Introduction to Java 8

Introduction to Java 8Introduction to Java 8
Manish Mishra,
Software Consultant,
Knoldus Software, LLP
Manish Mishra,
Software Consultant,
Knoldus Software, LLP

Agenda
● Lambda Expressions
● Functional Interfaces
● Default Methods
● Method References
● Streams API
● Why Streams?
● Operations on Streams

What is Lambda
Lambda is an anonymous function which can
be passed around in a concise way.

Lambda Expressions: An example
How about sorting a list of Apples?

To sort any object in Java, we need to
understand the Comparator Interface.

To sort any Object in Java, we need to
implement the Comparator Interface, which
defines logic for: How to order two objects.

Comparator<Apple> byWeight = new Comparator<Apple>() {
@Override
public int compare(Apple o1, Apple o2) {
return o1.getWeight().compareTo(o2.getWeight());
}
};

Comparator<Apple> byWeight1 = (Apple a1, Apple a2) ->
a1.getWeight().compareTo(a2.getWeight());

Constructing Lambda
A lambda expression is composed of three
parts:
A list of parameters:
(Apple a1, Apple a2)

Constructing Lambda
parts:
A list of parameters, an arrow:
(Apple a1, Apple a2) ->

Constructing Lambda
parts:
A list of parameters, an arrow and a body:
(Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight());

Valid Lambda Expressions
Here are some examples of valid lambda
expressions:
● (String anyString) -> anyString.length()

expressions:
● (Apple a) -> a.getWeight() > 150

expressions:
● () -> 47

Here are some examples of valid lambda expressions:
● () -> 47
● (int a, int b) -> {
System.out.println("Sqaure:");
System.out.println(a*b);
}

Functional Interfaces
A functional interface contains only a one
abstract method.
In Java 8 annotations, It is annotated as
@FunctionalInterface

Functional Interfaces: Examples
● Runnable is a functional interface with an
abstract method run().
● Comparator<T> is a functional interface with
an abstract method compareTo()

Why functional Interface
A functional interface let you describe a
lambda function. In other words,
Lambda expressions are used to instantiate a
functional interface.
interface Calculation {
int calculate(int a, int b);
}

Why functional Interface
A functional interface let you describe a lambda
function. In other words,
Lambda expressions are used to instantiate a
functional interface.
}
Calculation addition = (Int a, Int b) → (a+b)

Function Descriptor
The signature of an abstract method of a
functional interface is called function
descriptor.
}

Function Descriptor: Example
descriptor.
}
Calculation addition = (Int a, Int b) → (a+b)

Function Descriptor: Example
descriptor.
}
Calculation subtraction = (Int a, Int b) → (a-b)

Interface's default methods
An interface can declare a method and can
provide a default implementation for it.

Interface's default methods
An interface can declare a method and can
provide a default implementation for it.
private interface InterfaceOrTrait{
default String alreadyDone() {
return "Default implementation of method String";
}
}

Why Interface's default methods
● Adding enhancement to language without
hurting the backward compatibility.

Why Interface's default methods
● Adding enhancement to language without hurting the backward compatibility.
● Method for a default situation where any implementing class fails to implement
certain methods in the Interface e.g.
interface Iterator<T> {
boolean hasNext();
T next();
default void remove() {
throw new UnsupportedOperationException();
}
}

Method References
● Provides syntax to refer directly to existing
methods
● Can pass existing methods just like lambdas
to functions
● It is better to re use the existing methods
instead of creating lambda expressions for
them.

Method References: Example
Lambda Method reference equivalent
(Apple a) -> a.getWeight() Apple::getWeight
() -> Thread.currentThread().dumpStack() Thread.currentThread()::dumpStack
(str, i) -> str.substring(i) String::substring
(String s) -> System.out.println(s) System.out::println

Stream API
Streams are monads which help writing
programs in a declarative way, chaining
multiple operations which are safe and bug
free.

Why Streams
● Advantages over Collections
● No Storage
● Laziness-seeking
● Functional in nature
● No Bounds (infinite streams)
● Parallelizable

Streams API: An Example
Comparator<Apple> byWeight = new Comparator<Apple>() {
@Override
public int compare(Apple o1, Apple o2) {
return o1.getWeight().compareTo(o2.getWeight());
}
}

Streams API: An Example
listOfApples.add(new Apple("green",12.0));
listOfApples.add(new Apple("red",12.0));
listOfApples.add(new Apple("green",17.0));
listOfApples.add(new Apple("yellow",12.0));
listOfApples.stream()
.filter( apple -> apple.getColor().equals("green"))
.sorted(byWeight)
.forEach(System.out::println);

Creating Streams
● From a Collections
List<Apple> listOfApples = new ArrayList();
Stream<Apple> apples = listOfApples.stream()

Creating Streams
● From Primitive values
Stream<String> wordStream = Stream.of("Value1 ", "Value2", "Value3", "Value4");

Creating Streams
● From Primitive values
Stream<String> wordStream = Stream.of("Value1 ", "Value2", "Value3", "Value4");
● From An Array
int [] primitives = {2,4,6,8,10};
Stream<String> stream = Arrays.stream(primitives);

Operations on Streams
● Reduction
Double totalWeight = listOfApples.stream().mapToDouble(Apple::getWeight).sum();
Double totalReducedWeight =
listOfApples.stream().mapToDouble(Apple::getWeight).reduce(0,(a,b) -> a+b);

● Reduction
Double totalWeight = listOfApples.stream().mapToDouble(Apple::getWeight).sum();
Double totalReducedWeight =
listOfApples.stream().mapToDouble(Apple::getWeight).reduce(0,(a,b) -> a+b);
The reduce operation in this example takes two arguments:
● identity: The identity element is both the initial value of the reduction
● accumulator: The accumulator function takes two parameters: a partial result of the reduction
and the next element of the stream

● Grouping
Map<String, List<Apple>> applesByColor =
listOfApples.stream().collect(Collectors.groupingBy(Apple::getColor));
The above method uses groupingBy method to join the apples by color and returns a Map of
type <String, List<Apple>

● Stats
DoubleSummaryStatistics weightStats =
listOfApples.stream().mapToDouble(Apple::getWeight).summaryStatistics();
System.out.println("The max of Double" + weightStats.getAverage());
System.out.println("The min of Double" + weightStats.getMin());
System.out.println("The max of Double" + weightStats.getMax());
The DoubleSummaryStatistics class contain the stats like count, max, min etc.

References
● Java 8 In Action:
Book by: Mario-Fusco,Alan Mycroft, Raoul-
Gabriel Urma

Introduction to Java 8

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