public boolean add(E e) {

        return map.put(e, PRESENT)==null;

    }


Details:

1. map Variable:

The map variable seems to be an instance of a Map data structure, presumably a HashMap or some other class implementing the Map interface. 

This data structure is used to store the elements of the HashSet as keys. 

The associated value PRESENT might be a constant placeholder object, often used in custom Set implementations to save memory and indicate the presence of an element without needing to associate a separate value.


2.add(E e) Method:

The method signature indicates that the method accepts an object of type E, suggesting that this is a generic method capable of handling elements of any type.


3.map.put(e, PRESENT) Method Call:

The put method of the map is invoked, attempting to insert the element e as the key and the constant PRESENT as the value. If the key e is not already present in the map, the put method will return null.


4. Return Value Processing:

The returned value from the put method, which is either null or the previous value associated with the key if it already exists, is then checked.


Return Statement:

The method returns true if the returned value from put is null. This indicates that the element e was not previously present in the map, implying that the element was not already in the HashSet and was successfully added. 

On the other hand, if the return value from put is not null, it means the element was already present, and the method returns false, signifying that the element was not added.


This add(E e) method effectively utilizes the put method of the map to ensure the uniqueness of elements in the HashSet. It takes advantage of the behavior of the put method in a Map implementation where it returns null if the key was not previously present. By leveraging this behavior, the method is able to determine whether the element was successfully added to the HashSet without any duplicates.


Example:


import java.util.HashMap;
public class CustomHashSet<E> {
private static final Object PRESENT = new Object(); 
private HashMap<E, Object> map = new HashMap<>(); 
public boolean add(E e) {
return map.put(e, PRESENT) == null; 
 }
public static void main(String[] args) { 
CustomHashSet<String> customSet = new CustomHashSet<>();
 System.out.println(customSet.add("apple")); // true, as "apple" is added
        System.out.println(customSet.add("orange")); // true, as "orange" is added
        System.out.println(customSet.add("apple")); // false, as "apple" is already present 
 }
 }