2.3.1 HashSet Explained

The HashSet class in Java is a part of the Java Collections Framework and implements the Set interface. It is used to store a collection of unique elements and provides high-performance set operations. Understanding HashSet is crucial for managing unique data efficiently in Java SE 11 applications.

Key Concepts

1. Unique Elements

A HashSet does not allow duplicate elements. If you try to add a duplicate element, it will not be added to the set.

Example

        Set<String> hashSet = new HashSet<>();
        hashSet.add("Apple");
        hashSet.add("Banana");
        hashSet.add("Apple"); // Duplicate, will not be added
        System.out.println(hashSet); // Output: [Apple, Banana]
    

2. No Order Guarantee

A HashSet does not guarantee the order of elements. The elements are stored based on their hash code, which means the order in which elements are added may not be preserved.

Example

        Set<String> hashSet = new HashSet<>();
        hashSet.add("Dog");
        hashSet.add("Cat");
        hashSet.add("Bird");
        System.out.println(hashSet); // Output: [Cat, Dog, Bird] or any other order
    

3. Performance

A HashSet provides constant-time performance for basic operations like add, remove, and contains. This makes it highly efficient for large datasets.

Example

        Set<Integer> hashSet = new HashSet<>();
        hashSet.add(10);
        hashSet.add(20);
        hashSet.remove(10);
        System.out.println(hashSet.contains(20)); // Output: true
    

4. Null Elements

A HashSet allows the inclusion of a single null element. This can be useful in scenarios where you need to store optional data.

Example

        Set<String> hashSet = new HashSet<>();
        hashSet.add(null);
        hashSet.add("Apple");
        System.out.println(hashSet); // Output: [null, Apple]
    

Examples and Analogies

Think of a HashSet as a bag that can only hold one of each type of item. For example, if you have a bag of fruits, you can put an apple in it, but if you try to put another apple, it won't fit because the bag already has an apple. The order in which you put the fruits in the bag is not important, and the bag can also hold a single special item, like a golden coin (representing null).

By mastering HashSet, you can efficiently manage unique collections of data in your Java SE 11 applications, ensuring that each element is represented only once and optimizing performance for common operations.