为什么要写这样的迭代器呢？ [英] Why bother to write an iterator like this?

问题描述

我正在阅读关于内部类的Java教程

I was reading through the Java tutorial on inner class

http://java.sun.com/docs/books/tutorial/java/javaOO/innerclasses.html

它解释了在示例InnerEvenIterator内部类，它类似于标准的Java迭代器。所以我认为迭代器在Java中很常见？

It explains that in the example "The InnerEvenIterator inner class, which is similar to a standard Java iterator." So I take it that iterators are pretty common in Java?

我来自C编程背景。我不明白为什么这样的简单循环

I came from a C programming background. I don't understand why a simple loop like this

for(i=0;i <SIZE;i+2){
System.System.out.println(arrayOfInts[i]));
}

使用两种方法扩展到迭代器（内部类）。这里有什么意义？

got expanded to an iterators (inner class) with two methods. What's the point here?

public class DataStructure {
    //create an array
    private final static int SIZE = 15;
    private int[] arrayOfInts = new int[SIZE];

    public DataStructure() {
        //fill the array with ascending integer values
        for (int i = 0; i < SIZE; i++) {
            arrayOfInts[i] = i;
        }
    }

    public void printEven() {
        //print out values of even indices of the array
        InnerEvenIterator iterator = this.new InnerEvenIterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.getNext() + " ");
        }
    }

//inner class implements the Iterator pattern
    private class InnerEvenIterator {
        //start stepping through the array from the beginning
        private int next = 0;

        public boolean hasNext() {
            //check if a current element is the last in the array
            return (next <= SIZE - 1);
        }

        public int getNext() {
            //record a value of an even index of the array
            int retValue = arrayOfInts[next];
            //get the next even element
            next += 2;
            return retValue;
        }
    }

    public static void main(String s[]) {
        //fill the array with integer values and print out only values of even indices
        DataStructure ds = new DataStructure();
        ds.printEven();
    }
}

推荐答案

对数组进行简单循环，你不会（通常）在Java中使用Iterator。

For simple loops over arrays, you would not (usually) use an Iterator in Java.

for(int i=0;i < arrayOfInts.length ; i+2){
     System.out.println(arrayOfInts[i]));
}

迭代器的想法是解耦数据的存储方式（可能不会来自其使用者的代码（想要迭代它的代码）。

The idea of an iterator is to decouple how the data is stored (might not be an array) from its consumer (the code that wants to iterate over it).

你说Iterator是Java类库中一个非常核心的概念是正确的，从Java5开始就有这样的常见，就是支持它的for-each循环语言功能。使用此循环，用户甚至不再看到Iterator。

You are right to say that Iterator is a pretty core concept in the Java class library, so common that from Java5 on there is the for-each loop language feature to support it. With this loop, the user does not even see the Iterator anymore.

for(Something element: listOfSomething){
    System.out.println(element);
}

如果我要实现偶数步进迭代器，我会以它为基础在普通迭代器上，因此它可以与任何类型的迭代一起使用。

If I were to implement an "even-stepping iterator" I would base it on the normal iterator, so that it can be used with any kind of iterable.

public class EvenSteppingIterator<X> implements Iterator<X>{

      private final Iterator<X> source;

      public EvenSteppingIterator(Iterator<X> source){
          this.source = source;
          // skip the first one, start from the second
          if (source.hasNext()) source.next();
      }

      public boolean hasNext() {
          return source.hasNext();
      }

      public X next(){
          X n = source.next();
          // skip the next one
          if (source.hasNext()) source.next();
          return n;
      }


}

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