Streams API

The Streams API creates a sequence of items and iterates through data collections by collapsing iterations into more efficient and readable code. 

There are multiple ways to create a stream instance from a collection type and once created, the instance will not modify it’s source, enabling the creation of multiple instances from a single data source.

Create an empty stream

The empty() method is used when creating an empty stream to avoid returning null on a stream that has no elements.

Stream<String> emptyStream = Stream.empty();

Create a stream

Stream<String> groceries = Stream.of("apples","bananas","coffee","milk");

Create a stream from other collection types

Arrays

String[] groceries = new String[] {
    "apples",
    "bananas",
    "coffee",
    "milk"
};

Stream<String> groceryStream = Arrays.stream(groceries);

Lists

List<String> groceryList = List.of("apples","bananas","coffee","milk");
Stream<String> groceryListStream = groceryList.stream();

//print the list sequentially
groceryList.stream().forEach(System.out::println);

//print the list in parallel
groceryList.parallelstream().forEach(System.out::println);

//Using lambdas to print each element in the stream

//sequential stream
shoppingList.stream().forEach(s -> System.out.println(s));

//parallel stream
shoppingList.parallelStream().forEach(s -> System.out.println(s));

//Match
//Using a predicate, we can check if all, none, or any of the elements in the stream match a given condition.
boolean anyMatch = shoppingList.stream().anyMatch(s -> s.equals("Apple"));

//Filter
//Create a new stream with only the elements that match a given condition.
Stream<String> itemInAisle = shoppingList.stream()
     .filter(item -> item.startsWith("A"));

//Map
//Create a new stream with the results of applying a function to all elements of the stream.
Stream<String> upperCaseStream = shoppingList.stream()
      .map(item -> item.toUpperCase());

//Map
//Create a new stream with the results of applying a function to all elements of the stream.
List<Integer> numbers = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
Stream<Integer> doubleStream = numbers.stream()
      .map(n -> n +n );

//Collect the elements of the stream into a collection.
List<Integer> doubledList = numbers.stream()
      .map(n -> n * 2 )
      .collect(Collectors.toList());

The .stream().forEach() replaces a for loop in a single line of code and is used to perform an action on each element in the stream.

takeWhile() & doWhile()

//takeWhile
//Create a new stream with the elements that match a given condition.
Stream<Integer> takeWhileStream = numbers.stream()
      .takeWhile(n -> n < 5);
        
//dropWhile
//Create a new stream with the elements that don't match a given condition.
Stream<Integer> dropWhileStream = numbers.stream()
      .dropWhile(n -> n < 5);