In this article, we will discuss the difference between Collections and Streams in Java.

Learn and master Java Collections Framework at https://www.javaguides.net/p/java-collections-tutorial.html

Collections vs Streams in Java

Here is the summary of the difference between Collections and Streams in Java:

Let's understand each difference with an example.

1. Collections are used to store and group the data in a particular data structure like List, Set, or Map. Whereas Streams are used to perform complex data processing operations like filtering, matching, mapping, etc on stored data such as arrays, collections, or I/O resources.

That means, collections are mainly about data and streams are mainly about operations on data.

Example 1: List collection is used to store data

  // Creating an ArrayList of String using
     List<String> fruits = new ArrayList<>();
     // Adding new elements to the ArrayList
     fruits.add("Banana");
     fruits.add("Apple");
     fruits.add("mango");
     fruits.add("orange");

Example 2: Streams are used to perform operations like filtering, mapping, collection result, etc:

List<String> lines = Arrays.asList("java", "c", "python");

        List<String> result = lines.stream()       // convert list to stream
                .filter(line -> !"c".equals(line)) // we dont like c
                .collect(Collectors.toList());     // collect the output and convert streams to a List

        result.forEach(System.out::println);

2. You can add to or remove elements from collections. But, you can’t add to or remove elements from streams. Stream consumes a source, performs operations on it, and returns a result. They don’t modify even the source also.

For example:

        // Creating an ArrayList of String using
        List < String > fruits = new ArrayList < > ();
        // Adding new elements to the ArrayList
        fruits.add("Banana");
        fruits.add("Apple");
        fruits.add("Mango");
        fruits.add("Orange");
        fruits.add("Pineapple");
        fruits.add("Grapes");

        System.out.println(fruits);

        // Remove the element at index `5`
        fruits.remove(5);
        System.out.println("After remove(5): " + fruits);

In Streams, there are no such methods to add or remove elements.

3. Streams perform iteration internally behind the scene for us (using the forEach() method). We just have to mention the operations to be performed on a source. On the other hand, we have to do the iteration externally over collections using loops.

Example 1: External iterations of Collections using for loops

// Creating an ArrayList of String using
List < String > fruits = new ArrayList < > ();
// Adding new elements to the ArrayList
fruits.add("Banana");
fruits.add("Apple");
fruits.add("mango");
fruits.add("orange");
fruits.add("Watermelon");
fruits.add("Strawberry");

System.out.println("\n=== Iterate using for loop with index ===");
for (int i = 0; i < fruits.size(); i++) {
    System.out.println(fruits.get(i));
}

System.out.println("=== Iterate using Java 8 forEach and lambda ===");
fruits.forEach(fruit - > {
    System.out.println(fruit);
});

Example 2: Internal iteration of Streams. No more for loops:

List<String> lines = Arrays.asList("java", "c", "python");

        List<String> result = lines.stream()       // convert list to stream
                .filter(line -> !"c".equals(line)) // we dont like c
                .collect(Collectors.toList());     // collect the output and convert streams to a List

        result.forEach(System.out::println);

4. Streams are traversable only once. If you traverse the stream once, it is said to be consumed. To traverse it again, you have to get a new stream from the source again. But, collections can be traversed multiple times.

List<Integer> numbers = Arrays.asList(4, 2, 8, 9, 5, 6, 7);
         
Stream<Integer> numbersGreaterThan5 = numbers.stream().filter(i -> i > 5);
         
//Traversing numbersGreaterThan5 stream first time
         
numbersGreaterThan5.forEach(System.out::println);
         
//Second time traversal will throw error
         
//Error : stream has already been operated upon or closed
         
numbersGreaterThan5.forEach(System.out::println);

5. Collections are eagerly constructed i.e all the elements are computed at the beginning itself. But, streams are lazily constructed i.e intermediate operations are not evaluated until the terminal operation is invoked.