Java Technologies. Lecture IV. Valdas Rapševičius

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2 ŠMM-07-K-02-070, funded by The European Social Fund Agency and the Government of Lithuania.

1 Preparation of the material was supported by the project Increasing Internationality in Study Programs of the Department of Computer Science II, project number VP1 2.2 ŠMM-07-K , funded by The European Social Fund Agency and the Government of Lithuania. Java Technologies Lecture IV Valdas Rapševičius Vilnius University Faculty of Mathematics and Informatics

2 Session Outline Java 8 feature Lambda variations towards java.util.function interfaces Typed Generic Other Lambda scope Effectively final Method references Valdas Rapševičius. Java Technologies 2

3 Functional Programming Structure computer programs by evaluation of mathematical functions and avoids state and mutable data Emphasizes functions that produce results and depend only on their inputs and not on the program Programming is done with expressions In functional code, the output value of a function depends only on the arguments that are input to the function, so calling a function f twice with the same value for an argument x will produce the same result f(x) both times. Principles Avoiding Mutable State Functions as First-Class Values Higher-Order Functions Declarative Style Valdas Rapševičius. Java Technologies 3

Lambda expressions A new and important feature included in Java SE 8 Lambda comes from the use of the Greek lambda symbol λ to represent functions

4 Lambda expressions A new and important feature included in Java SE 8 Lambda comes from the use of the Greek lambda symbol λ to represent functions in lambda calculus A clear and concise way to represent one method interface using an expression Valdas Rapševičius. Java Technologies 4

5 Anonymous Class Definition static <T> void Arrays.sort(T[] array, Comparator<T> comp); public interface Comparator<T> { int compare(t arg1, T arg2); Implementation Class class ByNameLengthComparator implements Comparator<Person> { public int compare(person p1, Person p2) { return p1.getname().length() - p2.getname().length(); Arrays.sort(arr, new ByNameLengthComparator()); Annonymous class Arrays.sort(arr, new Comparator<Person>() { public int compare(person p1, Person p2) { return p1.getage() - p2.getage(); ); Valdas Rapševičius. Java Technologies 5

6 Lambda Expression 01 Monstrous anonymous class new Comparator<Person>() public int compare(person p1, Person p2) { return p1.getage() - p2.getage(); Replace with simple (lambda) function (Person p1, Person p2) -> { return p1.getage() - p2.getage(); Valdas Rapševičius. Java Technologies 6

7 Lambda Example 01 Arrays.sort(arr, (Person p1, Person p2) -> { return p2.getage() - p1.getage(); ); Arrays.sort(arr, (Person p1, Person p2) -> { return p2.getname().length() - p1.getname().length(); ); Arrays.sort(arr, (Person p1, Person p2) -> { ); int diff = p1.getname().length() - p2.getname().length(); if (diff == 0) { diff = p1.getage() - p2.getage() return diff; Valdas Rapševičius. Java Technologies 7

8 Lambda Expression 02 Monstrous anonymous class new Comparator<Person>() public int compare(person p1, Person p2) { return p1.getage() - p2.getage(); Replace with simple (lambda) function (p1,p2) -> { return p1.getage() - p2.getage(); Valdas Rapševičius. Java Technologies 8

9 Lambda Example 02 Arrays.sort(arr, (p1, p2) -> { return p2.getage() - p1.getage(); ); Arrays.sort(arr, (p1, p2) -> { return p2.getname().length() - p1.getname().length(); ); Arrays.sort(arr, (p1, p2) -> { ); int diff = p1.getname().length() - p2.getname().length(); if (diff == 0) { diff = p1.getage() - p2.getage() return diff; Valdas Rapševičius. Java Technologies 9

10 Lambda Expression 03 Monstrous anonymous class new Comparator<Person>() public int compare(person p1, Person p2) { return p1.getage() - p2.getage(); Replace with simple (lambda) function (p1,p2) -> p1.getage() - p2.getage() Valdas Rapševičius. Java Technologies 10

11 Lambda Example 03 Arrays.sort(arr, (p1, p2) -> p2.getage() - p1.getage() ); Arrays.sort(arr, (p1, p2) -> p2.getname().length() - p1.getname().length()); Arrays.sort(arr, (p1, p2) -> { ); int diff = p1.getname().length() - p2.getname().length(); if (diff == 0) { diff = p1.getage() - p2.getage() return diff; Valdas Rapševičius. Java Technologies 11

12 More Lambda Examples 03 // Pre Java 8 button.addactionlistener(new ActionListener() public void actionperformed(actionevent e) { button.setbackground(color.green); ); button.addactionlistener(new ActionListener() public void actionperformed(actionevent e) { JOptionPane.showMessageDialog(button, e.getactioncommand()); ); button.addactionlistener(new ActionListener() public void actionperformed(actionevent e) { System.out.println(this.getClass().getCanonicalName()); ); // Java 8! button.addactionlistener(e -> button.setbackground(color.red)); button.addactionlistener(e -> JOptionPane.showMessageDialog(button, e.getactioncommand())); button.addactionlistener(e -> System.out.println(this.getClass().getCanonicalName())); Valdas Rapševičius. Java Technologies 12

13 @FunctionalInterface Any single method of the functional interface can be used with lambdas One-method interfaces Functional interfaces Single Abstract Method (SAM) interfaces Declaration is NOT mandatory but recommended (just Compilation error if you add more than one abstract method Declares designers intent for lambdas Declaration of Object methods are public interface SimpleFuncInterface { public void dowork(); Valdas Rapševičius. Java Technologies 13

14 @FunctionalInterface Example Declare your intent for the interface MyFunction { double calc(double x); Declare a method with parameter private static double sumof(myfunction f, double a, double b, double step) { double sum = 0d; for (double i = a; i < b; i += step) { sum += f.calc(i); return sum; Usage double s1 = sumof(x -> x * x, 0d, 10d, 0.1d); double s2 = sumof(x -> 10 / x, -10d, 10d, 1.0d); double s3 = sumof(x -> { return 2 * x;, 1d, 100d, 2.0d); double s4 = sumof(x -> Math.PI * x, 0d, 10d, 0.1d); double s5 = sumof(x -> Math.cos(x), 0d, 10d, 0.1d); Valdas Rapševičius. Java Technologies 14

15 java.util.function Interfaces Provides a backbone for lambdas All-purpose Reusable Types Typed interfaces Takes parameters and/or return values as primitive types Generic interfaces Takes parameters and/or return values as generic objects Valdas Rapševičius. Java Technologies 15

16 Typed java.util.function Interfaces Typed unary operators: one in, one out Represents an operation on a single type-valued operand that produces a same type-valued result. DoubleUnaryOperator, IntUnaryOperator, LongUnaryOperator Typed binary operators: two in, one out Represents an operation upon two type-valued operands and producing a type-valued result. DoubleBinaryOperator, IntBinaryOperator, LongBinaryOperator Typed predicates: one in, boolean out Represents a predicate (boolean-valued function) of one type-valued argument. DoublePredicate, IntPredicate, LongPredicate Typed consumers: one in, void out Represents an operation that accepts a single type-valued argument and returns no result. DoubleConsumer, IntConsumer, LongConsumer Valdas Rapševičius. Java Technologies 16

17 Typed Interface Example Method declaration private static double sumof(doubleunaryoperator f, double a, double b, double step) { double sum = 0d; for (double i = a; i < b; i += step) { sum += f.calc(i); return sum; Usage (the same as in previous example slide!) double s1 = sumof(x -> x * x, 0d, 10d, 0.1d); double s2 = sumof(x -> 10 / x, -10d, 10d, 1.0d); double s3 = sumof(x -> { return 2 * x;, 1d, 100d, 2.0d); double s4 = sumof(x -> Math.PI * x, 0d, 10d, 0.1d); double s5 = sumof(x -> Math.cos(x), 0d, 10d, 0.1d); Valdas Rapševičius. Java Technologies 17

Typed Functional Interfaces in API 2015.04.

18 Typed Functional Interfaces in API Valdas Rapševičius. Java Technologies 18

19 Generic java.util.function Interfaces Predicate<T>: T in, boolean out Represents a predicate (boolean-valued function) of one argument. This is a functional interface whose functional method is test(object). Function<T,R>: T in, R out Represents a function that accepts one argument and produces a result. This is a functional interface whose functional method is apply(object). Consumer<T>: T in, nothing (void) out Represents an operation that accepts a single input argument and returns no result. This is a functional interface whose functional method is accept(object). Supplier<T>: nothing in, T out Represents a supplier of results. There is no requirement that a new or distinct result be returned each time the supplier is invoked. This is a functional interface whose functional method is get(). BinaryOperator<T>: two T s in, T out Represents an operation upon two operands of the same type, producing a result of the same type as the operands. This is a functional interface whose functional method is BiFunction.apply(Object, Object) Valdas Rapševičius. Java Technologies 19

20 Generic Interface Example Declaration private static <T> T findfirst(collection<t> collection, Predicate<T> pred) { for (T t: collection) { if (pred.test(t)) { return t; return null; Usage Person p1 = findfirstperson(persons, p -> p.getage() == 23); Person p2 = findfirst(persons, p -> p.getname().equalsignorecase("john")); Person p3 = findfirst(persons, p.getage() < 40 && p.getname().equals("john")); Valdas Rapševičius. Java Technologies 20

21 Other java.util.function Interfaces public interface BiConsumer<T,U> Represents an operation that accepts two input arguments and returns no result. This is the two-arity specialization of Consumer. public interface ToDoubleFunction<T> Represents a function that produces a double-valued result. public interface LongToDoubleFunction Represents a function that accepts a long-valued argument and produces a double-valued result. public interface ObjIntConsumer<T> Represents an operation that accepts an object-valued and a intvalued argument, and returns no result. (see java.util.functions for others, more info) Valdas Rapševičius. Java Technologies 21

22 Lambda Scope No new scope introduced! In contrast to anonymous class, lambda scope is on this class. // this variable Consumer<Person> c1 = (p) -> this.process(p); // Legal to access variables StringBuilder sb = new StringBuilder(); Consumer<Person> c2 = (p) -> sb.append(p.getname()); // Not legal to have same names Person p = new Person(); String s = ""; Consumer<Person> c3 = (p) -> { String s = ""; ); Valdas Rapševičius. Java Technologies 22

23 Effectively final To access outer variable it must be effectively final. // this variable Consumer<Person> c1 = (p) -> this.process(p); Consumer<Person> c2 = (p) -> this.name = p.getname(); // Legal to access effectively final variables StringBuilder sb = new StringBuilder(); Consumer<Person> c3 = (p) -> sb.append(p.getname()); // Legal to modify instance variables private boolean x = true; Consumer<Person> c4 = (p) -> x = p.ismanager(); // Not legal to modify local variable String s = ""; Consumer<Person> c3 = (p) -> { s = p.getname(); ; Valdas Rapševičius. Java Technologies 23

24 Method References For methods whose signature matches the signature of the method of a functional (SAM) interface you can use Class::method (or similar), rather than an explicit lambda MyClass::staticMethod Math::cos // x -> Math.cos(x) myvariable::instancemethod s::touppercase // () -> s.touppercase() MyClass::instanceMethod String::toUpperCase // s -> s.touppercase() MyClass::new Person::new // () -> new Person() Valdas Rapševičius. Java Technologies 24

25 Method References Examples Static class method Function<Double, Double> cos = Math::cos; // (x) -> Math.cos(x) Variable instance method String s = "Alma Matter Vilnensis"; Supplier<String> upper01 = s::touppercase; // (x) -> x.touppercase() Parameter instance method Function<String, String> upper02 = String::toUpperCase; // (x) -> x.touppercase() Constructor Supplier<String> str01 = String::new; // () -> new String() Function<String, String> str02 = String::new; // (x) -> new String(x) Valdas Rapševičius. Java Technologies 25

26 Session Conclusions Lambda expressions are much more concise than anonymous classes p -> p.getgender() == Person.Sex.MALE && p.getage() >= 18 to declare interfaces for lambda expressions java.util.function package provides a lot of useful reusable interfaces for your needs Learn these interfaces BEFORE creating yours Study the interface if using lambda with Java API Lambda do not introduce new scope Can access effectively final variable and modify its properties Use method references instead of simple lambdas Valdas Rapševičius. Java Technologies 26

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