for-each循环只能在java.lang.Iterable的数组或实例上进行迭代 [英] for-each loop can only iterate over an array or an instance of java.lang.Iterable

问题描述

以下代码不起作用.此代码有什么问题?编译器在for循环中抱怨 NumberList 不是 Iterable 类.

The following code doesn't work. What's wrong with this code? Compiler complains in the for loop that NumberList isn't a Iterable class.

在for-each循环中可以使用哪种类?如何使 NumberList 可迭代?我尝试使 NumberList实现Iterable ，但是它似乎不起作用，因为我不知道如何正确定义Iterator.

What kind of class can be used in for-each loop? How to make NumberList iterable? I tried making NumberList implement Iterable, but it doesn't seem to work because I don't know how to define the Iterator properly.

如果有人可以演示如何使此代码有效，或者将我链接到很棒的教程.

If someone could demonstrate how to make this code work, or link me to a tutorial that'd be great.

public class Test{
    public class NumberList{
        private int numItems;
        private Number[] numbers;

        public NumberList(int size){
            this.numbers = new Number[size];
            this.numItems=0;
        }

        public void add(Number n){
            this.numbers[this.numItems++]=n;
        }
    }

    public void printPairs() {
        ArrayList<Integer> num=new ArrayList<Integer>();

        NumberList numbers = new NumberList(50);
        numbers.add(4);
        numbers.add(5);
        numbers.add(6);

        for(Number n1: numbers){
            System.out.println(n1);
        }
    }
}

推荐答案

NumberList没有实现Iterable.就编译器而言，它只是任何其他类.

NumberList does not implement Iterable. As far as the compiler is concerned its just any other class.

您需要执行类似的操作

public class NumberList implements Iterable<Number> {

    private int numItems;
    private Number[] numbers;

    public NumberList(int size) {
        this.numbers = new Number[size];
        this.numItems = 0;
    }

    public void add(Number n) {
        this.numbers[this.numItems++] = n;
    }

    @Override
    public Iterator<Number> iterator() {
        return Arrays.asList(numbers).subList(0, numItems).iterator();
    }
}

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