迭代<？> vs java中的Iterator [英] Iterator<?> vs Iterator in java

问题描述

可能重复：

什么是原始类型，为什么我们不应该使用它？

这个问题几乎完全包含在描述中：

The question is pretty much self contained in the description: what is the difference between

Iterator<?>

和

Iterator

java中的对象？它们都在对象类型上进行迭代吗？

objects in java? They both iterate over Object types right?

推荐答案

不。 Iterator<？> 迭代？ s（某种类型派生自 Object ），而 Iterator 仅提供对象。这是Java中的泛型支持。

Nope. Iterator<?> iterates over ?s (some type that derives from Object), while Iterator only provides Objects. This is the generics support in Java.

什么是？类型？嗯，这是一个很好的问题。

What's a ? type? Well, that's a great question.

回到Java 5之前的旧时代，你有无类型的集合和无类型的迭代器。所以你的集合中有 Object s，你必须手动投射它们。例如：

Back in the bad old days before Java 5, you had untyped collections and untyped iterators. So you'd have Objects in your collections and you'd have to cast them manually. For example:

List foo = new ArrayList();
foo.add("Foo!");

for(Iterator i = foo.iterator(); i.hasNext(); )
{
    Object element = i.next();

    System.out.println((String)element);
}

请注意演员表？那是因为我们只是将 Object 填入列表中，我们必须自己做所有的内务处理。仅仅列表就不那么糟了，但想象一下包含地图的地图，其中包含一个包含......的地图，你明白了。你正在施展地狱，你很快就不知道你是否正在查看从 String 到整数或您的其他地图从 Integer 向后转移到 String 。

Notice that cast? That's because we're just stuffing Objects into lists, and we have to do all that housekeeping ourselves. Not so bad with just a List, but imagine a map that contains a map that contains a map that contains a ... well, you get the idea. You're in casting hell and you can pretty quickly not know whether you're looking at a map that goes from String to Integer or your other map that goes backwards from Integer to String.

在Java 5中，你可以使用泛型，所以现在你可以说：

In Java 5, you can use generics, so now you can say:

List<String> foo = new ArrayList();
foo.add("Foo!");

for(Iterator<String> i = foo.iterator(); i.hasNext(); )
{
    String element = i.next();

    System.out.println(element);
}

请注意，演员表是不必要的？当我们使用 List< String> 时，我们可以使用 Iterator< String> 并具有一些类型安全性。

Note that the cast was unnecessary? When we use a List<String>, we can use an Iterator<String> and have some type safety.

当您开始处理更复杂的数据结构时，这非常有用，例如：

This is really beneficial when you start doing things with more complex data structures, like:

Map<String, Map<String, Map<String, SomeRandomObject>>> complexMap;

这篇关于迭代<？> vs java中的Iterator的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！