I insert duplicate values in String and run set.size() it only counts the unique elements, but in Object, when I insert duplicate values it counts the total, including the duplicate itself. Why is this? In the First program duplicate values are not allowed but in the Second program set.size() should be 3 yet it shows 4

Your `Book` class should override `hashCode` and `equals` in order for `HashSet` to know which `Book` instances you consider to be identical. — Eran, Aug 05 '19 at 06:06

score 1 · Answer 1 · answered Aug 05 '19 at 06:10

Set stores only unique elements. It is using object's equals and hashCode methods to check is the object equal to another or not. String has already written proper equals and hashCode methods. Your own object probably doesn't have. You should override methods equals and hashCode derived from Object class and then your own object will behave same as String.

score 0 · Answer 2 · answered Aug 05 '19 at 06:21

For HashSet to work correctly you need to overide the hashCode(). When you are creating customized classes which contains several attributes you should override the hashcode accordingly to work properly. This is because HashSet uses hashcode() to add and get values from HashSet. In Java there's contract between hashcode() and equal() method which is if two objects are equal it should return the same HashCode. If you follow this contract these kind of implementation will work not only for HashSets but HashMaps also.

Why does set.size() show different different values for String and Java Object?

2 Answers2