英文:
why the java generic type is a must when I implement Interator<E>
问题
以下是翻译好的部分：
我写了这段代码，它运行良好。但我有一个问题：为什么它应该是 ```public class PeekingIterator<E> implements Iterator<E>```，而不是```public class PeekingIterator implements Iterator<E>```。我之所以问这个问题，是因为如果我在代码中将 E 替换为 Integer，它也能工作。我的意思是 ```public class PeekingIterator implements Iterator<Integer>``` 是正确的。import java.util.Iterator;public class PeekingIterator<E> implements Iterator<E> {private E nextElem;private boolean hasNextElem;private Iterator<E> iter;public PeekingIterator(Iterator<E> iterator) {iter = iterator;nextElem = next();hasNextElem = true;}public E peek() {return nextElem;}@Overridepublic E next() {if (!hasNextElem) {throw new RuntimeException();}E res = nextElem;if (hasNext()) {nextElem = iter.next();hasNextElem = true;} else {hasNextElem = false;}return res;}@Overridepublic boolean hasNext() {return hasNextElem;}}
英文:
I wrote this piece of code, and it works fine. but I have a question:
why it should be public class PeekingIterator<E> implements Iterator<E>, not
public class PeekingIterator implements Iterator<E>. I ask this because if I replace E with Integer in the code, it will work. I mean public class PeekingIterator implements Iterator<Integer> is correct.
import java.util.Iterator;public class PeekingIterator<E> implements Iterator<E> {private E nextElem;private boolean hasNextElem;private Iterator<E> iter;public PeekingIterator(Iterator<E> iterator) {iter = iterator;nextElem = next();hasNextElem = true;}public E peek() {return nextElem;}@Overridepublic E next() {if (!hasNextElem) {throw new RuntimeException();}E res = nextElem;if (hasNext()) {nextElem = iter.next();hasNextElem = true;} else {hasNextElem = false;}return res;}@Overridepublic boolean hasNext() {return hasNextElem;}}
答案1
得分: 4
E 是一个标识符。如果你声明 class PeekingIterator<E> implements Iterator<E>，编译器就知道这个标识符是一个泛型类型参数。
这使你能够实例化 PeekingIterator 如下：
PeekingIterator<Integer> it1 = new PeekingIterator<>();
或者
PeekingIterator<String> it2 = new PeekingIterator<>();
也就是说，你的 PeekingIterator 类可以迭代任何类型的元素。
如果你将 PeekingIterator 声明为 class PeekingIterator implements Iterator<E>，编译器会查找名为 E 的某种类型（类或接口）。如果找不到，就会导致编译错误。
如果你将 class PeekingIterator implements Iterator<Integer> 这样声明，这是有效的，因为 Integer 是一个类。这会限制你的 PeekingIterator 始终只能迭代 Integer 元素。
英文:
E is an identifier. If you say that class PeekingIterator<E> implements Iterator<E>, the compiler knows that this identifier is a generic type parameter.
This allows you to instantiate PeekingIterator as:
PeekingIterator<Integer> it1 = new PeekingIterator<>();
or
PeekingIterator<String> it2 = new PeekingIterator<>();
i.e. your PeekingIterator class can iterate over any type of elements.
If you declare PeekingIterator as class PeekingIterator implements Iterator<E>, the compiler searches for some type (class or interface) named E. If it doesn't find it, that's a compilation error.
If you declare class PeekingIterator implements Iterator<Integer>, this works, since Integer is a class. This limits your PeekingIterator to always iterate over Integer elements.
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