Question

我正试图在新类中使用for循环我创建了一个简单的java程序，它将从一个数字一直打印到另一个。我似乎无法掌握for循环以下是我测试它的示例代码：

import java.util.Iterator;
public class Try {
   public static void main(String[] args) {
      for (int i : new IntegerLoop(1, 9)) {
        System.out.print(i + " ");
      }

      System.out.println();

      for (int i : new IntegerLoop(-3, 3)) {
        System.out.print(i + " ");
      }

      System.out.println();

   }
}

这是IntergerLoop类，它只处理要打印的第一个和最后一个数字 import java.util.Iterator;

public class IntegerLoop implements Iterable<Integer>, Iterator<Integer> {
    int first, last;
    int nextInt;

    public IntegerLoop(int f, int l) {
        first = f;
        last = l;
        nextInt = first;
    }

    public boolean hasNext() {
        return nextInt >= first && nextInt <= last;
    }

    public Integer next() {
        if (hasNext()) {
            int result = nextInt; // needs Iterator
            nextInt++;
            return result;
        }
        return null;
    }

    @Override
    public void remove() {
        // TODO Auto-generated method stub

    }

    @Override
    public Iterator<Integer> iterator() {
        // TODO Auto-generated method stub
        return null;
    }
}

Answer 1

您正在获得NullPointerException因为您从null方法返回iterator ...

@Override
public Iterator<Integer> iterator() {
    // TODO Auto-generated method stub
    return null;
}

iterator方法每次调用时都应返回iterator的新实例。

您可以使用Iterator的内部类，但是对于此示例，我将其分开，因为这使得IntegerIterator更容易重复使用（恕我直言）...

IntegerLoop上课......

public class IntegerLoop implements Iterable<Integer> {

    int first, last;

    public IntegerLoop(int f, int l) {
        first = f;
        last = l;
    }

    @Override
    public Iterator<Integer> iterator() {
        return new IntegerIterator(first, last);
    }
}

IntegerIterator上课......

public class IntegerIterator implements Iterator<Integer> {

    int first, last;
    int nextInt;

    public IntegerIterator(int first, int last) {
        this.first = first;
        this.last = last;
        this.nextInt = first;
    }

    @Override
    public boolean hasNext() {
        return nextInt >= first && nextInt <= last;
    }

    @Override
    public Integer next() {
        if (hasNext()) {
            int result = nextInt; // needs Iterator
            nextInt++;
            return result;
        }
        return null;
    }

    @Override
    public void remove() {
        // TODO Auto-generated method stub

    }

}

使用for循环迭代java中的新类

1 个答案: