Java .addAll()

The .addAll() method is declared in the Set interface and is implemented in the HashSet class. It is used to add all of the elements in the specified collection to this set if they’re not already present. The function returns a boolean value true if the elements are added successfully, and false if not.

Syntax

The .addAll() method can be called on a HashSet instance and it requires one parameter, the collection which we need to add in the set:

hashSet.addAll(Collection<? extends E> c);

Collection<? extends E> c : Here c is a collection of type ? extends E. The method can take any collection of elements that are a subtype of E. This means that you can add elements of a subclass of E to the Set, but you cannot add elements of a superclass of E.

Example

In the example below, the .addAll() method is used to add elements of shoppingList1 and shoppingList2 in the combinedShoppingList set:

import java.util.HashSet;
import java.util.Set;

public class GroceryShopping {
  public static void main(String[] args) {

    // Creating two sets representing grocery shopping lists
    Set<String> shoppingList1 = new HashSet<>();
    Set<String> shoppingList2 = new HashSet<>();

     // Adding items to the first shopping list
    shoppingList1.add("Apples");
    shoppingList1.add("Bananas");
    shoppingList1.add("Milk");

    // Adding items to the second shopping list, including a common item with shoppingList1
    shoppingList2.add("Grapes");
    shoppingList2.add("Bananas"); // Common item
    shoppingList2.add("Bread");

    // Creating a new set to combine items from both shopping lists
    Set<String> combinedShoppingList = new HashSet<>();

    // Using addAll() to combine items from shoppingList1 and shoppingList2
    combinedShoppingList.addAll(shoppingList1);
    combinedShoppingList.addAll(shoppingList2);

    // Displaying the contents of the combined shopping list
    System.out.println("Combined Shopping List: " + combinedShoppingList);
  }
}

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The above example will give the following output:

Combined Shopping List: [Bread, Grapes, Bananas, Apples, Milk]

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In the above example, both sets contain Bananas, however, the combinedShoppingList set contains it only once.