for-each 循环只能遍历数组或 java.lang.Iterable 的实例

for-each loop can only iterate over an array or an instance of java.lang.Iterable

提问by turtlesoup

The following code doesn't work. What's wrong with this code? Compiler complains in the for loop that NumberListisn't a Iterableclass.

以下代码不起作用。这段代码有什么问题？编译器在NumberList不是Iterable类的 for 循环中抱怨。

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

在 for-each 循环中可以使用什么样的类？如何使NumberList可迭代？我试过 make NumberList implement Iterable，但它似乎不起作用，因为我不知道如何正确定义 Iterator。

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

如果有人可以演示如何使此代码工作，或者将我链接到一个很棒的教程。

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);
        }
    }
}

采纳答案by monkHyman

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

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

You need to do something like this

你需要做这样的事情

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();
    }
}

回答by DaoWen

You need to implementthe Iterable interface. In this case, that just means you need to add a method Iterator<T> iterator()to your NumberListclass. Since your list only contains numbers in this case, the generic type parameter Tis just Number.

你需要实现的可迭代接口。在这种情况下，你需要添加一个方法只是意味着Iterator<T> iterator()你的NumberList类。由于您的列表在这种情况下仅包含数字，因此泛型类型参数T只是Number.

回答by amatellanes

Your class NumberListneed to implement the Iterableinterface:

你的类NumberList需要实现Iterable接口：

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;

public class Test {
    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).iterator();
        }

    }

    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);
        }
    }
}