树结构的复制构造函数偶尔会删除叶子

Question

我有一个树状结构，我的复制构造函数似乎偶尔会掉一些“叶子”。

基本结构：

public class Arrow {
   ArrayList<Arrow> subArrows;
   Interval start;
   Interval end;
}

复制构造函数：

public Arrow(Arrow other) {
    this.start = new Interval(other.start);
    this.end = new Interval(other.end);
    if (other.subArrows != null) {
        this.subArrows = new ArrayList<Arrow>();
        for (Arrow sub : other.subArrows) {
            this.subArrows.add(new Arrow(sub));
        }
    } else {
        this.subArrows = new ArrayList<Arrow>();
    }
}

我希望这实际上是对我的树结构进行深度复制。相反，我偶尔会发现我的subArrows 数组之一是空的。除了它们往往位于我的树的最低“级别”之外，我没有注意到其他模式。

有什么想法吗？好久没用java了。

编辑：有几个人要求更多代码，所以这里是所有涉及 subArrows 的地方。这是来自一个相当大的算法/数据结构，因此将其全部发布是不合理的。

递归获取所有子箭头并返回一组。

Set<Arrow> allSubArrows(Arrow arrow) {
    Set<Arrow> arrowSet = new HashSet<Arrow>();
    if (arrow.subArrows != null && arrow.subArrows.size() > 0) {
        for (Arrow sub : arrow.subArrows) {
            arrowSet.addAll(allSubArrows(sub));
        }
        return arrowSet;
    } else {
        arrowSet.add(arrow);
        return arrowSet;
    }
}

Mathy 这样做的原因，但底部修改了 subArrows：

void enforceMonotonicity(Arrow arrow) {
    boolean changed = false;
    if (arrow.end != null && arrow.start != null) {
        if (arrow.start.isParallelTo(arrow.end)) {
            //either left to right or bottom to top
            if (arrow.start.isVertical()) {
                //left interval pointing to right interval
                if (arrow.start.startGraph.y > arrow.end.startGraph.y) {
                    arrow.end.startGraph.y = arrow.start.startGraph.y;
                    changed = true;
                }
                if (arrow.end.endGraph.y < arrow.start.endGraph.y) {
                    arrow.start.endGraph.y = arrow.end.endGraph.y;
                    changed = true;
                }
            } else {
                //bottom interval pointing to top interval
                if (arrow.start.startGraph.x > arrow.end.startGraph.x) {
                    arrow.end.startGraph.x = arrow.start.startGraph.x;
                    changed = true;
                }
                if (arrow.end.endGraph.x < arrow.start.endGraph.x) {
                    arrow.start.endGraph.x = arrow.end.endGraph.x;
                    changed = true;
                }
            }
        }
    }
    if (changed) {
        //check to make sure SOMETHING is still reachable, if not arrow = null
        if (arrow.start.isVertical()) {
            if (arrow.start.startGraph.y >= arrow.start.endGraph.y ||
                    arrow.end.startGraph.y >= arrow.end.endGraph.y) {
                arrow = null;
            }
        } else {
            if (arrow.start.startGraph.x >= arrow.start.endGraph.x ||
                    arrow.end.startGraph.x >= arrow.end.endGraph.x) {
                arrow = null;
            }
        }
        //if we changed the outer arrows, we need to recursively change the subarrows
        if (arrow != null && arrow.subArrows != null && arrow.subArrows.size() > 0) {
            for (Arrow sub : arrow.subArrows) {
                enforceMonotonicity(sub);
            }
        }
    }
}

合并算法的一部分：

HashSet<Arrow> mergeCells(Set<Arrow> first, Set<Arrow> second) {
    HashSet<Arrow> mergedCell = new HashSet<Arrow>();
    //loop through arrows in adjacent cells and find the ones that connect
    for (Arrow topArrow : first) {
        for (Arrow bottomArrow : second) {
            if(topArrow.start.intersects(bottomArrow.end)) {
                //merge arrows
                Interval middle = topArrow.start.intersection(bottomArrow.end);
                Arrow newArrow = new Arrow();
                if (middle != null) {
                    //if they connect, we copy the two arrows, modify their connection,
                    //create a new arrow with the constituents as subarrows, and add that to the mergedcell
                    //after the mergedcell is created, we can delete the base arrows
                    Arrow topCopy = new Arrow(topArrow);
                    topCopy.start = middle;
                    Arrow bottomCopy = new Arrow(bottomArrow);
                    bottomCopy.end = middle;

                    newArrow.subArrows.add(topCopy);
                    newArrow.subArrows.add(bottomCopy);
                    newArrow.start = bottomCopy.start;
                    newArrow.end = topCopy.end;

                    //if end and middle are parallel, we need to project monotonicity
                    //monotonicity is already enforced within a single cell
                    //and enforcemonotonicity knows whether or not start and end are parallel
                    enforceMonotonicity(newArrow);
                }
                //enforceMonotonicity could null out the new arrow
                if (newArrow != null && !newArrow.isNull()) {
                    mergedCell.add(newArrow);
                }
            }
        }
    }

    //keep the originals in case they can connect later on
    //(hashset doesn't allow duplicates, so no need to worry here)
    mergedCell.addAll(first);
    mergedCell.addAll(second);

    return mergedCell;
}

Answer 1

当(other.subArrows == null) 时，您的代码仍由new ArrayList<Arrow>() 创建this.subArrows。这不是真正的克隆（一个是null，另一个不是）。如果要确保 subArrow 始终为非空（便于在不检查 null 的情况下添加 subArrow），可以在字段定义中实例化它。

如果这仍然不起作用，您可能需要显示更多代码，因为我们不知道您如何在箭头中添加子箭头。另外，如果你的代码是在多线程环境下，而且bug很难重现，可能是同步问题。请注意，ArrayList 不是线程安全的。改用Vector 或Collections.synchronizedList，或正确同步关键块。同样，我们没有您的相关代码。

顺便说一句，声明 List<Arrow> subArrows 比 ArrayList<Arrow> subArrows 更好。