Java Set

Overview

The Set interface is present in java.util package and extends the Collection interface is an unordered collection of objects in which duplicate values cannot be stored. A Set is a Collection that cannot contain duplicate elements. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.

For official documentation on Set please refer oracle docs

Java Set vs List

• the same element cannot occur more than once in a Java Set. This is different from a Java List where each element can occur more than once.
• The elements in a Set has no guaranteed internal order. The elements in a List has an internal order, and the elements can be iterated in that order.

Java Set Example

Here is the example program for creating a Set

package com.techijournal.sets;

import java.util.HashSet;
import java.util.Set;
import java.util.TreeSet;
public class SetExample {
	public static void main(String[] args) {
		int count[] = { 5, 10, 15, 60, 65, 70 };
		Set<Integer> set = new HashSet<Integer>();
		try {
			for (int i = 0; i < 5; i++) {
				set.add(count[i]);
			}
			System.out.println(set);
			TreeSet sortedSet = new TreeSet<Integer>(set);
			System.out.println("The sorted list is:");
			System.out.println(sortedSet);
			System.out.println("The First element of the set is: " + (Integer) sortedSet.first());
			System.out.println("The last element of the set is: " + (Integer) sortedSet.last());
		} catch (Exception e) {}
	}
}

Output
[65, 5, 10, 60, 15]
The sorted list is:
[5, 10, 15, 60, 65]
The First element of the set is: 5
The last element of the set is: 65

Methods & Description

Set Implementations

Being a Collection subtype all methods in the Collection interface are also available in the Set interface.

Since Set is an interface you need to instantiate a concrete implementation of the interface in order to use it. You can choose between the following Set implementations in the Java Collections API:

• java.util.EnumSet
• java.util.HashSet
• java.util.LinkedHashSet
• java.util.TreeSet

Each of these Set implementations behaves a little differently with respect to the order of the elements when iterating the Set, and the time (big O notation) it takes to insert and access elements in the sets.

Add Element to Set

To add elements to a Set, call its add() method. This method is inherited from the Collection interface. Here are a few examples:

Set<String> setA = new HashSet<>();
setA.add("TechiJournal 1");
setA.add("TechiJournal 2");
setA.add("TechiJournal 3");

Iterate Set Using Iterator

To iterate the elements of a Set using an Java Iterator, you must first obtain an Iterator from the Set. You obtain an Iterator from a Set by calling the iterator() method. Here is an example of obtaining an Iterator from a Set:

Set<String> techSet = new HashSet<>();
techSet.add("TechiJournal 1");
techSet.add("TechiJournal 2");
techSet.add("TechiJournal 3");
Iterator<String> iterator = set.iterator();
while(iterator.hasNext(){
  String element = iterator.next();
}

Iterate Set Using the Java Stream API

To iterate a Java Set using the Java Stream API you must create a Stream from the Set. Here is an example of creating a Java Stream from a Set and iterate the Stream:

Set<String> set = new HashSet<>();
set.add("abc");
set.add("cde");
set.add("efg");
Stream<String> stream = set.stream();
stream.forEach((element) -> { System.out.println(element); });

Remove Elements From Set

Elements from a Java Set can be removed by calling the remove(Object o)method. Here is an example of removing an element from a Java Set:

set.remove("object-to-remove");

Remove All Elements From Set

All the elements from a Java Set can be removed using the clear() method which is shown below

set.clear();

Convert Java Set to List

Java Set can be converted to a Java List by creating a List and calling its addAll() method, passing the Set as parameter to the addAll() method. Here is an example of converting a Java Set to a List:

Set<String> set = new HashSet<>();
set.add("123");
set.add("456");
List<String> list = new ArrayList<>();
list.addAll(set);

Conclusion

There is a lot more you can do with a Set which will be discussed in further posts. A Set is a Collection that cannot contain duplicate elements.

Thanks, Note

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