如何使用LinkedHashMap获取子图？ [英] How to get a submap with LinkedHashMap?

问题描述

目前，我使用 TreeMap 来存储一些x和y坐标，但与 ArrayList 或 HashMap 。我正在使用它，因为我需要 subMap（）方法，所以我可以在确定的范围内获取X值，即使确切的X值（键）不存在。

Currently, I'm using TreeMap to store some x and y coordinates but the iteration is very slow compared to an ArrayList or HashMap. I'm using it because I need the subMap() method so I can get X values in a determined range even if the exact X value (key) doesn't exists.

LinkedHashMap 的速度与 HashMap 并且我可以迭代插入顺序中的键（我需要插入顺序或比较器的顺序，因为它在TreeMap中完成），但是我没有一个 submap（）方法。在TreeMap中，我可以非常快地生成子图。

LinkedHashMap has almost the same speed of a HashMap and I can iterate the keys in the insertion order (I need insertion order or order by comparator as it is done in TreeMap) but I don't have a submap() method. In TreeMap I can generate submaps very fast.

有没有任何数据结构或某种方式来存储顺序值（通过插入顺序或比较器）比TreeMap快，允许在范围内获取子图，即使确切的值不在地图上？我的意思是，也许我想要的值在2和25之间，但2不存在，最近的是3，所以它将返回一个子图从3到25.或某种方式添加这个功能到 LinkedHashMap ？

Is there any data structure or some way to store ordered values (by insertion order or comparator) faster than TreeMap that allows to get submaps in a range even if the exact value is not in the map? I mean, maybe I want values between 2 and 25 but 2 doesn't exist, the nearest is 3 so it will return a submap from 3 to 25. Or some way to add this functionality to LinkedHashMap?

推荐答案

今天我终于得到了我的问题的答案。经过几个测试 HashMap ， LinkedHashMap 和 TreeMap 比 ArrayList 慢，我想使用它们来创建 subMaps（）的能力。所以我创建了一个新类，扩展了 ArrayList ，这给了我很好的表现，并借助于这个答案我创建了通过值而不是索引获取子列表的快捷方式。这是完整的课程：

Today I finally got the answer to my problem. After a few test HashMap, LinkedHashMap and TreeMap are way slower than ArrayList and I wanted to use them just for the ability to create subMaps(). So I created a new class extending ArrayList which gives me a very good performance and with the help of this answer I created fast way of getting a sublist by values not by index. Here is the complete class:

/**
 * The purpose of this class is to be a faster replacement to a {@link java.util.TreeMap} with
 *  the ability to get sublist containing a range of x values. ArrayList access time is O(1) while
 *  {@link java.util.TreeMap} is O(log(n)). When large data is handled the impact on performance is
 *  noticeable.
 */
public class XYDataset extends ArrayList<PointValue> {

    private final float COMPARISON_THRESHOLD = 0.01f;

    final Comparator<PointValue> comparator = new Comparator<PointValue>() {
        @Override
        public int compare(PointValue lhs, PointValue rhs) {
            if (Math.abs(lhs.getX() - rhs.getX()) < COMPARISON_THRESHOLD) return 0;
            return lhs.getX() < rhs.getX() ? -1 : 1;
        }
    };

    public XYDataset(int capacity) {
        super(capacity);
    }

    public XYDataset() {
    }

    public XYDataset(Collection<? extends PointValue> collection) {
        super(collection);
    }

    @Override
    public List<PointValue> subList(int start, int end) {
        return super.subList(start, end);
    }

    /**
     * Generate a sublist containing the range of x values passed
     * @param x1 lower x value
     * @param x2 upper x value
     * @return sublist containing x values from x1 to x2
     */
    public List<PointValue> subList(float x1, float x2){
        /**
         * Collections.binarySearch() returns the index of the search key, if it is contained in the list;
         *  otherwise it returns (-(insertion point) - 1).
         * The insertion point is defined as the point at which the key would be inserted into the list:
         *  the index of the first element greater than the key, or list.size() if all elements in the list
         *  are less than the specified key. Note that this guarantees that the return value will be >= 0 if
         *  and only if the key is found.
         */
        int n1 = Collections.binarySearch(this, new PointValue(x1, 0), comparator);
        int n2 = Collections.binarySearch(this, new PointValue(x2, 0), comparator);

        /**
         * Example, we assume the list is sorted. Based on (http://stackoverflow.com/questions/19198586/search-sorted-listlong-for-closest-and-less-than)
         *
         * long X = 500;
         * List<Long> foo = new Arraylist<>();
         * foo.add(450L);
         * foo.add(451L);
         * foo.add(499L);
         * foo.add(501L);
         * foo.add(550L);
         *
         * If we search for something that isn't in the list you can work backward from the return value
         *  to the index you want. If you search for 500 in your example list, the algorithm would return (-3 - 1) = -4.
         * Thus, you can add 1 to get back to the insertion point (-3), and then multiply by -1 and subtract 1 to get
         *  the index BEFORE the first element GREATER than the one you searched for, which will either be an index that
         *  meets your 2 criteria OR -1 if all elements in the list are greater than the one you searched for.
         */
        if(n1 < 0) n1 = -n1-1;
        if(n2 < 0) n2 = -n2-1;

        return this.subList(n1, n2);
    }
}

PointValue 只是一个包含x和y坐标的类。所以现在我只是调用 subList（）传递我想要的x坐标的范围。在我的情况下，插入顺序也是排序的，为了使用 Collections.binarySearch（）

PointValue is a just a class that contains x and y coordinates. So now I just call subList() passing the range of x coordinates I want. In my case the insertion order is also sorted which is important in order to use Collections.binarySearch()

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