方法与构造器引用

1.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）

对象的引用 :: 实例方法名
类名 :: 静态方法名
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

 //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(com.compare(1, 2));

    System.out.println("-------------------------------------");

    Comparator<Integer> com2 = Integer::compare;
    System.out.println(com.compare(2, 2));
}

@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;
}

2.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

类名::new

//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());
}

3.数组引用

类型[] :: new

//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);
}