public class OutClass {

public void outFunc() {
// java8以前的寫法
// final String outFuncVar = "outFuncVar";
// java8以後可以這樣寫
String outFuncVar = "outFuncVar";

@SuppressWarnings("unused")
class NestClass {

public void NestFunc() {
// 開啟後報錯
// Local variable outFuncVar defined in an enclosing scope must be final or effectively final
// outFuncVar = "重新賦值";
System.out.println(outFuncVar);
}
}
}

// 特例
public static void outFunc2() {
final int k = 1;
// 下面這種寫法依然報錯
// int k = 1;
@SuppressWarnings("unused")
class LocalClass {

public void LocalFunc() {
int m = 1;
switch (m) {
case k:
break;
}
}
}
}
}
 

1、Java8引入一個effectively final的概念，當內部類訪問外部類變數時，只要內部類不對外部變數重新賦值，那麼外部類變數就是一個事實上的final變數，Java會隱含地將外部變數宣告為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.
 

effectively final有一個特殊例子，參考上面的outFunc2

參考