How HashSet works internally - Internal implementation

Internal implementation of any collection (HashMap, HashSet, LinkedhashMap, etc) is common interview question for senior java developer and it is highly recommended to brush-up internal implementations of collections before going for interview.Here I am discussing how HashSet works internally.Before reading this article you must know How HashMap works internally?, since HashMap is building block of internal working of HashSet.
Basic understanding of HashMap states that it stores key and value both in form of Map.Entry object. So our first befitting reply should be: "HashSet internally uses HashMap to store objects". HashSet constructor instantiate a HashMap when its constructor is being called. Below mentioned sample code from java.util.HashSet creates a HashMap named map.(Overloaded constructor of HashSet can be found here).

private transient HashMap<E,Object> map;
//Constructs a new, empty set; the backing HashMap instance 
//has default initial capacity (16) and load factor (0.75).
public HashSet() {
   map = new HashMap<E,Object>();
}

The very next question arises, since HashMap stores key/value pair while HashSet only stores object (set.add(someObject)) then "How Object is stored in HashSet":
Answer: HashMap stores object(Any value user is trying to store in set) as key and some pre-defined constant as value.By doing so, HashSet stores only unique values(Duplicate is not allowed).In other words, we call set.add(someObject) then call is redirected and map stores someObject as key and PRESENT(a constant) as value.Below mentioned sample code from java.util.HashSet, PRESENT is the the constatnt(marked final) value which is being used by HashMap.

// Dummy value to associate with an Object in the backing Map
 private static final Object PRESENT = new Object();
 //add operation 
 public boolean add(E e) {
   return map.put(e, PRESENT)==null; 
}

Please note the return type of add method is boolean: after adding object to map it is comparing with null, as it is expected( map will return the previous value associated with key, or null if there was no mapping for key).If returing true menas no previous element was there and it added successfully.
Similarly when we delete object form HashSet by using remove method, it redirects call to HashMap to remove object and return boolean based on object was removed or not.below is sample code for remove method:

public boolean remove(Object o) {
     return map.remove(o)==PRESENT;
  }

This is how HashSet works internally with the help of HashMap. Now try this question:

class TryHashSet{
 public static void main(String[] args) {
   HashSet<string> set = new HashSet();
   boolean stat1 = set.add("Topper of class");
   boolean stat2 = set.add("Second ranker");
   boolean stat3 = set.add("Topper of class");
   boolean stat4 = set.remove("Topper of class");
   boolean stat5 = set.remove("Topper of class");
   System.out.println(stat1 + " " + stat2 + " " +stat3 + " "+ stat4  + " " +stat5);
 }
}

Sample output : true true false true false

===============End of article==========================
Happy learning!!
Nikhil

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