public class OutClass {
 
 public void outFunc() {
 // java8以前的写法
 // final String outFuncVar = "outFuncVar";
 // java8以后可以这样写
 String outFuncVar = "outFuncVar";
 
 @SuppressWarnings("unused")
 class NestClass {
 
 public void NestFunc() {
 // 开启后报错
 // Local variable outFuncVar defined in an enclosing scope must be final or effectively final
 // outFuncVar = "重新赋值";
 System.out.println(outFuncVar);
 }
 }
 }
 
 // 特例
 public static void outFunc2() {
 final int k = 1;
 // 下面这种写法依然报错
 // int k = 1;
 @SuppressWarnings("unused")
 class LocalClass {
 
 public void LocalFunc() {
 int m = 1;
 switch (m) {
 case k:
 break;
 }
 }
 }
 }
}

1、Java8引入一个effectively final的概念，当内部类访问外部类变量时，只要内部类不对外部变量重新赋值，那么外部类变量就是一个事实上的final变量，Java会隐含地将外部变量声明为final。

However, starting in Java SE 8, a local class can access local variables and parameters of the enclosing block that are final or effectively final. 
A variable or parameter whose value is never changed after it is initialized is effectively final.

2、

effectively final有一个特殊例子，参考上面的outFunc2

参考

https://docs.oracle.com/javase/tutorial/java/javaOO/localclasses.html#accessing-members-of-an-enclosing-class

https://stackoverflow.com/questions/20938095/difference-between-final-and-effectively-final