当要传递给Lambda体的操作，已经有实现的方法了，可以使用方法引用！
（实现抽象方法的参数列表，必须与方法引用方法的参数列表保持一致！）
方法引用：使用操作符 “ ::” 将方法名和对象或类的名字分隔开来。
如下三种主要使用情况 ：
 对象 :: 实例方法
 类 :: 静态方法
 类 :: 实例方法

案例

方法

  2. import e_Lambda表达式.entity.Employee;
  3. import org.junit.Test;
  4. import java.io.PrintStream;
  5. import java.util.Comparator;
  6. import java.util.function.*;
  8. /*
  9. 案例:ZJJ_JavaBasic_2019/10/30_11:23:46_thcfi
  10.  * 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
  11.  *               （可以将方法引用理解为 Lambda 表达式的另外一种表现形式）
  12.  *
  13.  * 1. 对象的引用 :: 实例方法名
  14.  *
  15.  * 2. 类名 :: 静态方法名
  16.  *
  17.  * 3. 类名 :: 实例方法名
  18.  *
  19.  * 注意：
  20.  *      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！
  21.  *      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
  22.  *
  23.  * 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！
  24.  *
  25.  * 1. 类名 :: new
  26.  *
  27.  * 三、数组引用
  28.  *
  29.  *     类型[] :: new;
  30.  *
  31.  *
  32.  */
  33. public class TestMethodRef {
  34.     //数组引用
  35.     @Test
  36.     public void test8() {
  37.         Function<Integer, String[]> fun = (args) -> new String[args];
  38.         String[] strs = fun.apply(10);
  39.         System.out.println(strs.length);
  41.         System.out.println("--------------------------");
  43.         Function<Integer, Employee[]> fun2 = Employee[]::new;
  44.         Employee[] emps = fun2.apply(20);
  45.         System.out.println(emps.length);
  46.     }
  48.     //构造器引用
  49.     @Test
  50.     public void test7() {
  51.         Function<String, Employee> fun = Employee::new;
  53.         BiFunction<String, Integer, Employee> fun2 = Employee::new;
  54.     }
  56.     @Test
  57.     public void test6() {
  58.         Supplier<Employee> sup = () -> new Employee();
  59.         System.out.println(sup.get());
  61.         System.out.println("------------------------------------");
  63.         Supplier<Employee> sup2 = Employee::new;
  64.         System.out.println(sup2.get());
  65.     }
  67.     //类名 :: 实例方法名
  68.     @Test
  69.     public void test5() {
  70.         BiPredicate<String, String> bp = (x, y) -> x.equals(y);
  71.         System.out.println(bp.test("abcde", "abcde"));
  73.         System.out.println("-----------------------------------------");
  75.         BiPredicate<String, String> bp2 = String::equals;
  76.         System.out.println(bp2.test("abc", "abc"));
  78.         System.out.println("-----------------------------------------");
  81.         Function<Employee, String> fun = (e) -> e.show();
  82.         System.out.println(fun.apply(new Employee()));
  84.         System.out.println("-----------------------------------------");
  86.         Function<Employee, String> fun2 = Employee::show;
  87.         System.out.println(fun2.apply(new Employee()));
  89.     }
  91.     //类名 :: 静态方法名
  92.     @Test
  93.     public void test4() {
  94.         Comparator<Integer> com = (x, y) -> Integer.compare(x, y);
  95.         int compare = com.compare(1, 2);
  96.         System.out.println("compare = " + compare);
  99.         System.out.println("-------------------------------------");
  101.         Comparator<Integer> com2 = Integer::compare;
  102.         int compare1 = com2.compare(1, 2);
  103.         System.out.println("compare1 = " + compare1);
  104.     }
  106.     @Test
  107.     public void test3() {
  108.         BiFunction<Double, Double, Double> fun = (x, y) -> Math.max(x, y);
  109.         System.out.println(fun.apply(1.5, 22.2));
  111.         System.out.println("--------------------------------------------------");
  113.         BiFunction<Double, Double, Double> fun2 = Math::max;
  114.         System.out.println(fun2.apply(1.2, 1.5));
  115.     }
  117.     //对象的引用 :: 实例方法名
  118.     @Test
  119.     public void test2() {
  120.         Employee emp = new Employee(101, "张三", 18, 9999.99);
  122.         Supplier<String> sup = () -> emp.getName();
  123.         System.out.println(sup.get());
  125.         System.out.println("----------------------------------");
  127.         Supplier<String> sup2 = emp::getName;
  128.         System.out.println(sup2.get());
  129.     }
  131.     @Test
  132.     public void test1() {
  133.         PrintStream ps = System.out;
  134.         //消费性接口
  135.         Consumer<String> con = (str) -> ps.println(str);
  136.         con.accept("Hello World！");
  138.         System.out.println("--------------------------------");
  140.         Consumer<String> con2 = ps::println;
  141.         con2.accept("Hello Java8！");
  143.         Consumer<String> con3 = System.out::println;
  144.         con3.accept("11111111");
  145.     }
  147. }

实体类


public class Employee {

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

    //****************
    public Employee() {
    }

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

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

    public Status getStatus() {
        return status;
    }

    public void setStatus(Status status) {
        this.status = status;
    }

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

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


    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 enum Status {
        FREE, BUSY, VOCATION;
    }
}