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 + "]";

    }

}

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);

        }

    }

//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;

    }

//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);

}

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;

    }

}

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;

    }

}

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);

        }

    }

//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);

        }

    }

//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);

    }

//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);

    }

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();

    }

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！");

    }

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);

        };

    }

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);

    }

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);

}

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);

    }

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 + "]";

    }

}

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);

        }

    }

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);

}

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);

    }

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);

}

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));

    }

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);

    }

}

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;

    }

}

巴特西

Java8 新特性_Lambda 表达式

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