Java Collections Tutorial 5: Sets

A set is a member of the Java collections framework that only stores unique elements. So members of a Set implement the Set interface. Here’s how to declare a set:

HashSet

Set<String> nameSet = new HashSet<String>();

We can add several elements to the set using the add() method like this:

nameSet.add(“Veronicah”);

nameSet.add(“Brian”);

nameSet.add(“Tyrese”);

nameSet.add(“Gibson”);

nameSet.add(“Anthony”);

Now, if we want to output the contents of the set, we can just use System.out.println(nameSet); because sets have a toString() method. Your output will be as shown in the figure below.

Figure 1: Adding items to a set

Remember we said that sets only store unique elements. So what happens when we try to add duplicate items like this:

nameSet.add(“Brian”);

 Figure 2: Trying to add a duplicate item to a set

Well, it does nothing. Java will only output the unique elements. This means that instead of outputting two elements “Brian”, it will only output one “Brian.”

HashSet doesn’t remember any kind of order. Therefore, HashSets will rearrange themselves randomly from time to time. HashSets are, therefore, not ideal when you want some order in the elements added to it.

LinkedHashSet

Use a LinkedHashSet if you want items put in a particular order. With a LinkedHashSet, you have unique elements placed in a specific order. A LinkedHashSet is just like a HashSet, only that it has a doubly-linked list between all the elements.

Set<String> nameSet = new LinkedHashSet<String>();

We can add several elements to the set using the add() method like this:

nameSet.add(“Veronicah”);

nameSet.add(“Brian”);

nameSet.add(“Tyrese”);

nameSet.add(“Gibson”);

nameSet.add(“Anthony”);

Now, if we want to output the contents of the set, we can just use System.out.println(nameSet); because sets have a toString() method. Your output will be as shown in the figure below.

Remember we said that sets only store unique elements. So what happens when we try to add duplicate items like this:

nameSet.add(“Brian”);

Well, it does nothing. Java will only output the unique elements. This means that instead of outputting two elements “Brian”, it will only output one “Brian.”

 Figure 3: Adding items to a LinkedHashSet, adding duplicate items to a LinkedHashSet

TreeSet

If you want your set to be ordered in natural order, use a TreeSet. What a TreeSet does, is that it allows you to store unique elements, but in a natural order. 

Set<String> nameSet = new TreeSet<String>();

We can add several elements to the set using the add() method like this:

nameSet.add(“Veronicah”);

nameSet.add(“Brian”);

nameSet.add(“Tyrese”);

nameSet.add(“Gibson”);

nameSet.add(“Anthony”);

Now, if we want to output the contents of the set, we can just use System.out.println(nameSet); because sets have a toString() method. Your output will be as shown in the figure below.

Remember we said that sets only store unique elements. So what happens when we try to add duplicate items like this:

nameSet.add(“Brian”);

Well, it does nothing. Java will only output the unique elements. This means that instead of outputting two elements “Brian”, it will only output one “Brian.”

As you can see in the figure below, the TreeSet outputs unique items, but in an ordered manner, which in this case is an ascending numerical order.

 Figure 4: A TreeSet displays unique items in their natural order

Iterating through Sets

You can iterate through any set using a foreach loop like this:

for(String name: nameSet) {

            System.out.println(name);

}

 Figure 5: Iterating through sets using a foreach loop

Finding out whether a Set has a Particular Item

Sets are optimized for quickly finding whether an item exists. We use the contains() method where we place the element we are searching for in the parentheses. So let’s say we are looking to see whether the name “Brian” is actually in the set:

if(nameSet.contains(“Brian”) ){

            System.out.println (“nameSet contains “Brian”);

}

Figure 6: Finding out if an item exists in a set

If the item is indeed there, then the above sentence will be output in the console. Otherwise, nothing will be printed.

Finding out if a Set is Empty

If you want to find out if a set is empty, you can use the isEmpty() method. 

if (nameSet.isEmpty()) {

            System.out.println (“nameSet is empty.”);

}

If the set is indeed empty, then the above text will be output to the console. Otherwise, nothing will be displayed.

Figure 7: Finding out if a set is empty

Intersection of Sets

An intersection refers to the element or elements in two sets that are equal. In other words, it is where the two sets meet. So to demonstrate this, we are going to create another set.

Set<String> nameSet1 = new TreeSet<String>();

We can add several elements to the set using the add() method like this:

nameSet1.add(“Veronicah”);

nameSet1.add(“Dan”);

nameSet1.add(“Bob”);

nameSet1.add(“Mark”);

nameSet1.add(“Bean”);

The only element that is common in both nameSet and nameSet1 is “Veronicah”. 

It is advisable that before checking whether two sets have an intersection or not, you create a copy of one of the sets to avoid messing up your two sets. So what we can do is create a new HashSet and in its constructor, pass in the first set, i.e. nameSet1 like this:

Set<String> intersection = new HashSet<String>(nameSet);

So what we do in the above line is create a new HashSet and make intersection a copy of nameSet. 

Now, we want to find out which elements are in the set labeled “intersection” that are also in nameSet1. We use the retainAll() method. So we can say:

intersection.retainAll(nameSet1);

So we can surround the above statement with a System.out.println() to output the result to the console.

System.out.println(intersection.retainAll(nameSet1));

The results are as shown in the figure below.

 Figure 8: Finding out if two sets have common items, i.e. intersection

Finding the Difference between Two Sets

A difference refers to the element or elements in one set that are not in the other set. In other words, it is where the two sets meet. So to demonstrate this, we are going to create another set.

Set<String> nameSet1 = new TreeSet<String>();

We can add several elements to the set using the add() method like this:

nameSet1.add(“Veronicah”);

nameSet1.add(“Dan”);

nameSet1.add(“Bob”);

nameSet1.add(“Mark”);

nameSet1.add(“Bean”);

The only element that is common in both nameSet and nameSet1 is “Veronicah”. 

It is advisable that before checking whether two sets have an difference or not, you create a copy of one of the sets to avoid messing up your two sets. So what we can do is create a new HashSet and in its constructor, pass in the first set, i.e. nameSet1 like this:

Set<String> difference = new HashSet<String>(nameSet);

So what we do in the above line is create a new HashSet and make difference a copy of nameSet. 

Now, we want to find out which elements are in the set labeled “difference” that are also in nameSet1. We use the removeAll() method. So we can say:

difference.removeAll(nameSet1);

So we can surround the above statement with a System.out.println() to output the result to the console.

System.out.println(difference.removeAll(nameSet1));

The results are as shown in the figure below.

Figure 9: Finding the difference between two sets

In the next tutorial, we are going to look at how to use your own objects in sets and as keys in HashMap, which will require us to implement the hashCode() and equals() methods.

If you have any questions or comments, please post them in the comments section below and I will address them. 

Until next time, take care.