目前,我使用TreeMap
来存储一些x和y坐标,但与ArrayList
或HashMap
相比,迭代速度非常慢。我使用它是因为我需要subMap()
方法,所以即使确切的X值(键)不存在,我也可以在确定的范围内得到X值。
LinkedHashMap
与HashMap
几乎具有相同的速度,我可以按插入顺序迭代键(我需要在TreeMap中按照比较器执行插入顺序或顺序)但是我没有&# 39; t有submap()
方法。在TreeMap中,我可以非常快速地生成子图。
是否有任何数据结构或某种方式来存储有序值(通过插入顺序或比较器)比TreeMap更快,即使精确值不在地图中,也允许获取范围内的子图?我的意思是,也许我想要2到25之间的值,但是2不存在,最近的是3,所以它将从3到25返回一个子图。或者某种方式将此功能添加到LinkedHashMap
?
答案 0 :(得分:1)
听起来你需要一个TreeMap,它的迭代速度并不比LinkedHashMap慢得多,并且可以满足您的需求。由于HashMap是无序的,因此subMap没有意义。
答案 1 :(得分:0)
今天我终于得到了问题的答案。经过一些测试HashMap
后,LinkedHashMap
和TreeMap
的速度比ArrayList
慢,我只想将它们用于创建subMaps()
的能力。所以我创建了一个扩展ArrayList
的新类,它给了我一个非常好的表现,在this答案的帮助下,我创建了快速获取子列表而不是索引的方法。这是完整的课程:
/**
* 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 (https://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。所以现在我只需要调用subList()
传递我想要的x坐标范围。在我的情况下,插入顺序也是排序的,这对于使用Collections.binarySearch()