Lambda Expressions CSC 203 Christopher Siu 1 / 12 Lambda - - PowerPoint PPT Presentation

Lambda Expressions

CSC 203 Christopher Siu

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Lambda Expressions

A function is a mapping of inputs to outputs: input, x

f

• utput, f (x)

Defjnition

A lambda expression defjnes a function that can be passed and returned as though it were a value. Lambda expressions are also called anonymous functions, because they are not bound to one particular name. Java does not allow standalone functions — every method must be contained within a class.

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Lambda Expressions

Example

Consider the following class:

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import java.util.Comparator;

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public class PointComparator

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implements Comparator<Point>

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{

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public int compare(Point pt1, Point pt2) {

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return ((Double)pt1.getX())

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.compareTo(pt2.getX());

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}

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}

A Comparator class is boilerplate that allows a single method to be passed around.

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Functional Interfaces

Defjnition

A functional interface defjnes exactly one abstract method. Functional interfaces can be targets for lambda expressions. A lambda expression in Java has the form: (PARAMETERS …) -> VALUE

Parameter and return types are omitted. If the method has one parameter, the parentheses are optional.

Example

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Comparator<Point> compareByY =

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(pt1, pt2) -> ((Double)pt1.getY())

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.compareTo(pt2.getY());

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Functional Interfaces

In order to use a lambda in Java, an appropriate functional interface must already have been defjned.

Example

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@FunctionalInterface

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public interface PointStringifier

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{

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public String stringify(Point pt);

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}

Functional interfaces don’t truly solve the problem of lambdas in Java, they only hide it behind interfaces and generics. The java.util.function package defjnes functional interfaces for common forms of functions.

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The Function Interface

Defjnition

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@FunctionalInterface

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public interface Function<T, R>

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{

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R apply(T t);

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}

A java.util.function.Function encapsulates a method that takes one argument and returns a value.

Example

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Function<Point, String> stringifyPoint =

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pt -> "(" + pt.getX() + ", " + pt.getY() + ")";

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The Predicate Interface

Defjnition

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@FunctionalInterface

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public interface Predicate<T>

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{

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boolean test(T t);

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}

A java.util.function.Predicate encapsulates a method that takes one argument and returns true or false.

Example

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Predicate<Point> inFirstQuadrant =

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pt -> pt.getX() > 0 && pt.getY() > 0;

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The Consumer Interface

Defjnition

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@FunctionalInterface

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public interface Consumer<T>

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{

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void accept(T t);

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}

A java.util.function.Consumer encapsulates a method that takes one argument and does not return anything.

Example

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Consumer<Point> printX =

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pt -> System.out.println("pt.x: " + s.getX());

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The Supplier Interface

Defjnition

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@FunctionalInterface

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public interface Supplier<T>

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{

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T get();

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}

A java.util.function.Supplier encapsulates a method that takes no arguments and returns a value.

Example

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Supplier<Point> createOrigin =

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() -> new Point(0.0, 0.0);

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Method References

Example

Consider the following lambda:

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Function<Point, String> stringifyPoint =

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pt -> pt.toString();

This lambda expression merely calls an existing method. A method reference in Java has the form: CLASS::METHOD

Example

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Function<Point, String> stringifyPoint =

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Point::toString;

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Composing Lambda Expressions

Some functional interfaces provide additional methods for combining lambda expressions.

Example

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Function<Point, String> stringifyPoint =

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pt -> "(" + pt.getX() + ", " + pt.getY() + ")";

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Function<String, String> shoutString =

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String::toUpperCase;

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Function<Point, String> shoutPoint =

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shoutString.compose(stringifyPoint);

1 shoutPoint fjrst stringifjes the given Point. 2 It then converts the string representation to upper case.

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Composing Lambda Expressions

Example

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Comparator<Point> compareByX =

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(pt1, pt2) -> ((Double)pt1.getX())

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.compareTo(pt2.getX());

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Comparator<Point> compareByY =

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(pt1, pt2) -> ((Double)pt1.getY())

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.compareTo(pt2.getY());

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Comparator<Point> compareByXthenY =

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compareByX.thenComparing(compareByY);

1 compareByXthenY fjrst compares x-coordinates. 2 It then breaks any ties by comparing y-coordinates.

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