Java 8 Lambda Built-in Functional Interfaces

JAVA 8: LAMBDA BUILT-IN
FUNCTIONAL INTERFACES
ganesh samarthyam
ganesh.samarthyam@gmail.com

Functional interfaces
@FunctionalInterface
interface LambdaFunction {
void call();
}
Functional interface
Abstract method providing the signature of the
lambda function
Annotation to explicitly state that it is a functional
interface

Java 8 lambdas - “Hello world!”
void call();
}
class FirstLambda {
public static void main(String []args) {
LambdaFunction lambdaFunction = () -> System.out.println("Hello world");
lambdaFunction.call();
}
}
Functional interface - provides
signature for lambda functions
Lambda function/expression
Call to the lambda
Prints “Hello world” on the console when executed

Older Single Abstract Methods (SAMs)
// in java.lang package
interface Runnable { void run(); }
// in java.util package
interface Comparator<T> { boolean compare(T x, T y); }
// java.awt.event package:
interface ActionListener { void actionPerformed(ActionEvent e) }
// java.io package
interface FileFilter { boolean accept(File pathName); }

Functional interfaces: Single abstract methods
void call();
// Single Abstract Method (SAM)
}

Using built-in functional interfaces
// within Iterable interface
default void forEach(Consumer<? super T> action) {
Objects.requireNonNull(action);
for (T t : this) {
action.accept(t);
}
}
// in java.util.function package
public interface Consumer<T> {
void accept(T t);
// the default andThen method elided
}

Using built-in functional interfaces
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");
Consumer<String> printString = string -> System.out.println(string);
strings.forEach(printString);
List<String> strings = Arrays.asList("eeny", "meeny", "miny", "mo");
strings.forEach(string -> System.out.println(string));

Built-in functional interfaces

Built-in functional interfaces are a
part of the java.util.function
package (in Java 8)

Built-in interfaces
Predicate<T> Checks a condition and returns a
boolean value as result
In filter() method in
java.util.stream.Stream
which is used to remove elements
in the stream that don’t match the
given condition (i.e., predicate) asConsumer<T> Operation that takes an argument but
returns nothing
In forEach() method in
collections and in
java.util.stream.Stream; this
method is used for traversing all
the elements in the collection orFunction<T,
R>
Functions that take an argument and
return a result
In map() method in
java.util.stream.Stream to
transform or operate on the passed
value and return a result.
Supplier<T> Operation that returns a value to the
caller (the returned value could be
same or different values)
In generate() method in
java.util.stream.Stream to
create a inﬁnite stream of
elements.

Predicate interface
Stream.of("hello", "world")
.filter(str -> str.startsWith("h"))
.forEach(System.out::println);
The ﬁlter() method takes a Predicate
as an argument (predicates are
functions that check a condition and
return a boolean value)

Predicate interface
A Predicate<T> “afﬁrms” something as true or
false: it takes an argument of type T, and returns a
boolean value. You can call test() method on a
Predicate object.
public interface Predicate<T> {
boolean test(T t);
// other methods elided
}

Predicate interface: Example
import java.util.function.Predicate;
public class PredicateTest {
Predicate<String> nullCheck = arg -> arg != null;
Predicate<String> emptyCheck = arg -> arg.length() > 0;
Predicate<String> nullAndEmptyCheck = nullCheck.and(emptyCheck);
String helloStr = "hello";
System.out.println(nullAndEmptyCheck.test(helloStr));
String nullStr = null;
System.out.println(nullAndEmptyCheck.test(nullStr));
}
}
Prints:
true
false

Predicate interface: Example
import java.util.List;
import java.util.ArrayList;
public class RemoveIfMethod {
List<String> greeting = new ArrayList<>();
greeting.add("hello");
greeting.add("world");
greeting.removeIf(str -> !str.startsWith("h"));
greeting.forEach(System.out::println);
}
}
Prints:
hello

Consumer interface
Stream.of("hello", "world")
// void forEach(Consumer<? super T> action);
Prints:
hello
world

Consumer interface
A Consumer<T> “consumes” something: it takes
an argument (of generic type T) and returns
nothing (void). You can call accept() method on a
Consumer object.
public interface Consumer<T> {
void accept(T t);
// the default andThen method elided
}

Consumer interface: Example
Consumer<String> printUpperCase =
str -> System.out.println(str.toUpperCase());
printUpperCase.accept("hello");
Prints:
HELLO

Consumer interface: Example
import java.util.stream.Stream;
import java.util.function.Consumer;
class ConsumerUse {
Stream<String> strings = Stream.of("hello", "world");
Consumer<String> printString = System.out::println;
strings.forEach(printString);
}
}
Prints:
hello
world

Function interface
import java.util.Arrays;
public class FunctionUse {
Arrays.stream("4, -9, 16".split(", "))
.map(Integer::parseInt)
.map(i -> (i < 0) ? -i : i)
}
}
Prints:
4
9
16

Function interface
A Function<T, R> “operates” on something and
returns something: it takes one argument (of
generic type T) and returns an object (of generic
type R). You can call apply() method on a Function
object.
public interface Function<T, R> {
R apply(T t);
// other methods elided
}

Function interface: Example
Function<String, Integer> strLength = str -> str.length();
System.out.println(strLength.apply("supercalifragilisticexpialidocious"));
Prints:
34

Function interface: Example
import java.util.Arrays;
import java.util.function.Function;
public class CombineFunctions {
Function<String, Integer> parseInt = Integer:: parseInt ;
Function<Integer, Integer> absInt = Math:: abs ;
Function<String, Integer> parseAndAbsInt = parseInt.andThen(absInt)
Arrays.stream("4, -9, 16".split(", "))
.map(parseAndAbsInt)
.forEach(System. out ::println);
}
}
Prints:
4
9
16

Supplier interface
import java.util.stream.Stream;
import java.util.Random;
class GenerateBooleans {
Random random = new Random();
Stream.generate(random::nextBoolean)
.limit(2)
}
}
Prints two boolean
values “true” and “false”
in random order

Supplier interface
A Supplier<T> “supplies” takes nothing but
returns something: it has no arguments and
returns an object (of generic type T). You can call
get() method on a Supplier object
public interface Supplier<T> {
T get();
// no other methods in this interface
}

Supplier interface: Example
Supplier<String> currentDateTime = () -> LocalDateTime.now().toString();
System.out.println(currentDateTime.get());
Prints current time:
2015-10-16T12:40:55.164

Summary of built-in interfaces in
java.util.function interface
❖ There are only four core functional interfaces in this
package: Predicate, Consumer, Function, and Supplier.
❖ The rest of the interfaces are primitive versions, binary
versions, and derived interfaces such as
UnaryOperator interface.
❖ These interfaces differ mainly on the signature of the
abstract methods they declare.
❖ You need to choose the suitable functional interface
based on the context and your need.

Check out our book!
❖ “Oracle Certiﬁed Professional Java
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email sgganesh@gmail.com
website ocpjava.wordpress.com
twitter @GSamarthyam
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Java 8 Lambda Built-in Functional Interfaces

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