Java 8 lambdas expressions

Lambda Expressions pt1
1.1 Getting Started with Java 8
1.2 Java 8 References
1.3 Quick review of Pre-Lambda Handlers
1.4. Functional Interface Annotation
1.5 Lambda Handlers
1.6 Why Java 8?
1.7 Wra-Up
Lars A. Lemos
1

1.Getting Started,Setup and
Review
1.4. Functional Interface Annotation
1.5 Lambda Handlers
1.6 Why Java 8?
1.7 Wra-Up
Lars A. Lemos
2

Starting point
 Jdk 8 download, Jdk 8 Demos and Samples
http://www.oracle.com/technetwork/java/javase/downloads/jdk8-
downloads-2133151.html
 More Info
http://docs.oracle.com/javase/8/docs/
http://docs.oracle.com/javase/tutorial/java/javaOO/lambda
expressions.html
 IDE
• IntelliJ IDEA 13 - : http://www.jetbrains.com/idea/download/
• Netbeans 8 : https://netbeans.org/downloads/
• Eclipse Kepler : https://www.eclipse.org/downloads/
Lars A. Lemos 3

 Books
Functional Programming in Java - Venkat Subramanian
Java 8 Lambdas: Pragmatic Functional Programming – Richard Warburton
Java SE 8 for the Really Impatient – Cay S. Horstman
Java The Complete Reference 9th Ed – Herbert Schildt
Lars A. Lemos 4

 Anonymous inner classess
 Idea
You need code to respond a button click, to run in the background
for threads, or to handle sorting comparisons.
 Pre-lambda alternatives
 Separete class
• Arrays.sort(theArray, new SeparateClass(...));
 Main class(which implements interface)
• Arrays.sort(theArray, this);
 Named inner class
• Arrays.sort(theArray, new InnerClass(...));
 Anonymous inner class
• Arrays.sort(theArray, new
Comparator<String>(){...});
Lars A. Lemos 5

 Separete class
public class StringSorter1 {
public void doTests() {
String[] testStrings = {"one","two","three","four"};
System.out.println("Original: ");
ArrayUtils.printArray(testStrings);
Arrays.sort(testStrings, new StringLengthComparator());
System.out.println("After sorting by length: ");
Arrays.sort(testStrings, new LastLetterComparator());
System.out.println("After sorting by last letter: ");
}}
Example: StringSorter1.java
 Advantages
• Flexible: can pass arguments to class constructor
• More reusable: loosely coupled
 Disadavantages
• Need extra stpes to call method in main app. How does handler call method in main
app?
• It needs reference to the main app to do so.
Lars A. Lemos 6

 Implementing Interface
 Advantages
• No extra steps need to call methods in the main app.
o The code is part of the main app, so it can call any
method or access any instance variable, even private
ones.
• Simple
o Widely used in real life for button handlers, when you
know you will have only one button, or for threading
code, when you need one method to run in the
background.
 Disadavantages
• Inflexible: hard to have multiple different versions, since
you cannot pass argumens to the handler.
o For example, if you want to sort array using two or more
ways? If you pass “this” to the second argument in
Arrays.sort, it will refer to the same “compare” method both
times. Lars A. Lemos 7

 Named Inner Class
 Advantages
• No extra steps need to call methods in the main app.Inner class have
full access to all methods and instances variables of surrounding class,
including private ones.
o However if there is a name conflict, it is necessary to use the
unpopular OuterClass.this.name syntax.
• Flexible: can define constructor and pass arguments.
 Disadavantages
o A bit harder to understand.
 Anonymous Inner Class
 Advantages
• Full access to code of surrounding class (even private methods and
variables).
• Slightly more concice than named inner class(but still bulk and
verbose).
 Disadavantages
o Harder to understand
o Not reusable(cannot use the anonymous class definition in more).
Example: StringSorter3.java Lars A. Lemos 8

1.4 @FunctionalInterface Annotation
 @FunctionalInterface
 Tells the compiler you intend this to be a functional Single Abstract Method (SAM)
interface
 One with exactly one abstract method
 Thus, for which lambdas can be used.
 Check performed at compile time
 Also informs others developers that lambdas can be used for this interface
 Analogous to @Override
 Does extra work at compile time, but no difference at run time (assuming it passes
the check)
 Goals
 Simple numerical integration using rectangle (mid-point) rule
 Use lambdas for function to be integrated. Convenient and succinct.
Example: SimpleInterface.java , TestInterface.java
Lars A. Lemos 9

1.3 Lambda Handlers
 Desired features
 Full access to code from surrounding class
 No confusion about meaning of “this”
 Much more concise, succinct, and readable
 Enourage a functional programming style
 Lambda
Arrays.sort(testStrings, (s1, s2) -> s1.length() –
s2.length());
 What is the under the hood implementation
Arrays.sort(testStrings, new Comparator<String>() {
@Override
public int compare(string s1, String s2) {
s1.length() – s2.length());
}
});
Lars A. Lemos 10

1.4 Why Java 8 and Lambdas ?
 Weakly (and usually dynamically ) typed
 JavaScript, Lisp, Scheme, etc.
 Strongly typed
 Ruby, Scala, Clojure, etc.
 Functional approach proven concise, useful, and
parallelizable
 Concise syntax(clear than anonymous inner classes)
 Convienent for new streams library
 Shapes.forEach(s -> s.setColor(Color.RED));
 Similar constructs used in other languages
 Callbacks, closures, map/reduce idiom
 Step toward true functional programming
 When funct. Prog approach is used, many classes of problems
are easier to solve and result in code that is clearer to read and
simpler to maintain. Lars A. Lemos 11

2. Lambdas Basics
2.1 Lambdas Expressions
2.2 Type Inferencing
2.3 Implied Return Values
2.4 Omitting Parents for One-Arg Lambdas
2.5 Effectively Final Local Variables
2.7 Method References
2.8 The java.util.function Package
2.9 Wrap Up
Lars A. Lemos
12

2.1 Lambada Expressions
 Replace this
new SometInterface () {
@Override
public someType someMethod(args) { body; }
}
 With this
(args) - > { body }
 Example
Array.sort(testStrings, new Comparator<String> () ) {
@Override
public int compare(String s1, String s2) {
return (s1.length() – s2.length());
}
Array.sort(testStrings,
(String s1, String s2) - > {
return (s1.length() – s2.length() ) ; } ) ;
Example: JavaLambdas.java
Lars A. Lemos 13

2.1 Lambada Expressions Part
1Basics
 Old
button.addActionListener (new ActionListener () {
@Override
public void actionPerformed(ActionEvent event) {
action();
}
});
 New
button.addActionListener(event -> action());
 You get an Instance of an inner class that
implements interface
 The expected type must be an interface that has exactly one
(abstract).
 It’s called “Functional Interface” or “Single Abstract Method” (SAM)
Example: ButtonFrame1.java
Lars A. Lemos 14

 Types in argument list can usually be omitted
 Since Java usually already knows the expected parameter types
for the single method of the functional interface (SAM interface)
 Basic lambda
(Type1 var1, Type2 var 2 …) - > { method body }
 Lambda with type inference
( var1, var 2 …) - > { method body }
Lars A. Lemos 15

 Replace this
new SometInterface () {
@Override
public someType someMethod( T1 v1, T2 v2 ) { body; }
}
 With this
(v1, v2) - > { body }
Array.sort(testStrings, new Comparator<String> () ) {
@Override
public int compare(String s1, String s2) {
return (s1.length() – s2.length());
}
Array.sort(testStrings,
(s1, s2) - > {
return (s1.length() – s2.length() ) ; } ) ;
Lars A. Lemos 16

2.3 Implied Return Values
 For body, use expression instead of block
 Value of expression will be the return value, with no explicit “return” needed
 If method has a void return type, then automatically no return value
 Since lambdas are usually used only when method body is short, this
approach (using expression instead of block ) is very common.
 Previous Version
(var1, var2) - > { return (something); }
 Lambda with expression for body
(var1, var2) - > { something }
Lars A. Lemos 17

2.4 Omitting Parents for One-Arg
Lambdas
 If method take single argument, parents
optional
 No type should be used: you must let Java infer the type.
 Ommiting types is normal practice
 Previous Version
( varName ) - > someResult()
 Lambda with parentheses omitted
var - > someResult()
Lars A. Lemos 18

2.5 Effectively Final Local Variables
 Lambdas can refer to local variables that are not
declared final (but are never modified)
 This is known as “effectively final” – variables where it would have been legal
to declare them final.
 You can still refer to mutable instance variables.
 “this” in a lambda refers to main class, not inner class that was created for
the lambda. There is no OuterClass.this. Also, no new level of scoping.
 With explicit declaration
final String s = “…”;
doSomething(someArg - > use(s) );
 Effectively final
String s = “…”;
doSomething(someArg - > use(s) );
Lars A. Lemos 19

MathUtilities.integrationTest(x -> x*x, 10, 100);
MathUtilities.integrationTest(x -> Math.pow(x,3), 50, 500);
MathUtilities.integrationTest(x -> Math.sin(x), 0, Math.PI);
MathUtilities.integrationTest(x -> Math.exp(x) , 2, 20);
Example: MathUtilities.java
Lars A. Lemos 20

 Use ClassName:: staticMethodName or
variable::instanceMethodName for lambdas
 Ex: Math::cos or myVar::myMethod
 The function must match signature of method in functional (SAM)
interface to which it is assigned.
 The type is found only from the context
 The type of a method reference depends on what it is assigned to.
This is always true with lambdas, but more obvious here.
 Ex: no predifined type for Math::cos
Lars A. Lemos 21

 In previous example, replace
MathUtilities.integrationTest(x-> Math.sin(x), 0, Math.PI );
MathUtilities.integrationTest(x-> Math.exp(x), 2, 20 );
 With these
MathUtilities.integrationTest( Math::sin(x), 0, Math.PI );
MathUtilities.integrationTest(Math::exp(x), 2, 20 );
 Type of a method reference?
Is not known until you try to assign it to a variable, in which case its
type is whatever interface that variable expected.
Ex: Math::sin could be different types in different contexts, but all the
types would be single-method interfaces whose method could accept
a single double as an argument.
Example: IntegrationTest1.java
Lars A. Lemos 22

2.8 The java.util.function
Package
 Interfaces like Integratable widely used
 So, Java 8 should build in many common cases
 Java.util.function defines many simple functional(SAM)
interfaces
 Named according to arguments and return values
Ex: replace my Integratable with builtin DoubleUnaryOperator
 Look in API for the method names
Althoug lambdas don’t refer to method names, your code that uses
the lambdas will need to call the methods.
 Type of a method reference?
Lars A. Lemos 23

Package
 Types given
 Samples
 IntPredicate(int n, boolean out)
 LongUnaryOperator(long in, long out)
 DoubleBinaryOperator(two double in, double out)
 Example
 DoubleBinaryOperator f = (d1, d2) -> Math.cos(d1 + d2);
 Genericized
 There are also generic interfaces (Function<T,R> , Predicate<T>)
with widespread applicability.
 And concrete methods like “compose” and “negate”
Lars A. Lemos 24

Package
 In previous example, replace this
public static double integrate(Integratable function, ...) {
... function.eval(...);
....
 With this
public static double integrate(DoubleUnaryOperator function, ...) {
... function.applyAsDouble(...);
....
}
Example: MathUtils.java
 Then, omit definition of Integratable entirely
 Because DoubleUnaryOperator is a functional (SAM) interface
containing a method with same signature as the method of the
Integratable interface.
Lars A. Lemos 25

Package
 General Case
 If you are tempted to create an interface purely to be used as
target for a lambda
 Look through java.util.function and see if one of the functional
(SAM) interfaces can be used instead.
• DoubleUnaryOperator, IntUnaryOperator, LongUnaryOperator
o double/int/long in, same type out
• DoubleBinaryOpeartor, IntBinaryOperator, LongBinaryOperator
o Two doubles/ints/longs in, same type out
• DoublePredicate, IntPredicate, LongPredicate
o double/int/long in, boolean out
• DoubleConsumer, IntConsumer , LongConsumer
o double/int/long in, void return type
• Genericized interfaces: Function, Predicate, Consumer, etc
Lars A. Lemos 26

2.9 Wrap Up
 We have lambdas
 Concise and succint
 Familiar to developers that know functional programming
 Fits well with new streams API
 Have method references
 Many functional interfaces built in
 Still not real functional programming
 Type is inner class that implements interface, not a function
 Must create or find interface 1st, must know method name.
 Cannot use mutable local variables
Lars A. Lemos 27

End of part I
Part II – Streams
Thank you for your patience….
larslemos@gmail.com
Lars Lemos
toplars
Lars A. Lemos 28

Java 8 lambdas expressions

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