Java Collections Framework – The Collection Interface

In this guide, we will learn about the Java Collection interface of the Java Collections framework. The Collection interface is considered the root interface of the Collection Framework

Important Key Points about Java Collection Interface

Basic Unit of Collection Framework: 

The Collection interface is a member of the Java Collections Framework and serves as the base interface for most of the other collection interfaces. 

Data Manipulation: 

The Collection interface provides methods for basic operations such as add, remove, contains, size, and iterator, which allow you to manipulate and traverse collections. 

Standardized Behavior: 

All the classes that implement the Collection interface will have a set of common methods. 

Extendable: 

The Collection interface extends the Iterable interface, which means the elements of the Collection can be iterated in a sequence. 

Polymorphic Arguments: 

If a method takes the Collection interface as an argument, it means it can accept any object of any class that implements this interface. This makes methods more flexible and scalable. 

Subinterfaces: 

Some of the commonly used subinterfaces of the Collection interface are Set, List, and Queue. These subinterfaces provide more specialized data structures for use in a variety of applications. 

Stream Support: 

From Java 8, the Collection interface has methods to support streams like a stream() and parallelStream() to return sequential or parallel streams respectively. 

Collection Interface Class Diagram

Here is the class diagram of the Collection interface.

Collection Interface Example

The following example demonstrates the usage of important Collection interface methods:

import java.util.ArrayList;
import java.util.Collection;

public class CollectionDemo {

    public static void main(String[] args) {

        Collection <String> fruitCollection = new ArrayList<>();
        fruitCollection.add("banana");
        fruitCollection.add("apple");
        fruitCollection.add("mango");
        System.out.println(fruitCollection);

        fruitCollection.remove("banana");
        System.out.println(fruitCollection);

        System.out.println(fruitCollection.contains("apple"));

        fruitCollection.forEach((element) -> {
            System.out.println(element);
        });

        fruitCollection.clear();

        System.out.println(fruitCollection);
    }
}

Output:

[banana, apple, mango]
[apple, mango]
true
apple
mango
[]

In the above example, we can clearly see that the Collection is an interface and we used it as a reference type:

        Collection <String> fruitCollection = new ArrayList<>();

Collection Interface Important Methods with Examples

boolean add(): Adds a specific element to the collection. Returns true if the collection is modified, false otherwise.

Collection<String> collection = new ArrayList<>();
boolean isAdded = collection.add("Hello");
System.out.println(isAdded);  // Output: true

boolean addAll(Collection<? extends E> c): Adds all elements from a specified collection to the current collection. Returns true if the collection is modified, false otherwise.

Collection<String> collection1 = new ArrayList<>();
Collection<String> collection2 = new ArrayList<>();
collection2.add("Hello");
collection2.add("World");
boolean isAllAdded = collection1.addAll(collection2);
System.out.println(isAllAdded);  // Output: true

void clear(): Removes all elements from the collection.

Collection<String> collection = new ArrayList<>();
collection.add("Hello");
collection.clear();
System.out.println(collection.size());  // Output: 0

boolean contains(Object o): Checks if the collection contains the specified element. Returns true if it does, false otherwise.

Collection<String> collection = new ArrayList<>();
collection.add("Hello");
boolean contains = collection.contains("Hello");
System.out.println(contains);  // Output: true

boolean containsAll(Collection<?> c): Checks if the collection contains all elements from a specified collection. Returns true if it does, false otherwise.

Collection<String> collection1 = new ArrayList<>();
Collection<String> collection2 = new ArrayList<>();
collection2.add("Hello");
collection2.add("World");
collection1.add("Hello");
collection1.add("World");
boolean containsAll = collection1.containsAll(collection2);
System.out.println(containsAll);  // Output: true

boolean isEmpty(): Checks if the collection is empty. Returns true if it is, false otherwise.

Collection<String> collection = new ArrayList<>();
boolean isEmpty = collection.isEmpty();
System.out.println(isEmpty);  // Output: true

Iterator iterator(): Returns an iterator that can be used to traverse the collection.

Collection<String> collection = new ArrayList<>();
collection.add("Hello");
Iterator<String> iterator = collection.iterator();
while(iterator.hasNext()) {
    System.out.println(iterator.next());  // Output: Hello
}

default Stream parallelStream(): Returns a possibly parallel Stream with the collection as its source.

Collection<String> collection = new ArrayList<>();
collection.add("Hello");
collection.parallelStream().forEach(System.out::println);  // Output: Hello

boolean remove(Object o): Removes a single instance of the specified element from the collection, if present. Returns true if the collection is modified, false otherwise.

Collection<String> collection = new ArrayList<>();
collection.add("Hello");
boolean isRemoved = collection.remove("Hello");
System.out.println(isRemoved);  // Output: true

boolean removeAll(Collection<?> c): Removes all elements in the collection that are also contained in the specified collection. Returns true if the collection is modified, false otherwise.

Collection<String> collection1 = new ArrayList<>();
Collection<String> collection2 = new ArrayList<>();
collection2.add("Hello");
collection1.add("Hello");
collection1.add("World");
boolean isAllRemoved = collection1.removeAll(collection2);
System.out.println(isAllRemoved);  // Output: true

boolean retainAll(Collection<?> c): Retains only the elements in the collection that are also contained in the specified collection. Returns true if the collection is modified, false otherwise.

Collection<String> collection1 = new ArrayList<>();
Collection<String> collection2 = new ArrayList<>();
collection2.add("Hello");
collection1.add("Hello");
collection1.add("World");
boolean isRetained = collection1.retainAll(collection2);
System.out.println(isRetained);  // Output: true

int size(): Returns the number of elements in the collection.

Collection<String> collection = new ArrayList<>();
collection.add("Hello");
int size = collection.size();
System.out.println(size);  // Output: 1

default Spliterator spliterator(): Returns a Spliterator over the elements in the collection.

Collection<String> collection = new ArrayList<>();
collection.add("Hello");
Spliterator<String> spliterator = collection.spliterator();
spliterator.forEachRemaining(System.out::println);  // Output: Hello

default Stream stream(): Returns a sequential Stream with the collection as its source.

Collection<String> collection = new ArrayList<>();
collection.add("Hello");
collection.stream().forEach(System.out::println);  // Output: Hello

What's next?

In this guide, we have learned all about the Collection interface, its methods, and the usage of a few common methods with examples.

In the next guide, we will learn about the List interface of the Java collections framework.

Related Guides

• Collections Framework - The Collection Interface
• Collections Framework - The Set Interface
• Collections Framework - The SortedSet Interface
• Collections Framework - The List Interface
• Collections Framework - The Queue Interface
• Collections Framework - The Deque Interface
• Collections Framework - The Map Interface
• Collections Framework - The SortedMap Interface