在jdk 1.8开始出现lambda 表达式，而双冒号:: 和lambda表达式一样都是简化写法，可以相互转化。

类名::方法名
这种双冒号语法糖是最常见的：
例如：

1. x -> System.out.println(x) 可以替换成 System.out::println
   2.user-> user.getName();可以替换成User::getName
   就是和lambda表达时互换

this::方法名 ，本质上是对函数型接口的简化
这里的this代表当前对象，调用当前对象的方法，例如如下

package function;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test {
    private void test1() {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        executorService.execute(this::testPrint);
    }
    private void testPrint() {
        System.out.println("kang");
    }
}

然在我们看下是怎么一步步简化成this::testPrint
1.executorService.execute 需要的是一个Runnable对象，因此可以写个匿名内部类

package function;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test {
    private void test1() {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
//        executorService.execute(this::testPrint);
        executorService.execute(new Runnable() {
            @Override
            public void run() {
                System.out.println("kang");
            }
        });
    }
}

2.在Ruunbale接口是个函数式接口，因此可以用lambda表达式简化：

package function;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test {
    private void test1() {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
//        executorService.execute(this::testPrint);
        executorService.execute(()->System.out.println("kang"));
    }
}

3.抽出方法: testPrint()

package function;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test {
    private void test1() {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
//        executorService.execute(this::testPrint);
        executorService.execute(()->testPrint());
    }


    private void testPrint() {
        System.out.println("kang");
    }
}

4.最终版本：

package function;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test {
    private void test1() {
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        executorService.execute(this::testPrint);
    }
    private void testPrint() {
        System.out.println("kang");
    }
}