Java：遍历泛型哈希集时出现类型不匹配错误<；E>；

解决了。如有疑问，请参阅底部编辑

我正在为我的计算机科学课程做一个编程作业，我们正在为一个使用四叉树的2D图形游戏创建一个实现。我在四叉树实现的特定部分遇到了问题

这是属性和构造函数：

public class QuadTreeNode<E extends BoxIntersectable> {

protected Box region;

protected HashSet<E> items;

protected QuadTreeNode<E> northEast; // Quadrant 2, left 2
protected QuadTreeNode<E> northWest; // Quadrant 1, left 1
protected QuadTreeNode<E> southEast; // Quadrant 4, right 2
protected QuadTreeNode<E> southWest; // Quadrant 3, right 1

protected int depth;

public static final int MAX_items = 20; // # of items per node
public static final int MAX_DEPTH = 20;

public QuadTreeNode(Box bbox, int depth) {
    items = new HashSet<E>();
    northEast = northWest = southEast = southWest = null;
    this.region = bbox;
    this.depth = depth;
}

allItems方法将属性项作为一个集合返回。我认为这很好，因为这是通过一组Es进行迭代，但是编译器要求不同。我得到以下编译时间错误：

类型不匹配：无法从元素类型对象转换为E

如何修复此问题，以便编译器批准我尝试执行的操作

谢谢你的帮助

---

Edit：我的问题是递归方法调用没有泛型类型root。我是个笨蛋。我向我制作facepalm的所有人致歉。

您在findIntersectingHelper中声明root的类型是什么？我认为如果您对for循环执行以下操作，它将起作用<代码>用于（框可相交项：root.allItems（））{。由于从根目录返回的所有项目都将是类型/子类型为

BoxIntersectable

。我找到了答案。我的问题是我在递归函数调用中没有使用泛型类型root。现在工作正常。您在findIntersectingHelper中声明根目录的类型是什么？我想如果您按照下面的方法执行for lo，它会工作op.

for（BoxIntersectable项：root.allItems（））{

。因为从root返回的所有项都将是

BoxIntersectable

的类型/子类型。我解决了这个问题。我的问题是在递归函数调用中没有泛型root。现在工作正常。

    /**
 * Find all items intersecting a given bounding box.
 * 
 * <p>
 * Hints: Use a recursive algorithm with a helper. Create a HashSet to store
 * the items and pass it to the recursive calls.
 * </p>
 * 
 * <p>
 * Recurse on any child node whose region intersects the box. At a leaf node
 * test each item stored in the node to see if it intersects the box. If it
 * does, add it to the HashSet that is returned.
 * </p>
 * 
 * @param box
 *            The box to intersect objects with.
 * @return A set containing all of the items.
 */
public Set<E> findIntersecting(Box box) {
    Set<E> intersectingBoxes = new HashSet<>();
    if (getBoxRegion().intersects(box)) {
        findIntersectingHelper(intersectingBoxes, this, box);
    }
    return intersectingBoxes;
}

/**
 * Helper method for findIntersecting.
 * 
 * @param intersectingBoxes the set of boxes intersecting box.
 * @param root the node 
 * @param box the box to be checked for intersection.
 */
private void findIntersectingHelper(Set<E> intersectingBoxes, QuadTreeNode root, Box box) {
    // No need to check if root box region intersects box, method is only called if such occurs.
    if (root.isLeaf()) {
        for (E item : root.allItems()) {
            intersectingBoxes.add(item);
        }
    } else {
        // Call recursion on any child who intersects box.
        if (northWest.getBoxRegion().intersects(box)) {
            findIntersectingHelper(intersectingBoxes, northWest, box);
        }
        if (northEast.getBoxRegion().intersects(box)) {
            findIntersectingHelper(intersectingBoxes, northEast, box);
        }
        if (southWest.getBoxRegion().intersects(box)) {
            findIntersectingHelper(intersectingBoxes, southWest, box);
        }
        if (southEast.getBoxRegion().intersects(box)) {
            findIntersectingHelper(intersectingBoxes, southEast, box);
        }
    }
}

for (E item : root.allItems()) {