stream.foreach中的java 8局部变量[重复]

Question

我想在lambda函数中使用局部变量，但我得到错误：请参见1。和2。代码中的点。

class Foo {
    int d = 0; // 1. It compiles, but ugly, 2. doesnt compile
    public void findMax(List<List<Route>> routeLists) {
        int d = 0; // 2.Error : Local variable dd defined in an enclosing scope must be final or effectively final
        routeLists.forEach(e-> {
            e.forEach(ee -> {
                d+=ee.getDistance();    
            });

        });
        ... doing some other operation with d
    }
}

Answer 1

foreach是错误的作业工具。

int d =
    routeLists.stream()                // Makes a Stream<List<Route>>
        .flatMap(Collection::stream)   // Makes a Stream<Route>
        .mapToInt(Route::getDistance)  // Makes an IntStream of distances
        .sum();

或者只使用嵌套for循环：

int d = 0;
for (List<Route> rs : routeLists) {
  for (Route r : rs) {
    d += r.getDistance();
  }
}