Cover Basic concept for Functional Programming in Java. Define new functional interfaces, lambda expressions, how to translate lambda expression, JVM deal with new byte code etc. This is not the perfect slides for functional programming, but trying cover simple basic functional programming.

1. Functional Programming
In JAVA
By Harmeet Singh(Taara)
(Java EE Developer)
Email: harmeetsingh.0013@gmail.com
Blog: http://harmeetsingh13.blogspot.in
Skype: harmeetsingh0013
Contact: Via Email or Skype

2. ➢ Introduction
➢ Functional Programming.
➢ Single Abstract Method(SAM).
➢ Functional Interface.
➢ Declare Lambda Expressions.
➢ Code Refactor Using Lambda Expression.
➢ Predefined Functional Interfaces in Java 8.
➢ User Defined Functional Interface in Java 8.
➢ Type Inference.
➢ Translation Of Lambda Expressions.
➢ Runtime Translation Strategies.
➢ Leftover: The Things We Didn’t Cover.
Contents

3. Acknowledgement
❖ Thanks To My Parents.
❖ Thanks To All Who Support Me or Not.
❖ Dedicated To My Teacher “Mr. Kapil Sakhuja”.

4. Introduction
In Java 8, the functional programming is the important
evolution in Object Oriented Programming world of Java.
Now, Java use lambdas, Simple Abstract Method (SAM)
etc for allow functional programming in Object Oriented
World.
There are so many new features are added in Java 8 like
Streams, Default methods etc. But Today’s we only
discuss about “Functional Programming In Java”.

5. What is Functional Programming ?
Immutable Object: In Object Oriented and Functional
Programming, An Immutable Object is an object whose
state cannot be modified after it is created.
Functional Programming: Functional Programming
involves writing code that does not change state. One of
the key feature of Functional Programming is the First-
Class Functions.

6. Single Abstract Method(SAM)
Interfaces have one Abstract Method are called SAM.
In Java there are lots of interfaces which only have one
abstract method(by default interface have abstract
method) like Runnable, Closeable etc.
These Single Abstract Method(SAM) Interfaces are also
called “Functional Interface”.

7. Functional Interface
Functional Interface: In Java 8, If Functional Interface
have more than one methods, one is different and rest of
the methods have same signature of Object class
methods. These types of Interfaces also called Functional
Interfaces like Comparator etc in Java.
If interface have one abstract method and one or more
than one Default and Static methods are also called
Functional Interface.

8. Declare Lambda Expression
Syntax :
lambda = ArgList “->” Body
ArgList = Identifier && Body = Expression
Examples:
(int x, int y) -> { return x+y }
(x, y) -? { return x+y }
(x) -> { return x+1 }
() -> { System.out.println(“Hello World”) }

9. Code Refactoring Using Lambda

10. Predefined Functional Interfaces in Java 8
In Java 8, There are lots of Predefined Functional
Interface, But today’s we only cover some important
Functional interfaces are :-

11. Predefined Functional Interfaces in Java 8
Predicate<T>: Represents a predicate (boolean-value-
function) of one argument.
Example:

12. Predefined Functional Interfaces in Java 8
Consumer<T>: Represents an operation that accept a
single input argument and returns no result.
Example:

13. Predefined Functional Interfaces in Java 8
Function<T, R>: Represents a function that accept a one
argument and produces a result.
Example:

14. Predefined Functional Interfaces in Java 8
Supplier<T>: Represents a supplier of results
Example:

15. Predefined Functional Interfaces in Java 8
UnaryOperator<T>: Represents an operations on a single
operands that produces a result of the same type as its
operand.
Example:

16. Predefined Functional Interfaces in Java 8
BinaryOperator<T, T>: Represents an operations upon
two operands of same type, producing a result of the
same type as the operands.
Example:

17. User Defined Functional Interfaces In Java 8
IN Java 8, it is also possible to create our custom user
define Functional Interface with the help of
“@FunctionalInterface” annotation. Our Functional
Interfaces also have default methods and static methods.
@FunctionalInterface: @FunctionalInterface annotation
indicates that the type declaration is intended to be a
functional interface, as defined by Java Language
Specification.

18. User Defined Functional Interfaces In Java 8
Example:

19. Type Inference
Type Inference is not the new topic in Java. In Java 7 we
create the Collections object with the help of Diamond
Operator(<>) like :
List<Integer> list = new ArrayList<>();
Under the hood of this code, Compiler use the “Type
Inference” technique. In this compiler pick the type
information from the left side of type declaration.

20. Type Inference
For Lambda expressions in Java 8, compiler use the same
“Type Inference” technique with some improvements.
Examples:
Predicate<Integer> predicate = (x) -> x > 5; //code
compile
Predicate predicate = (x) -> x > 5; // Compile time error
// Predicate is a raw type. References to generic type
//Predicate<T> should be parameterized

21. Translation Of Lambda Expression
Translate the lambda expression to bytecode is major
challenge for Java, because the important thing is to
maintain backward compatibility in bytecode. There are
so many challenges were faced for conversion like:
1. How to deal with Functional Interfaces?
2. For Lambdas use Inner classes?
3. Maintain Lambda information at runtime? etc.

22. Translation Of Lambda Expression
There are so many things are used and maintain for
lambdas in JVM, But for JVM the lambdas are not a new,
Because some languages are already use JVM for Runtime
Environment that have “Lambda Expression” like
Groovy, Scala, Clojure etc.
Today we only discuss the brief steps for translate Lambda
Expression after compilation.

23. Translation Of Lambda Expression
Followings are the Steps:
➔ Read “Lambda Expression” and desugars the lambda
body into a method whose argument list and return
type match that of “Lambda Expression”.
Example :
list.forEach( s -> { System.out.println(s); } );
//before compile
static void lambda$1(String s) { //After compile
System.out.println(s);
}

24. Translation Of Lambda Expression
Followings are the Steps:
➔ At the point at which the “Lambda Expression” would
be captured, it generates an “invokedynamic”
CallSite like :
list.forEach( s -> { System.out.println(s); } );
list.forEach( [lambda for lambda$1 as Block] );

25. Translation Of Lambda Expression
Followings are the Steps:
➔ This CallSite is called Lambda Factory for Given
Lambda.
➔ The Dynamic Arguments to the lambda factory are the
values captured from lexical scope.
➔ The Bootstrap Method for “Lambda Factory” is called
“Lambda Metafactory”.

26. Runtime Translation Strategies
➔ Generate Inner Classes Dynamically.
➔ Generate per-SAM wrapper class
◆ One per SAM type, not per lambda expression
◆ Use method handles for invocation.
◆ Use ClassValue to cache wrapper for SAM.
➔ Use Dynamic Proxies.
➔ Use MethodHandleProxies.asInterfaceInstance.
➔ Use VM private API to build object from scratch.

27. Leftover: The Important Topics We Didn’t Cover
1. Lexical Scoping
2. Method References
3. Method Handlers
4. Default Methods and Static methods in Interface
5. Streams in Java 8
6. Java 8 Date API
7. invokedynamic
8. Concurrency in lambda
9. Advanced Collections
10. Design and Architecture Principles

28. References:
❖ Special Thanks to Richard Warburton for “Java 8
lambdas”.
❖ “Implementing Lambda Expressions in Java” by Brian
Goetz.
❖ “Lambda Expressions in Java” by Simon Ritter.
❖ Oracle Java 8 Api Docs.
❖ Translation Of Lambda Expression by OpenJDK.(http:
//cr.openjdk.java.net/~briangoetz/lambda/lambda-
translation.html)
And many more like, DZone, StackOverflow etc.