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Introduction to Java 8: Lambda Expressions and Streams

- Ways of representing functions
- pre-built function building blocks
- Set of function composition methods

 - Wrappers around collections
 - map, reduce, filter, forEach
 - Lazy Evaluation
 - Can be made parallel

Lambda is not lisp
Stream is not Hadoop

Why Lambda's in Java
  Ability to pass functions around (Javascript, Lisp, Ruby, Scala etc)
  Functional approach proven concise, useful, and parallelizable

Java 7 version
String[] testStrings = {"", "", ""};
Arrays.sort(testStrings, new Comparator() {
   public int compare(String s1, String s2) {
      return (s1.length() - s2.length());

Lambda version
Arrays.sort(testStrings, (s1, s2) -> s1.length() - s2.length());

 - replaces anonymous inner class' single method
 - No type should be used (Type inferencing)
 - Parens optional for single arg (Parens omitting)
     button.addActionListener(e -> setBg(Color.RED));
 - can refer to local, instance variable
 - are not allowed to modify local variables
 - allowed to modify instance variables
 - this is outer class

  - tells compiler is a functional interface
  - only one abstract method
  - other's won't be able to add more methods, compile time check

Method References
 - E.g., Math::cos or myVar::myMethod


Stream methods
 forEach, map, filter, findFirst, findAny

how to make stream
 - Stream.of(val, val)
 - don't use int use wrappers Integer array

  - strm.toArray(EntryType[]::new)
  - strm.collect(Collectors.toList())
  - strm.collect(Collectors.joining(delim)).toString()


String[] arr = {"a", "b", "c"};

Double[] nums = {1.0, 2.0, 3.0, 4.0, 5.0};
        Double[] squars = Stream.of(nums).map(n -> n * n).toArray(Double[]::new);

Double[] nums1 = {1.0, 2.0, 3.0, 4.0, 5.0};
        Double[] squars1 = Stream.of(nums).filter(n -> n % 2 == 0).toArray(Double[]::new);

Optional squars2 = Stream.of(nums).filter(n -> n % 2 == 0)
                .filter(n -> n > 10).findFirst();

System.out.println(Stream.of(arr).reduce("", String::concat));
System.out.println(Stream.of(nums).reduce(1.0, (n1, n2) -> n1 * n2));

comma, space delimiter.
StringJoiner j = new StringJoiner(", ");
j.add("").add("") etc

Limiting Stream Size
limit(n) - returns a Stream of first n elements
substream(n) - starts with nth element

Comparisons - sorted, min, max, distinct

- Sorting by salary,filter(..).limit(..)
   .sorted( (e1, e2) -> e1.getSalary() - e2.getSalary())

- Richest employee,filter(..).limit(..)
   .max( (e1, e2) -> e1.getSalary() - e2.getSalary()).get()

Checking Matches - allMatch, anyMatch, noneMatch

Predicate, Function & BiFunction, Consumer, Supplier and binaryOperator


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