Set

A Set is an interface provided in the java.util package that implements collections with zero duplicate elements. Some implementations will have restrictions on the elements the Set can contain, such as excluding null elements. Adding an ineligible element will typically throw an unchecked NullPointerException or a ClassCastException.

Examples of classes implementing the Set interface are HashSet and TreeSet.

Syntax

SetClass<DataType> mySet = new SetClass<DataType> ();

The SetClass is a class that implements the Set interface, and is initialized with a generic type inside angle brackets < ... >.

Methods

The Set interface utilizes the following methods:

  • .add() : .add(item) adds item to the Set if it is not already present.
  • .clear() : Removes all the elements from the Set
  • .contains() : .contains(item) returns true if item is a member of the Set.
  • .remove() : .remove(item) will remove item from the Set.
  • .size() : Returns the number of items in the Set.

Example

This is an example of a Set interface being implemented by a HashSet:

import java.util.HashSet;
import java.util.Set;
public class Main {
public static void main(String[] args) {
Set<String> food = new HashSet<String>();
food.add("Cabbage");
food.add("Pizza");
food.add("Sausage");
food.add("Potatoes");
food.add("Salad");
food.remove("Sausage");
System.out.println(food.contains("Sausage"));
System.out.println(food);
System.out.println(food.size());
}
}

This will output the following:

false
[Pizza, Potatoes, Cabbage, Salad]
4
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