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Java HashSet Class & Set Interface

1. Java hashSet and Set

Before we learn about Java hashSet class, we will learn about the Set interface. Because the HashSet class implements Java Collection’s Set interface. The Set Interface ensures the implementing class to hold uniqueness in the collection. Means, it does not allow duplicate items. In Java Collection framework, the below classes implement the Set interface:

  1. HashSet
  2. EnumSet
  3. TreeSet

There were other classes as well. But here we listed frequently used ones. In this example, we will explore about the HashSet collection class.

2. Set Interface Methods

Since hashSet signs the contract for the Set interface, it supports the methods exposed by the Set interface. Below table shows commonly used Set Interface methods and its description:

MethodDescription
boolean add(E e)Adds Element E to the Set. The interface does not allow adding a duplicate element
boolean remove(E e)Removes an Element E from the Set.
void clear()Removes all the elements. Keeps the Set Empty.
boolean isEmpty()Tells the Set is empty or not.
int size()Returns number of elements in the Set.
boolean contains(Object o)Searches the Unique elements in the Set and returns true when ‘o’ is found.
Iterator<E> iteratorReturns an iterator to iterate over unique elements present in this set.
Java Set Interface Frequently Methods

3. Java Collection Class – hashSet

The hashSet class in Java Collection implements both Collection & Set Interfaces. Java hashSet does not keep insertion order. Say, for example, if we insert Item1, Item2 and Item3 into HashSet, the hashSet does not assure that it will store the items in the same insertion order. The HashSet operates on Hash Table, which we will study later in more details. For now, you can imagine a hash table as Buckets with a collection of items in it. And Java’s algorithm makes sure the items are spread evenly among these buckets by using the hashCode function. 

The items stored in the hashSet collection must override the hashCode and equals methods. This is pointed out in the below two videos.

Comparing Two Objects

Override hashCode Method

4. Java HashSet Example

In our previous example, we used the Product Class instance as the collection item. Here also we will use the same product class. Refer the code for the product class in the Java ArrayList Example.

4.1 Override equals Method

The hashSet class works perfectly when we override the equals and hashCode methods. First, let us override the equals method. We must override this method on the Product class as we store the instance of it in the hashSet. Now have a look at the override below:

The method returns true when the current instance’s ProdId matches with the passed-in ProdId. The hashSet class will use this method to know how to compare two objects. Now hashSet class says two products are equals when their product IDs match. Note, our implementation does not compare the product names.

4.2 Override hashCode Method

Our Product class needs one more override. It is hashCode method. Java’s hashing algorithm depends heavily on this and we will learn more about this hashing later. For now, we will see how to override this method for our Product class. Have a look at the below code:

Java recommends overriding the hashCode method whenever you override the equals method. In the past section we already overrode the equals method for the Product class. The equal method involves only the ProdId for the comparison. So, our hashCode generation method also should depend only on the Product ID. Since, we are using the Java 8, it is easy to set up the hashCode. In the above code at line 1, we call the hashCode static method on the Objects class and pass only the ProdId to make a hash code out of it. Now our product class is eligible for any hashing-based collection class like hashSet.

4.3 Create hashSet and Add Items

We set our Product class with required overrides. Now, we will create the instances of it and add to the hashSet. Below is the code snippet:

At line 4, we create an instance of the hashSet and hold its reference as Set type. Note, hashSet implements Set interface. Next, we create six products and add it to the hashSet using the add method. Finally, we print the content of the hashSet using the custom function printProducts. We will write it in the next section.

4.4 Print hashSet

The code below iterates the hashSet of products and print each product in the console window. Here, we use the for each construct to list the products in the console output window.

4.5 Test Adding Duplicate Items to Java hashSet

Remember, the Set interface implementation does not allow duplicates. In this section, we will try to add duplicate products and see what happens. Have a look at the below code:

At line 2,3 we create two new products in the heap. Our hashSet already contains a product [104, Writing Pad]. So, adding the new product p7 is not allowed. But hashSet above does not allow adding the product p8 also, even though the name is not Writing Pad.

Since hashSet need to maintain unique products, it checks for duplicate before adding any new product. It uses our equals override to know the products are same. Our equals override compares only Product ID. This is how product p8 is rejected by the hashSet as its id is 104 and it already exists in it. You can look at the console window to see how it invokes the overrides.

4.6 Java hashSet remove Method

Does removing a product require any comparison? The code snippet is below:

In the above code, we try to remove product p3 two times. We call remove method of the hashSet and pass a Product instance to remove. The hashSet class compares the collection items with the passed-in parameter. When there is a match, it removes that specific item. So, the remove method of the hashSet also depends on our equals method override. In the above code, remove method call at Line No 8 fails.

4.7 The contains Method Of Java hashSet

The contains method of the hashSet is used to check whether a specific product exists in the collection. The method returns true when the item exists in the collection. In our code below, we check product p2 and p5 exists in the collection. Since we removed p5 in the previous section, the second call to the contains method will fail.

5. Complete Code Example – Java hashSet

5.1 Modified Product Class

5.2 The HashSetTest Class

6. Watch hashSet Example as YouTube

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