java Lambda Expressions

Lambda是JDK 1.8新特性，用Lambda代替匿名类，可以使代码更加间接，干练。

语法规则：(param1, param2,...) ->(return param1+param2); return可以省略
function interface（函数接口）: it contains only one abstract method.
the JDK defines several standard functional interfaces, which you can find in the package java.util.function.
lambda Expressions可以和函数接口配合使用，具体demo参考代码

eg:匿名类和lambda比较

package lambda;

import java.time.LocalDate;
import java.util.LinkedList;
import java.util.List;

/**
 * 
 * @Description:
 * @author: chenlh
 * @date: 2018-07-05
 */
public class Person {

    public enum Sex {
        MALE, FEMALE
    }

    String name;
    LocalDate birthday;
    Sex gender;
    String emailAddress;

    public int getAge() {
        return 20;
    }

    public void printPerson() {
        System.out.println("printPerson");
    }

    /**
     * @return
     */
    public static List<Person> createRoster() {
        List p = new LinkedList();
        p.add(new  Person());
        p.add(new  Person());
        p.add(new  Person());
        return p;
    }

    /**
     * @return
     */
    public Sex getGender() {
        return Sex.MALE;
    }

    /**
     * @return
     */
    public String getEmailAddress() {
        return this.getEmailAddress();
    }
}

测试类:

package lambda;


import java.util.List;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;


/**
 * 
 * @Description:匿名类和lambda测试比较
 * @author: chenlh
 * @date: 2018-07-05
 */
public class LambdaTest {

    interface CheckPerson {
        boolean test(Person p);
    }

    // Approach 1: Create Methods that Search for Persons that Match One
    // Characteristic

    public static void printPersonsOlderThan(List<Person> roster, int age) {
        for (Person p : roster) {
            if (p.getAge() >= age) {
                p.printPerson();
            }
        }
    }

    // Approach 2: Create More Generalized Search Methods

    public static void printPersonsWithinAgeRange(
        List<Person> roster, int low, int high) {
        for (Person p : roster) {
            if (low <= p.getAge() && p.getAge() < high) {
                p.printPerson();
            }
        }
    }

    // Approach 3: Specify Search Criteria Code in a Local Class
    // Approach 4: Specify Search Criteria Code in an Anonymous Class
    // Approach 5: Specify Search Criteria Code with a Lambda Expression

    public static void printPersons(
        List<Person> roster, CheckPerson tester) {
        for (Person p : roster) {
            if (tester.test(p)) {
                p.printPerson();
            }
        }
    }

    // Approach 6: Use Standard Functional Interfaces with Lambda Expressions

    public static void printPersonsWithPredicate(
        List<Person> roster, Predicate<Person> tester) {
        for (Person p : roster) {
            if (tester.test(p)) {
                p.printPerson();
            }
        }
    }

    // Approach 7: Use Lambda Expressions Throughout Your Application

    public static void processPersons(
        List<Person> roster,
        Predicate<Person> tester,
        Consumer<Person> block) {
        for (Person p : roster) {
            if (tester.test(p)) {
                block.accept(p);
            }
        }
    }

    // Approach 7, second example

    public static void processPersonsWithFunction(
        List<Person> roster,
        Predicate<Person> tester,
        Function<Person, String> mapper,
        Consumer<String> block) {
        for (Person p : roster) {
            if (tester.test(p)) {
                String data = mapper.apply(p);
                block.accept(data);
            }
        }
    }
    
    // Approach 8: Use Generics More Extensively

    public static <X, Y> void processElements(
        Iterable<X> source,
        Predicate<X> tester,
        Function<X, Y> mapper,
        Consumer<Y> block) {
            for (X p : source) {
                if (tester.test(p)) {
                    Y data = mapper.apply(p);
                    block.accept(data);
                }
            }
    }

    public static void main(String... args) {

        List<Person> roster = Person.createRoster();

        for (Person p : roster) {
            p.printPerson();
        }

        // Approach 1: Create Methods that Search for Persons that Match One
        // Characteristic

        System.out.println("Persons older than 20:");
        printPersonsOlderThan(roster, 20);
        System.out.println();

        // Approach 2: Create More Generalized Search Methods

        System.out.println("Persons between the ages of 14 and 30:");
        printPersonsWithinAgeRange(roster, 14, 30);
        System.out.println();

        // Approach 3: Specify Search Criteria Code in a Local Class

        System.out.println("Persons who are eligible for Selective Service:");

        class CheckPersonEligibleForSelectiveService implements CheckPerson {
           public boolean test(Person p) {
                return p.getGender() == Person.Sex.MALE
                    && p.getAge() >= 18
                    && p.getAge() <= 25;
            }
        }

        printPersons(
            roster, new CheckPersonEligibleForSelectiveService());


        System.out.println();

        // Approach 4: Specify Search Criteria Code in an Anonymous Class

        System.out.println("Persons who are eligible for Selective Service " +
            "(anonymous class):");

        printPersons(
            roster,
            new CheckPerson() {
                public boolean test(Person p) {
                    return p.getGender() == Person.Sex.MALE
                        && p.getAge() >= 18
                        && p.getAge() <= 25;
                }
            }
        );

        System.out.println();

        // Approach 5: Specify Search Criteria Code with a Lambda Expression

        System.out.println("Persons who are eligible for Selective Service " +
            "(lambda expression):");

        printPersons(
            roster,
            (Person p) -> p.getGender() == Person.Sex.MALE
                && p.getAge() >= 18
                && p.getAge() <= 25
        );

        System.out.println();

        // Approach 6: Use Standard Functional Interfaces with Lambda
        // Expressions

        System.out.println("Persons who are eligible for Selective Service " +
            "(with Predicate parameter):");

        printPersonsWithPredicate(
            roster,
            p -> p.getGender() == Person.Sex.MALE
                && p.getAge() >= 18
                && p.getAge() <= 25
        );

        System.out.println();

        // Approach 7: Use Lamba Expressions Throughout Your Application

        System.out.println("Persons who are eligible for Selective Service " +
            "(with Predicate and Consumer parameters):");

        processPersons(
            roster,
            p -> p.getGender() == Person.Sex.MALE
                && p.getAge() >= 18
                && p.getAge() <= 25,
            p -> p.printPerson()
        );

        System.out.println();

        // Approach 7, second example

        System.out.println("Persons who are eligible for Selective Service " +
            "(with Predicate, Function, and Consumer parameters):");

        processPersonsWithFunction(
            roster,
            p -> p.getGender() == Person.Sex.MALE
                && p.getAge() >= 18
                && p.getAge() <= 25,
            p -> p.getEmailAddress(),
            email -> System.out.println(email)
        );

        System.out.println();

        // Approach 8: Use Generics More Extensively

        System.out.println("Persons who are eligible for Selective Service " +
            "(generic version):");

        processElements(
            roster,
            p -> p.getGender() == Person.Sex.MALE
                && p.getAge() >= 18
                && p.getAge() <= 25,
            p -> p.getEmailAddress(),
            email -> System.out.println(email)
        );

        System.out.println();

        // Approach 9: Use Bulk Data Operations That Accept Lambda Expressions
        // as Parameters

        System.out.println("Persons who are eligible for Selective Service " +
            "(with bulk data operations):");

        roster
            .stream()
            .filter(
                p -> p.getGender() == Person.Sex.MALE
                    && p.getAge() >= 18
                    && p.getAge() <= 25)
            .map(p -> p.getEmailAddress())
            .forEach(email -> System.out.println(email));
     }
}