Java 8 stream and c# 3.5

Java 8 Stream & C# 3.5
Trần Duy Quang – May 2014

Agenda
1. Some notable new things in Java 8
2. Comparison with LINQ in C# 3.5!
2

Notable things
 Lambda expressions
 Default methods
 Method reference
 Stream
 Optional
 New Date & Time API
 Functional programming
 Reference (blog post): Everything about Java 8
4

Lambda expression
5
(x, y) -> x + y

Passing function around
 Dynamically (usually weakly) typed: Javascript
 Strongly typed: Ruby, Scala, Closure
6
Concise Useful
Paralel
lizable
Functional approach

Why lambda in Java now?
 Clearer than anonymous inner class
 Basic for stream library
 shapes.forEach(s -> s.setColor(Color.RED));
 A step toward functional programming !
7

Main advantage of lambdas
 Concise & expressive
8

New way of thinking
 Encourage functional programming
 Many classes of problems are easier to solve
 Code is clearer to read, simpler to maintain
9

What we actually get?
 An instance of a class that implements the
interface that was expected in that place
 Remember this?
 Interface that has exactly one abstract method!
 Functional Interface | Single Abstract Method
10

Shortening lambda syntax
 Omit parameter types
 Expressions instead of blocks
 Single arguments? Omit parenthesis!
11

Even better with high-order function
 High-order function: method return lambdas
 comparing method of Comparator need function
specifying how to extract the key
12

Core idea behind
 Make lambda more succinct!
 Rather than
 angles.map(x -> Math.sin(x));
 Can be shorthand
 angles.map(Math::sin);
14

Core idea behind
 Boolean test(T t)
 A function to test condition
16

Benefit: Flexibility
 Even better with stream (later slides)
17

Similar things
 Predicate: boolean test(T t)
 Check a condition
 Consumer: void consume(T t)
 Perform action with the given object
 BiConsumer: with two parameters
 Function: R change(T t)
 Take an object type T and return new object type R
 BiFunction: with two parameters
 Supplier: T supply(T t)
 Return an object with the same type
18

More convenient helper classes
 IntConsumer
 IntFunction<R>
 IntPredicate
 IntSupplier
19

Stream
 Wrapper around data sources: arrays, collections…
 Use lambdas
 Support map/reduce
 Lazy evaluation
 Made parallel atutomatically by compiler
21

Making streams
 From individual values
 Stream.of(val1, val2,…)
 From array
 Stream.of(names), Arrays.stream(names)
 From List (and other collections)
 names.stream()
 From a StreamBuilder
 builder.build()
 From String
 String.chars, Arrays.stream(s.split())
 From another stream
 distinct, filter, map, limit, sorted, substream
22

Turn stream to other data structures
 Array
 employees.toArray(Employee[]::new);
 List
 names.collect(Collectors.toList());
 Set
 names.collect(Collectors.toSet());
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1. Filtering
 Find names where “am” occurs
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2. Indexed Filtering (tricky)
 Find names where length <= index + 1
(generate an indexed stream out of the original array)
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3. Selecting/Mapping
 Add “Hello “ in front of each names
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4. Selecting Many/Flattening
 Project all the elements in a single collection
 Java: Transform to entry set, then flatten
30

5. Taking an Arbitrary Number of Items
 Obtain the first 4 items
 Java: convert IntStream into Stream<Integer>
32

6. Taking Items Based on Predicate
 Take while having the string that start with “S”
 Java: don’t have the short-circuited ability, have to
create Boolean map
33
Different meaning from the above!

7. Skipping an Arbitrary Number of Items
 Skip top items, take the rest
 Java:
34

8. Skipping Items Based on Predicate
 LINQ: SkipWhile
 Sadly, no way in Java 
35

9. Ordering/Sorting Elements
 Order the elements of a collection alphabetically
 Java:
37

10. Ordering/Sorting Elements by
Specific Criterium
 Ordering by the length of the string
 Java
 Shorthand:
38

11. Ordering/Sorting Elements by
Multiple Criteria
 Sort by length, then by order
 Java:
39

12.Grouping by a Criterium
 Group collection of strings by their length
41

13. Filter Distinct Elements
 Obtain all the distinct elements from a collection
43

14. Union of Two Sets
 Join together two sets of items
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15. First Element
 Obtain the first element of a collection
 Java: Maybe better!
46

16. Generate a Range of Numbers
 Generate a range of no that are multiples of 11
48

17. All
 Do all elements in a collection satisfy a predicate?
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18. Any
 Do any elements in a collection satisfy a predicate?
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19.Zip
 Combine two collections into a single collection
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Still left behind by C#! Gambatte, Java!

Make IntelliJ IDEA work with Java 8
 Make sure you have the latest version (>=13.1.2)
 Change project language level to 8.0 (F4)
56

Reference
 Blog post: Java Streams Preview vs .Net High-
Order Programming with LINQ
 Slide: Evolution of Java
 Slide: Lambda expressions & Stream in Java 8
 Slide: Java 8 Stream Tutorial Part 1
 Slide: Java 8 Stream Tutorial part 2
 Just for fun (Youtube Video 20”):
 Javapocalypse ^^
57

Java 8 stream and c# 3.5

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