Basics of Streams In Java8

Streams are introduced in java 8. It is defined in package. A stream is sequence of objects which supports various methods. That can be pipelined to produce the result.

Features of Stream

  • Stream is not a data structure. It takes input from various sources (like array, collection, data structure) performs an operation and gives output.
  • It does not store element.
  • It does not change the original data.
  • The elements of streams can be visited only once like an iterator, if you want to view them again then you have to create a new stream.
  • It support different operations like filter, map, reduce, limit etc.

A stream can be composed of multiple operations/functions that create a pipeline of data that flows. That is, the original data structure doesn’t change.

The stream is a pipeline of functions or operations. These operations can be classified as intermediate operations or terminal operations.

The difference between these two is in the output-

  • If an operation results into another stream, to which you can apply another operation, then that operation is called as intermediate operation.
  • If an operation results into a concrete type, it is called terminal operation.

Intermediate Operation

Intermediate operations return stream which allows you to call multiple operations in a form of a query. Intermediate operations execute after terminal operations are invoked. Intermediate operations are lazy operations.

Lets look at some popular intermediate operations used in stream:-

  • filter: The filter operation returns a stream of elements that satisfy the predicate passed in as a parameter to the operation.
List names = Arrays.asList("John","Sam","Jordan","George").stream().filter(s->s.startsWith("S")).collect(Collectors.toList());

In the above example the stream will perform the filter operation on list and gives the sorted list in the end.


  • map: The map operation returns a stream of the element after they have processed by the function passed in as a parameter.
List number = Arrays.asList(2,3,4,5).stream().map(s->s+s).collect(Collectors.toList());

In the above line of code, the map will perform an operation which is passed to it that is addition like 2+2=4, 3+3=6.


  • sorted : The sorted method is used to sort the stream.
List num = Arrays.asList(7,5,9,2).stream().sorted().collect(Collectors.toList());

In above example, stream will perform the sort operation and gives the list in sorted order.


Terminal Operations:

Terminal operations are preceded by intermediate operations which return another stream which allows operations to be connected in a form of a query.

  • collect : The Stream.collect terminal operation will convert the stream into some other container such as a list.
Set stringSet = Stream.of("hello","world").collect(Collectors.toSet());

In the above example, it will convert the stream into set.

  • foreach: The Stream.forEach method will perform an action for each element in the stream. It is a simplified inline way to write a for loop. 
Stream.of("Hello", "World").forEach(str -> System.out.println(str));

In above example, foreach is used to iterate, it will go through each element.

  • reduce: Stream.reduce operations are commonly found in statistic operations such as long summary statistics and combine the stream elements into one using a Binary Operator.
int sum = IntStream.of(1, 2, 3, 4).reduce(0, (a, b) -> a + b);

In above example, stream will be reduced into int.



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