Java Functional Programming

Foundation of the Java Stream API

Functional Interfaces

• A Lambda Expression implements a Functional Interface.
• A Lambda Expression is not another way to write instances of anonymous classes.

What is a Functional Interface?

• An interface.
• Has only one abstract method.
• Default and static methods do not count.
• Methods from Object Class do not count. I.e. hashcode(), equals(), toString().
• May have one abstract methods.
• May have as many default methods
• May have as many static methods
• May have methods from the Object Class.
• May be annotated with the @FunctionalInterface annotation.
  • This just tells the compiler to help me determine if the interface is a functional interface or not.
  • If not, it will throw a compiler error.

Writing Suppliers, Consumers, Predicates and Functions

Supplier Interface

public interface Supplier<T> {
    T get();
}

• The above is a functional interface.
• A Supplier does not take any kind of object and produces any kind of object as long as it matches the T type you define when you define the supplier.
• The above can be implemented as follows:

public class SupplierExample {
    public String getGreeting() {
        Supplier<String> supplier = () -> "Hello Artemas";
        return supplier.get();
    }
}

Consumer Interface

public interface Consumer<T> {
    void accept(T t);
}

• A consumer, consumes any kind of object which is defined on the parameter.
• Has a single method called accept(T t) that takes any kind of object and that does not produce or return anything.
• The above can be implemented as:

public class ConsumerExample {
    public void printValue(String name) {
        Consumer<String> nameConsumer = System.out::println;
        nameConsumer.accept(name);
    }
}

Predicate Interface

public interface Predicate<T> {
    boolean test(T t);
}

• A Predicate will take some object T and perform a test with it to produce a boolean value true or false.
• Used in the filtering operations in the Stream API.
• The above can be implemented as the following lambda:

public class PredicateExample {
    public List<String> stringNotStartingWithLetterT(List<String> strings) {
        Predicate<String> filter = (String someString) -> someString.startsWith("t");
        strings.removeIf(filter);

        return strings;
    }
}

Function Interface

public interface Function<T, R> {
    R apply(T t);
}

• Like the Predicate which returns a boolean value, a Function will take in any type of object and return another type of object.
• Used in the map() operation in the Stream API.
• The above can be implemented as:

public class FunctionExample {
    public List<String> getNames(List<User> users) {
        Function<User, String> userNames = User::getName;
        return users.stream()
                .map(userNames)
                .collect(toList());
    }
}

Invoking a Lambda Expression on Objects and Primitive Types

How fast Are Lambda Expressions?

• What you can put in a lambda, can be returned also in an instance of an anonymous class.
• Lambda expressions are compiled using specific byte code instructions called invokedynamic introduced in Java 7.
• Ta make lambdas even faster, you want to avoid autoboxing on lambdas (converting int to INT and auto-unboxing which is converting INT to int).

Introducing the Specialized Interfaces

• The java.util package has interfaces that are tailored to with primitive types instead of wrapping types.
• E.g. you have the IntPredicate that takes an int and returns a boolean.

IntSupplier Interface

@FunctionalInterface
public interface IntSupplier {
    int getAsInt();
}

• This can be implemented as follows:

public class IntSupplierExample {
    public int getValue() {
        IntSupplier supplier = () -> 10;
        return supplier.getAsInt();
    }
}

DoubleToIntFunction

@FunctionalInterface
public interface DoubleToIntFunction {
    int applyAsInt(double value);
}

• This can be implemented as follow:

public class DoubleToIntFunctionExample {
    public int getIntValueBy(double input) {
        DoubleToIntFunction intFunction = value -> (int) value * 2;
        return intFunction.applyAsInt(input);
    }
}