Java迭代器和for-each-loop。 任何方式访问底层迭代器？

Question

I very much like the for-each-loop construction ( for(T e : iterable) ) in Java which works on any Iterable<T> because it makes in many cases very easy to read and to write code.

I wonder though if there is any way that I can access the underlying iterator from such a loop. This can be mandatory if I want to use the remove() from the iterator.

Answer 1

No, the whole point of the for-each-loop is to abstract away the underlying iterator.

If you need it, you must declare it.

Answer 2

No, you cannot remove objects in a for each loop.

Use this instead:

Iterator<Type> it = collection.iterator();

while (it.hasNext()) {
   if (it.next().shouldBeRemoved()) {
       it.remove();
   }
}

Answer 3

请改用for循环。

Answer 4

If the collection is reasonably small, you can alternatively use a copy of the collection to iterate if you want to be able to remove elements so you won't have to act as if you have two collections.

for(T e : collection.clone())
    if(e.shouldBeRemoved())
        collection.remove();

Even better, Apache CollectionUtils (and there is probably a Google alternative and a generics alternative) provides filter(java.util.Collection collection, Predicate predicate) . This example returns the whole list. You can store a predicate for reuse.

CollectionUtils.filter(collection, new Predicate(){
        boolean evaluate(Object object){return true;}
    });

Answer 5

You are correct, using an Iterator supports the ability to remove an object from a source collection safely, by calling remove() on the Iterator itself. The point here is to avoid a ConcurrentModifiedException which implies that a collection was modified while an Iterator was open against it. Some collections will let you get away with removing or adding elements to a Collection while iterating across it, but calling remove() on the Iterator is a safer practice.

But Iterator supports a derived and more powerful cousin ListIterator , only available from Lists, supports both adding and removing from a List during iteration, as well as bidirectional scrolling through Lists.

Answer 6

修改数据有一种错误的感觉，只需创建一个新的Iterable（或集合）并添加你想要保留的项目，其他时间在for循环中丢失（（有点**）