[JAVA] Introduction to lambda expression

Why use lambda expressions?

Java, which is used for large-scale development, requires that the amount of code be reduced as much as possible and that it be easy to read. The "lambda expression" used in Java 8 is useful for such occasions. A feature of lambda expressions is that you can treat methods like variables.

How to write:

[Interface declaration]= (argument)->{Execution block}

Let's check it from the source code at once. First of all, the traditional expression:


public class LambdaTest {

    public static void main(String[] args) {
        //Traditional expression
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("threading" + Thread.currentThread().getId());
            }
        }).start();
    }
}

The following is a representation of a Lambda expression.


public class LambdaTest {

    public static void main(String[] args) {
        //Lambda expression
        new Thread(() -> {
            System.out.println("Lambda threading" + Thread.currentThread().getId());
        }).start();
    }
}

Java functional interface

Java's functional interface is almost the same as a traditional interface, but the only feature is that the functional interface is "an interface with only one abstract method defined". Static methods and default methods can be included. (It's a good idea to annotate java.lang.FunctionalInterface).

See the detailed introduction to the Java function interface. https://qiita.com/Jenny1025/items/470e304ff77c44874fc8

Example:

@FunctionalInterface
public interface UserIdentify {
    String verifyUser(String userName);
}

Benefits of Lambda expressions

The advantage of Java's Lambda expression is that it can simplify the description of functional interfaces using anonymous classes.

Anonymous class: A class that is not used only once

Anonymous class:

public class App 
{
    public static void main( String[] args ) {

        //Functional interface
        UserIdentify ic = new UserIdentify() {
            @Override
            public String verifyUser(String userName) {
                return "admin".equals(userName)? "admin":"member";
            }
        };

        System.out.println(ic.verifyUser("admin"));
        System.out.println(ic.verifyUser("manager"));
        }
    }

So how do you express it in a Lambda expression?

public class App {
    public static void main( String[] args ) {
        
        UserIdentify ic1 = (String userName) -> {
                return "admin".equals(userName)? "admin":"member";
            };
        System.out.println(ic1.verifyUser("admin"));
        System.out.println(ic1.verifyUser("manager"));
        }
    }

Specific explanation of how to write a lambda expression

Declaration: Lambda-style interface type
Arguments: Enter in (). Same as the number and order of interface arguments
Expression:->
Execution block: Implemented in {}

Let's start with a concrete example.

No arguments, no return value

public class App {
    public static void main( String[] args ) {

        //No arguments, no return value
        Lambda1 lambda1 = () -> System.out.println("Hello world");
        lambda1.test();
    }
    interface Lambda1 {
        void test();
    }
}

Implementation result: Hello world

With arguments, no return value

public class App {
    public static void main( String[] args ) {

        //With arguments, no return value
        Lambda2 lambda2 = (n, a) -> System.out.println(n + " is " + a + " years old");
        lambda2.test("Sally",18);
    }
    interface Lambda2 {
        void test(String name, Integer age);
    }
}

Implementation result: Sally is 18 years old

With arguments, with return value

public class App {
    public static void main( String[] args ) {

        //With arguments, with return value
        Lambda3 lambda3 = (a, b) -> a + b;
        System.out.println(lambda3.test(100,3000));
    }
    interface Lambda3 {
        int test(int a, int b);
    }
}

Implementation result: 3100