Java8 Streams

Streams are functional programming design pattern for processing of elements of a data structure sequentially or parallely.

Example:

 
List<Order> orders = getOrders();
int qtySum = orders.stream()
                .filter(o -> o.getType().equals("ONLINE"))
                .mapToInt(o -> o.getQuantity())
                .sum();

Above code can be explained as below:

• Create a stream from source java collection
• Add a filter operation to the stream “intermediate operations pipeline”
• Add a map operation to the stream “intermediate operations pipeline”
• Add a terminal operation that kicks off the stream processing

A Stream has

• Source: that stream can pull objects from
• Pipeline: of operations that can execute on the elements of the stream
• Terminal: operation that pull values down the stream

Streams are lazily evaluated and the stream lifecycle is

• Creation: from source
• Configuration: from collection of pipeline operations
• Execution: (terminal operation is invoked)
• Cleanup

Few java stream sources

 
// Number Stream
LongStream.range(0, 5).forEach(System.out::println)

// Collection Streams
List<String> cities = Arrays.asList("nyc", "edison", "livingston");
cities.stream().forEach(System.out::println)

// Character Srream
int cnt = "ABC".chars().count()

// File Streams
Path filePath = new Path("/user/ntallapa/test");
Files.list(filePath).forEach(System.out::println);

Stream Terminal Operations

• Reduction Terminal Operations: results in single result
  • reduce(f(x)), sum(f(x)), min(f(x)), max(f(x)), etc
• Mutable Reduction Terminal Operations: returns multiple results in a container data structure
  • List integers = Arrays.asList(1,2,3,3);
  • Set integersSet = integers.stream().collect(Collectors.toSet()); // returns 1,2,3
• Search Terminal Operations: returns result as soon as match is found
  • findFirst(), findAny(), anyMatch(f(x))
• Generic Terminal Operations: do any kind of processing on every element

Stream Pipeline Rule: Since streams are meant to process elements sequentially/parallely, stream source is not allowed to modify

Intermediate Pipeline operations can be of two types

• Stateless: filter(f(x)), summaryStatistics()
• Statefull: distinct(), limit(n), skip(n)

References:
https://www.youtube.com/watch?v=MLksirK9nnE
https://www.youtube.com/watch?v=8pDm_kH4YKY