您不能将int用作变量，因为必须是最终的才能在流中使用。

You can't use an int as variable because it must be final to be used in a stream.

但是你可以创建一个包装int的类。

But You can create a class wrapping the int.

然后声明保持此类为最终的变量。

Then declare the variable holding this class as final.

更改内部int变量的内容。

Changing the content of the inner int variable.

public void findMax(List<List<Route>> routeLists) {
        final IntWrapper dWrapper = new IntWrapper();
        routeLists.forEach(e-> {
            e.forEach(ee -> {
                dWrapper.value += ee.getDistance();    
            });

        });

        int d = dWrapper.value;

        ... doing some other operation with d
    }

 public class IntWrapper {
    public int value;
 }