Java Note - 5: Lambda

Lamda

Lambda Best Practices

Use Interfaces

The most common misstep taken by an over-eager functional programmer is the use of functional interfaces in type signatures. In general, you should avoid using the functional interface types directly and instead provide single-method interfaces as arguments to your methods. These interfaces become a way to create self-documenting code and to provide meaningful type information, as well as leaving open the opportunity for your user to provide an actual Java type.

Use Method Reference

As much as possible, use a method reference instead of a lambda. Method references are not only shorter and easier to read, but using method references will get you thinking directly about the methods as values.

Define Lambdas Inline

When you do use lambdas, define them inline. Unless you are doing some kind of fancy manipulation of your lambda, there is no reason to be assigning them to a variable. The reason that you want to define your lambdas inline is that it will allow your code to be more flexible when types change: you are letting type inference do more heavy lifting for you, and adapting your code to changing contexts.

Lambdas Should Always Be Threadsafe

As we go through the rest of this book, we will see many places where lambdas make concurrent programming much easier. Many of the structures built off of lambdas will perform concurrent executions, sometimes without much warning. Because of this, your lambdas always need to be threadsafe. Pay particular attention to this with instance method handles, since thread-dangerous state can often be hiding within those instances.

Don’t Use Null

The null keyword should never be used in your code. Now that Java has the Optional type, there is simply no need for it. Whenever you have a method, you should be explicit about whether or not you accept null, and you generally shouldn’t accept it. This will save you from NullPointerException cropping up in obnoxious places, far from the site of the actual error. This is an especially painful problem when you start working with streams and lambdas, because the stack trace may not be very useful for you when you go to debug. The solution is to never accept null and to aggressively check for it, exploding loudly as soon as it occurs.

Don’t Release Zalgo

• Don’t mix asynchronous and synchronous execution in the same lamda expressoin.

Build Complexity from Simple Parts

Use Types and the Compiler to Your Advantage

Common functional Interfaces

Functional Interface	Parameter Types	Return	Abstract Description Method	Description	Other Methods
Runnable	none	void	run	Runs an action
Supplier	none	T	get	Supplies a value of type T
Consumer	T	void	accept	Consumes a value of type T	chain
BiConsumer	T, U	void	accept	Consumes a value of type T and U	chain
Function	T	R	apply	A function with argument oftype T	compose, andThen,identity
BiFunction	T, U	R	apply	A function with argument of type T and U	andThen
UnaryOperator	T	T	apply	A unary operator on type T	compose, andThen, identity
BiUnaryOperator	T,T	T	apply	A binary operator on type T	andThen
Predicate	T	boolean	test	A Boolean-valued function	and, or, negate, isEqual
BiPredicate	T,T	boolean	test	A Boolean-valued function with tow arguments	and, or, negate

Method Reference

Syntax	Description
TypeName::staticMethod	A method reference to a static method of a class, an interface, or an enum
objectRef::instanceMethod	A method reference to an instance method of the specified object
ClassName::instanceMethod	A method reference to an instance method of an arbitrary object of the specified class
TypeName.super::instanceMethod	A method reference to an instance method of the supertype of a particular object
ClassName::new	A constructor reference to the constructor of the specified class
ArrayTypeName::new	An array constructor reference to the constructor of the specified
array type

Lambda Demo

import java.util.Locale;
import java.util.Arrays;
import java.util.List;
import java.util.ArrayList;
import java.util.function.Supplier;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.BiFunction;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
import java.util.function.BinaryOperator;
import java.util.function.IntFunction;

public class LambdaDemo {

    public static void main(String[] args) {

        // FunctionalInterface 
        System.out.println("x + y:" + engine((x, y) -> x + y)); // 6
        System.out.println("x * y:" + engine((x, y) -> x * y)); // 8
        System.out.println("x / y:" + engine((x, y) -> x / y)); // 0 
        System.out.println("x % y:" + engine((x, y) -> x % y)); // 2

        String[] strArray = new String[] { "abc", "klm", "xyz", "pqr" };
        List list = Arrays.asList(strArray);

        // Default Methods
        list.forEach(System.out::println);

        // abc
        // klm
        // xyz
        // pqr

        Arrays.sort(strArray, (first, second) -> first.compareToIgnoreCase(second));
        list = Arrays.asList(strArray);

        System.out.println("After sorting ... ");
        list.forEach(System.out::println);

        // After sorting ...
        // abc
        // klm
        // pqr
        // xyz

        // Common Functional Interfaces
        // Runnable
        repeat(5, () -> System.out.println("Hello"));

        UnaryOperator<String> upperCase = str -> str.toUpperCase();
        BinaryOperator<String> concat = (left, right) -> left + right;

        System.out.println(" UnaryOperator upperCase " + upperCase.apply("hello"));
        System.out.println("  BinaryOperator<String> concat " + concat.apply("hello", "world"));

        // Function 
        Function<Long, Long> square = x -> x * x;
        Function<Long, Long> plusOne = x -> x + 1;
        // Function with andThen, 
        Function<Long, Long> squarePlusOne = square.andThen(plusOne);
        Function<Long, Long> plusOneSquare = square.compose(plusOne);
        System.out.println(" 5 squarePlusOne is " + squarePlusOne.apply(5L)); // 26 
        System.out.println(" 5 plusOneSquare is  " + plusOneSquare.apply(5L)); // 36

        // Predicate
        Predicate<Integer> divisibleByThree = x -> x % 3 == 0;
        Predicate<Integer> divisibleByFive = x -> x % 5 == 0;
        Predicate<Integer> isNegative = x -> x < 0;
        // Predicate with AND , OR , NOT
        Predicate<Integer> divisibleByThreeAndFive = divisibleByThree.and(divisibleByFive);
        Predicate<Integer> divisibleByThreeOrFive = divisibleByThree.or(divisibleByFive);
        Predicate<Integer> isPositive = isNegative.negate();

        System.out.println(" 15 is divisibleByThreeAndFive " + divisibleByThreeAndFive.test(15));
        System.out.println(" 7 is divisibleByThreeAndFive " + divisibleByThreeOrFive.test(7));
        System.out.println(" -1 is isPositive " + isPositive.test(7));

        // static method reference 
        Function<Integer, String> toBinary = x -> Integer.toBinaryString(x);
        System.out.println(toBinary.apply(19));

        // Using a method reference
        Function<Integer, String> toBinary2 = Integer::toBinaryString;
        System.out.println(toBinary2.apply(19));

        // static method lambda expression
        BiFunction<Integer, Integer, Integer> sum = (a, b) -> Integer.sum(a, b);
        System.out.println(sum.apply(3, 4));

        // Instance method
        Supplier<Person> personSup = () -> new Person();
        Function<String, Person> personFunc = (x) -> new Person(x);

        BiFunction<String, String, Person> personBiFunc = (x, y) -> new Person(x, y);
        // Consumer<String> personCon = (Person p) -> p.setTitle;

        System.out.println(personSup.get());
        // Person() constructor called
        // name = Unknown, title = Unknown
        System.out.println(personFunc.apply("John Doe"));
        // Person( fullName ) constructor called
        // name = John Doe, title = Unknown

        System.out.println(personBiFunc.apply("John", "Doe"));
        // Person(firstName, lastName ) constructor called
        // name = John, title = Unknown


        // Recursive Lambda Expressions
        IntFunction<Long> factorialCalc = new IntFunction<Long>() {
            @Override
            public Long apply(int n) {
                if (n < 0) {
                    String msg = "Number must not be negative.";
                    throw new IllegalArgumentException(msg);
                }
                if (n == 0) {
                    return 1L;
                } else {
                    return n * this.apply(n - 1);
                }
            }
        };

        int n = 5;
        long fact = factorialCalc.apply(n);
        System.out.println("Factorial of " + n + " is " + fact);
        // Factorial of 5 is 120   
    }

    private static int engine(Calculator calculator) {
        int x = 2, y = 4;
        return calculator.calculate(x, y);
    }

    public static void repeat(int n, Runnable action) {
        for (int i = 0; i < n; i++)
            action.run();
    }
}

@FunctionalInterface
interface Calculator {
    int calculate(int x, int y);
}

final class Person {
    String firstName;
    String lastName;
    String fullName;
    String title;

    public Person() {
        System.out.println(" Person() constructor called ");
    }

    public Person(String fullName) {
        this.fullName = fullName;
        System.out.println(" Person( fullName ) constructor called ");
    }

    public Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
        System.out.println(" Person(firstName, lastName ) constructor called ");
    }

    public void setTitle(String t) {
        this.title = t;
        System.out.println(" Person setTitle ( t ) called ");
    }

    public String getFirstName() {
        return firstName;
    }

    public String getFullName() {
        return fullName == null ?( firstName != null ? firstName : "Unknown" ): fullName;
    }

    @Override
    public String toString() {
        return "name = " + getFullName() + ", title = " + (title != null ? title : "Unknown");
    }
}