Java8 新特性_Lambda 表达式

1. Java8新特性_简介

Lambda 表达式
函数式接口
方法引用与构造器引用
Stream API
接口中的默认方法与静态方法
新时间日期 API
减少空指针异常的容器 Optional

2. Java8新特性_为什么使用 Lambda 表达式

减少实现功能的代码行数，增加源码的可读性。

导入项目代码后，发现有感叹号提示丢失jdk依赖的解决。

导入项目代码后，发现有源码中文注释乱码的解决。

Employee是一个实体类，具有id、name、age、salary成员属性，需求是实现根据一些限定条件对容器进行检索并打印输出。

public class Employee

package com.atguigu.java8;

public class Employee {

    private int id;
    private String name;
    private int age;
    private double salary;

    public Employee() {
    }

    public Employee(String name) {
        this.name = name;
    }

    public Employee(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public Employee(int id, String name, int age, double salary) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.salary = salary;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    public String show() {
        return "测试方法引用！";
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + id;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        long temp;
        temp = Double.doubleToLongBits(salary);
        result = prime * result + (int) (temp ^ (temp >>> 32));
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Employee other = (Employee) obj;
        if (age != other.age)
            return false;
        if (id != other.id)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
            return false;
        return true;
    }

    @Override
    public String toString() {
        return "Employee [id=" + id + ", name=" + name + ", age=" + age + ", salary=" + salary + "]";
    }

}

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public class TestLambda1

使用 Arrays.asList(…数组元素…）方法返回一个List接口的对象

//需求：获取公司中年龄小于 35 的员工信息

public List<Employee> filterEmployeeAge(List<Employee> emps){

public void test3(){

    List<Employee> emps = Arrays.asList(
            new Employee(101, "张三", 18, 9999.99),
            new Employee(102, "李四", 59, 6666.66),
            new Employee(103, "王五", 28, 3333.33),
            new Employee(104, "赵六", 8, 7777.77),
            new Employee(105, "田七", 38, 5555.55)
    );

    //需求：获取公司中年龄小于 35 的员工信息
    public List<Employee> filterEmployeeAge(List<Employee> emps){
        List<Employee> list = new ArrayList<>();
        
        for (Employee emp : emps) {
            if(emp.getAge() <= 35){
                list.add(emp);
            }
        }
        
        return list;
    }
    
    @Test
    public void test3(){
        List<Employee> list = filterEmployeeAge(emps);
        //增强for循环遍历集合
        for (Employee employee : list) {
            System.out.println(employee);
        }
    }

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//需求：获取公司中工资大于 5000 的员工信息

可以先对需求1调用方法得到结果list，然后在把结果list作为参数传到实现需求2的方法里。

新的需求产生后，又得重复大量的代码，这谁顶得住啊。组合数。

    //需求：获取公司中工资大于 5000 的员工信息
    public List<Employee> filterEmployeeSalary(List<Employee> emps){
        List<Employee> list = new ArrayList<>();
        
        for (Employee emp : emps) {
            if(emp.getSalary() >= 5000){
                list.add(emp);
            }
        }
        
        return list;
    }

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//优化方式一：策略设计模式

创建一个函数式接口 public interface MyPredicate<T> {

package com.atguigu.java8;

//抽象方法有且仅有一个
@FunctionalInterface
public interface MyPredicate<T> {
    
    boolean test(T t);
    
}

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public class FilterEmployeeForAge implements MyPredicate<Employee>{

package com.atguigu.java8;

public class FilterEmployeeForAge implements MyPredicate<Employee>{

    @Override
    public boolean test(Employee t) {
        return t.getAge() <= 35;
    }

}

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public class FilterEmployeeForSalary implements MyPredicate<Employee> {

package com.atguigu.java8;

public class FilterEmployeeForSalary implements MyPredicate<Employee> {

    @Override
    public boolean test(Employee t) {
        return t.getSalary() >= 5000;
    }

}

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编写一个抽取冗余代码的方法 public List<Employee> filterEmployee(List<Employee> emps, MyPredicate<Employee> mp){

用接口实例化对象的方法，替换硬编码在if(###)小括号里的判断条件。

    //优化方式一：策略设计模式
    public List<Employee> filterEmployee(List<Employee> emps, MyPredicate<Employee> mp){
        List<Employee> list = new ArrayList<>();
        
        for (Employee employee : emps) {
            if(mp.test(employee)){
                list.add(employee);
            }
        }
        
        return list;
    }
    
    @Test
    public void test4(){
        List<Employee> list = filterEmployee(emps, new FilterEmployeeForAge());
        for (Employee employee : list) {
            System.out.println(employee);
        }
        
        System.out.println("------------------------------------------");
        
        List<Employee> list2 = filterEmployee(emps, new FilterEmployeeForSalary());
        for (Employee employee : list2) {
            System.out.println(employee);
        }
    }

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//优化方式二：匿名内部类

//需求：获取公司中id小于等于 103 的员工信息

不用创建类.java文件，更进一步优化编码。

    //优化方式二：匿名内部类
    @Test
    public void test5(){
        List<Employee> list = filterEmployee(emps, new MyPredicate<Employee>() {
            @Override
            public boolean test(Employee t) {
                //需求：获取公司中id小于等于 103 的员工信息
                return t.getId() <= 103;
            }
        });
        
        for (Employee employee : list) {
            System.out.println(employee);
        }
    }

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//优化方式三：Lambda 表达式

    //优化方式三：Lambda 表达式
    @Test
    public void test6(){
        List<Employee> list = filterEmployee(emps, (e) -> e.getAge() <= 35);
        list.forEach(System.out::println);
        
        System.out.println("------------------------------------------");
        
        List<Employee> list2 = filterEmployee(emps, (e) -> e.getSalary() >= 5000);
        list2.forEach(System.out::println);
    }

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//优化方式四：Stream API

    //优化方式四：Stream API
    @Test
    public void test7(){
        emps.stream()
            .filter((e) -> e.getAge() <= 35)
            .forEach(System.out::println);
        
        System.out.println("----------------------------------------------");
        
        emps.stream()
            .map(Employee::getName)
            .limit(3)
            .sorted()
            .forEach(System.out::println);
    }

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3. Java8新特性_Lambda 基础语法

一、Lambda 表达式的基础语法：Java8中引入了一个新的操作符 "->"

Lambda 表达式的参数列表的数据类型可以省略不写，因为JVM编译器通过上下文推断出数据类型，即“类型推断”

语法格式一：无参数，无返回值
() -> System.out.println("Hello Lambda!");

    @Test
    public void test1(){
        int num = 0;//jdk 1.7 前，必须是 final的，现在可以省略
        
        Runnable r = new Runnable() {
            @Override
            public void run() {
                System.out.println("Hello World!" + num);
            }
        };        
        r.run();
        
        System.out.println("-------------------------------");
        
        Runnable r1 = () -> System.out.println("Hello Lambda!");
        r1.run();
    }

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语法格式二：有一个参数，并且无返回值

(x) -> System.out.println(x);

语法格式三：若只有一个参数，小括号可以省略不写
x -> System.out.println(x);

    @Test
    public void test2(){
        Consumer<String> con = x -> System.out.println(x);
        con.accept("我们程序员很威武！");
    }

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语法格式四：有两个以上的参数，有返回值，并且 Lambda 体中有多条语句

->右侧需要有大括号方法体

    @Test
    public void test3(){
        Comparator<Integer> com = (x, y) -> {
            System.out.println("函数式接口");
            return Integer.compare(x, y);
        };
    }

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语法格式五：若 Lambda 体中只有一条语句， return 和大括号都可以省略不写
Comparator<Integer> com = (x, y) -> Integer.compare(x, y);

    @Test
    public void test4(){
        Comparator<Integer> com = (x, y) -> Integer.compare(x, y);
    }

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二、Lambda 表达式需要“函数式接口”的支持

函数式接口：接口中只有一个抽象方法的接口，称为函数式接口。可以使用注解 @FunctionalInterface 修饰
可以检查是否是函数式接口

//需求：对一个数进行运算

先创建一个函数式接口

package com.atguigu.java8;

@FunctionalInterface
public interface MyFun {

    Integer getValue(Integer num);
    
}

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再用Lambda表达式实现接口

    //需求：对一个数进行运算
    @Test
    public void test6(){
        Integer num = operation(100, (x) -> x * x);
        System.out.println(num);
        
        System.out.println(operation(200, (y) -> y + 200));
    }
    
    public Integer operation(Integer num, MyFun mf){
        return mf.getValue(num);
    }

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4. Java8新特性_Lambda 练习

public class Employee {

package com.atguigu.java8;

public class Employee {

    private int id;
    private String name;
    private int age;
    private double salary;

    public Employee() {
    }

    public Employee(String name) {
        this.name = name;
    }

    public Employee(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public Employee(int id, String name, int age, double salary) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.salary = salary;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    public String show() {
        return "测试方法引用！";
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + id;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        long temp;
        temp = Double.doubleToLongBits(salary);
        result = prime * result + (int) (temp ^ (temp >>> 32));
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Employee other = (Employee) obj;
        if (age != other.age)
            return false;
        if (id != other.id)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        if (Double.doubleToLongBits(salary) != Double.doubleToLongBits(other.salary))
            return false;
        return true;
    }

    @Override
    public String toString() {
        return "Employee [id=" + id + ", name=" + name + ", age=" + age + ", salary=" + salary + "]";
    }

}

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问题1解答

List<Employee> emps = Arrays.asList(

Collections.sort(emps,实现了接口的Lambda表达式);

    List<Employee> emps = Arrays.asList(
            new Employee(101, "张三", 18, 9999.99),
            new Employee(102, "李四", 59, 6666.66),
            new Employee(103, "王五", 28, 3333.33),
            new Employee(104, "赵六", 8, 7777.77),
            new Employee(105, "田七", 38, 5555.55)
    );
    
    @Test
    public void test1(){
        Collections.sort(emps, (e1, e2) -> {
            //成员变量age是基本数据类型
            if(e1.getAge() == e2.getAge()){
                    return e1.getName().compareTo(e2.getName());
            }else{
               //return Integer.compare(e1.getAge(), e2.getAge());
                return -Integer.compare(e1.getAge(), e2.getAge());
            }
        });
        
        for (Employee emp : emps) {
            System.out.println(emp);
        }
    }

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问题2解答

先创建一个函数式接口

package com.atguigu.exer;

@FunctionalInterface
public interface MyFunction {
    
    public String getValue(String str);

}

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写一个方法输入上面的接口作第二个形参

    @Test
    public void test2(){
        String trimStr = strHandler("\t\t\t 我们程序员很威武   ", (str) -> str.trim());
        System.out.println(trimStr);
        
        String upper = strHandler("abcdef", (str) -> str.toUpperCase());
        System.out.println(upper);
        
        String newStr = strHandler("我们程序员很威武", (str) -> str.substring(2, 5));
        System.out.println(newStr);
    }
    
    //需求：用于处理字符串
    public String strHandler(String str, MyFunction mf){
        return mf.getValue(str);
    }

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问题3解答

public interface MyFunction2<T, R> {

package com.atguigu.exer;

public interface MyFunction2<T, R> {

    public R getValue(T t1, T t2);
    
}

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public void op(Long l1, Long l2, MyFunction2<Long, Long> mf){

    @Test
    public void test3(){
        op(100L, 200L, (x, y) -> x + y);
        
        op(100L, 200L, (x, y) -> x * y);
    }
    
    //需求：对于两个 Long 型数据进行处理
    public void op(Long l1, Long l2, MyFunction2<Long, Long> mf){
        System.out.println(mf.getValue(l1, l2));
    }

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5. Java8新特性_四大内置核心函数式接口

public class TestLambda3 {

package com.atguigu.java8;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

import org.junit.Test;

/*
 * Java8 内置的四大核心函数式接口
 * 
 * Consumer<T> : 消费型接口
 *         void accept(T t);
 * 
 * Supplier<T> : 供给型接口
 *         T get(); 
 * 
 * Function<T, R> : 函数型接口
 *         R apply(T t);
 * 
 * Predicate<T> : 断言型接口
 *         boolean test(T t);
 * 
 */
public class TestLambda3 {
    
    //Predicate<T> 断言型接口：
    @Test
    public void test4(){
        List<String> list = Arrays.asList("Hello", "atguigu", "Lambda", "www", "ok");
        List<String> strList = filterStr(list, (s) -> s.length() > 3);
        
        for (String str : strList) {
            System.out.println(str);
        }
    }
    
    //需求：将满足条件的字符串，放入集合中
    public List<String> filterStr(List<String> list, Predicate<String> pre){
        List<String> strList = new ArrayList<>();
        
        for (String str : list) {
            if(pre.test(str)){
                strList.add(str);
            }
        }
        
        return strList;
    }
    
    //Function<T, R> 函数型接口：
    @Test
    public void test3(){
        String newStr = strHandler("\t\t\t 我大尚硅谷威武   ", (str) -> str.trim());
        System.out.println(newStr);
        
        String subStr = strHandler("我大尚硅谷威武", (str) -> str.substring(2, 5));
        System.out.println(subStr);
    }
    
    //需求：用于处理字符串
    public String strHandler(String str, Function<String, String> fun){
        return fun.apply(str);
    }
    
    //Supplier<T> 供给型接口 :
    @Test
    public void test2(){
        List<Integer> numList = getNumList(10, () -> (int)(Math.random() * 100));
        
        for (Integer num : numList) {
            System.out.println(num);
        }
    }
    
    //需求：产生指定个数的整数，并放入集合中
    public List<Integer> getNumList(int num, Supplier<Integer> sup){
        List<Integer> list = new ArrayList<>();
        
        for (int i = 0; i < num; i++) {
            Integer n = sup.get();
            list.add(n);
        }
        
        return list;
    }
    
    //Consumer<T> 消费型接口 :
    @Test
    public void test1(){
        happy(10000, (m) -> System.out.println("你们刚哥喜欢大宝剑，每次消费：" + m + "元"));
    } 
    
    public void happy(double money, Consumer<Double> con){
        con.accept(money);
    }
}

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6. Java8新特性_方法引用与构造器引用

* 一、方法引用：若 Lambda 体中的功能，已经有方法提供了实现，可以使用方法引用
* （可以将方法引用理解为 Lambda 表达式的另外一种表现形式）
*
* 1. 对象的引用 :: 实例方法名
*
* 2. 类名 :: 静态方法名
*
* 3. 类名 :: 实例方法名

package com.atguigu.java8;

import java.io.PrintStream;
import java.util.Comparator;
import java.util.function.BiFunction;
import java.util.function.BiPredicate;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;

import org.junit.Test;

/*
 * 一、方法引用：若 Lambda 体中的功能，已经有方法提供了实现，可以使用方法引用
 *                   （可以将方法引用理解为 Lambda 表达式的另外一种表现形式）
 * 
 * 1. 对象的引用 :: 实例方法名
 * 
 * 2. 类名 :: 静态方法名
 * 
 * 3. 类名 :: 实例方法名
 * 
 * 注意：
 *      ①方法引用所引用的方法的参数列表与返回值类型，需要与函数式接口中抽象方法的参数列表和返回值类型保持一致！
 *      ②若Lambda 的参数列表的第一个参数，是实例方法的调用者，第二个参数(或无参)是实例方法的参数时，格式： ClassName::MethodName
 * 
 * 二、构造器引用 :构造器的参数列表，需要与函数式接口中参数列表保持一致！
 * 
 * 1. 类名 :: new
 * 
 * 三、数组引用
 * 
 *     类型[] :: new;
 * 
 * 
 */
public class TestMethodRef {
    //数组引用
    @Test
    public void test8(){
        Function<Integer, String[]> fun = (args) -> new String[args];
        String[] strs = fun.apply(10);
        System.out.println(strs.length);
        
        System.out.println("--------------------------");
        
        Function<Integer, Employee[]> fun2 = Employee[] :: new;
        Employee[] emps = fun2.apply(20);
        System.out.println(emps.length);
    }
    
    //构造器引用
    @Test
    public void test7(){
        Function<String, Employee> fun = Employee::new;
        
        BiFunction<String, Integer, Employee> fun2 = Employee::new;
    }
    
    @Test
    public void test6(){
        Supplier<Employee> sup = () -> new Employee();
        System.out.println(sup.get());
        
        System.out.println("------------------------------------");
        
        Supplier<Employee> sup2 = Employee::new;
        System.out.println(sup2.get());
    }
    
    //类名 :: 实例方法名
    @Test
    public void test5(){
        BiPredicate<String, String> bp = (x, y) -> x.equals(y);
        System.out.println(bp.test("abcde", "abcde"));
        
        System.out.println("-----------------------------------------");
        
        BiPredicate<String, String> bp2 = String::equals;
        System.out.println(bp2.test("abc", "abc"));
        
        System.out.println("-----------------------------------------");
        
        
        Function<Employee, String> fun = (e) -> e.show();
        System.out.println(fun.apply(new Employee()));
        
        System.out.println("-----------------------------------------");
        
        Function<Employee, String> fun2 = Employee::show;
        System.out.println(fun2.apply(new Employee()));
        
    }
    
    //类名 :: 静态方法名
    @Test
    public void test4(){
        Comparator<Integer> com = (x, y) -> Integer.compare(x, y);
        
        System.out.println("-------------------------------------");
        
        Comparator<Integer> com2 = Integer::compare;
    }
    
    @Test
    public void test3(){
        BiFunction<Double, Double, Double> fun = (x, y) -> Math.max(x, y);
        System.out.println(fun.apply(1.5, 22.2));
        
        System.out.println("--------------------------------------------------");
        
        BiFunction<Double, Double, Double> fun2 = Math::max;
        System.out.println(fun2.apply(1.2, 1.5));
    }

    //对象的引用 :: 实例方法名
    @Test
    public void test2(){
        Employee emp = new Employee(101, "张三", 18, 9999.99);
        
        Supplier<String> sup = () -> emp.getName();
        System.out.println(sup.get());
        
        System.out.println("----------------------------------");
        
        Supplier<String> sup2 = emp::getName;
        System.out.println(sup2.get());
    }
    
    @Test
    public void test1(){
        PrintStream ps = System.out;
        Consumer<String> con = (str) -> ps.println(str);
        con.accept("Hello World！");
        
        System.out.println("--------------------------------");
        
        Consumer<String> con2 = ps::println;
        con2.accept("Hello Java8！");
        
        Consumer<String> con3 = System.out::println;
    }
    
}

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来源：https://www.cnblogs.com/MarlonKang/p/12357202.html

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