Java Lambda表达式学习-1

背景介绍

• Lambda是JAVA SE8中的一个重要的新特性,允许使用表达式来代替功能接口,提供了一个正常的参数列表和一个使用这些参数的主题(可以是一个表达式或者一个代码块).例如代替匿名内部类,Runnable.

基本语法:

(parameters)->expression    /*表达式*/
(parameters)->{statements}  /*代码块*/
/-------简单示例------------/
()->a  //不需要参数,返回a
a->2*a //接受一个参数(int类型),返回其与2的乘积,一个参数已不写括号
(x, y) ->x-y //接受x,y参数,返回两者的差,虽然编译器会去自动推导类型,但最好还是自己将类型加上

• 简单例子,Lambda表达式代替new 匿名内部类:

public interface TestLambda {
    void test(int x);
}

public class TestLambdaImpl {
    public void func(TestLambda test) {
        test.test(10);
    }
}

public void testFunc() {
    TestLambdaImpl t = new TestLambdaImpl();
    //普通匿名内部类写法
    t.func(new TestLambda() {
        @Override
        public void test(int x) {
            System.out.println(x);
        }
    });
    //Lambda表达式,实现了接口的方法,变得更加简洁,等于在在此处就实现了接口的方法
    t.func((int x)->System.out.println(x));//表达式
    t.func((int x)->{
        System.out.println(x);
        System.out.println("I am your father!");});//代码块
}

• 例子2 for循环

String[] atp = {"Rafael Nadal", "Novak Djokovic",  
       "Stanislas Wawrinka",  
       "David Ferrer","Roger Federer",  
       "Andy Murray","Tomas Berdych",  
       "Juan Martin Del Potro"};
List list = Arrays.asList(atp);
//以前的循环方式
for(String player : list) {
    System.out.println(player);
}

//Lambda表达式
list.forEach((String s)->System.out.println(s));

• 例子3:Runnable接口

//使用匿名内部类
new Thread(newRunnbale() {
    @Override
    public void run() {
        System.out.println("hello world");
    }
});

Runnable r1 = new Runnable() {
    @Override
    public void run() {
        System.out.println("hello world");
    }
};

//Lambda表达式
new Thread(()->System.out.println("hello world"));

Runnable r2 = ()->System.out.println("hello world");
new Thread(r2);

• 例子4:集合的排序

String[] players =  {"Rafael Nadal", "Novak Djokovic",  
       "Stanislas Wawrinka",  
       "David Ferrer","Roger Federer",  
       "Andy Murray","Tomas Berdych",  
       "Juan Martin Del Potro"};
//匿名内部类排序
Arrays.sort(players, new Comparator() {
    @Override
    public int compare(String s1, String s2) {
        return (s1.compareTo(s2));//从大到小的排序
    }
});
//Lambda表达式
Comparator comparator = (String s1, String s2) -> s1.compareTo(s2);
Arrays.sort(player, comparator);
Arrays.sort(players, (String s1, String s2) -> return s1.compareTo(s2));

使用Lambdas和Streams

• Stream是对集合的包装,通常与Lambda一起使用,支持多种操作,如 map, filter, limit, sorted, count, min, max, sum, collect 等等。
• 代码

public class Person {
    private String firstName, lastName, job, gender;
    private int salary, age;
    public Person(String firstName, String lastName, String job, String gender, int age, int salary) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.job = job;
        this.gender = gender;
        this.age = age;
        this.salary = salary;
    }
}

List javaProgrammers = new ArrayList() {  
  {  
    add(new Person("Elsdon", "Jaycob", "Java programmer", "male", 43, 2000));  
    add(new Person("Tamsen", "Brittany", "Java programmer", "female", 23, 1500));  
    add(new Person("Floyd", "Donny", "Java programmer", "male", 33, 1800));  
    add(new Person("Sindy", "Jonie", "Java programmer", "female", 32, 1600));  
    add(new Person("Vere", "Hervey", "Java programmer", "male", 22, 1200));  
    add(new Person("Maude", "Jaimie", "Java programmer", "female", 27, 1900));  
    add(new Person("Shawn", "Randall", "Java programmer", "male", 30, 2300));  
    add(new Person("Jayden", "Corrina", "Java programmer", "female", 35, 1700));  
    add(new Person("Palmer", "Dene", "Java programmer", "male", 33, 2000));  
    add(new Person("Addison", "Pam", "Java programmer", "female", 34, 1300));  
  }  
}; 

List phpProgrammers = new ArrayList() {  
  {  
    add(new Person("Jarrod", "Pace", "PHP programmer", "male", 34, 1550));  
    add(new Person("Clarette", "Cicely", "PHP programmer", "female", 23, 1200));  
    add(new Person("Victor", "Channing", "PHP programmer", "male", 32, 1600));  
    add(new Person("Tori", "Sheryl", "PHP programmer", "female", 21, 1000));  
    add(new Person("Osborne", "Shad", "PHP programmer", "male", 32, 1100));  
    add(new Person("Rosalind", "Layla", "PHP programmer", "female", 25, 1300));  
    add(new Person("Fraser", "Hewie", "PHP programmer", "male", 36, 1100));  
    add(new Person("Quinn", "Tamara", "PHP programmer", "female", 21, 1000));  
    add(new Person("Alvin", "Lance", "PHP programmer", "male", 38, 1600));  
    add(new Person("Evonne", "Shari", "PHP programmer", "female", 40, 1800));  
  }  
}; 

//lambda 表达式
//forEach
javaProgrammers.forEach((List p)->System.out.println("FirstName:"p.getFirstName() + ", LastName:" + p.getLastName()));

phpProgrammers.forEach((List p)->System.out.println("FirstName:"p.getFirstName() + ", LastName:" + p.getLastName()));

//过滤器filter
phpProgrammers.stream().filter((List p)->(p->getSalary()>1000))
                                          .forEach((List p)->System.out.println("FirstName:"p.getFirstName() + ", LastName:" + p.getLastName()));

//限制器limit
phpProgrammers.stream().filter((List p)->(p->getSalary()>1000))
                                          .limit(3)
                                          .forEach((List p)->System.out.println("FirstName:"p.getFirstName() + ", LastName:" + p.getLastName()));