Using HashMap to improve search performance in Java

straight left 2021-02-23 16:03:00
using hashmap improve search performance

Java in ,HashMap, It's actually a key value pair . One Key, Corresponds to a value ; When writing data , Appoint Key Write the corresponding value ; Read by Key Find the corresponding value . It feels like Redis almost .

// establish HashMap object Sites
HashMap<Integer, String> Sites = new HashMap<Integer, String>();
// Add key value pair 
Sites.put(1, "Google");
Sites.put(2, "Runoob");
Sites.put(3, "Taobao");
Sites.put(4, "Zhihu");
// Read 
String val = Sites.get(1);// obtain Google

Why can I use HashMap To improve performance ? The reason is not that HashMap This kind of data structure has better storage performance than others , For example, array , How much more advanced . My main focus is , It's about knowing Key Under the circumstances , It's very fast to find the corresponding value . If you use arrays , The simplest , Use the cycle ; Be careful , Arrange order well , Search in half ( Two points search ). It's not as good as Key stay HashMap Read directly from . do not know why HashMap Why is the search so fast , It's probably the storage structure , What tree was used , And for Key It's indexed . This is another subject , I'll learn about it later . yesterday , I just took advantage of this feature , Will run for several hours without the end of the problem , It only took a dozen seconds .

Questions as follows :
Yes 25 Ten thousand records , Each record contains latitude and longitude ; There are different records with the same coordinates . Now I want to put records with the same coordinates together .

If the data is stored in a database , Then use SQL Coordinate grouping , It should solve the problem . However, there is no database , Data is from gdb It's read from the file .

ok , Save the data in an array , Create a new set ; Then loop the array , Compare one by one with the records in the new set , If the coordinates are the same, merge them into a new set , If not, insert a new set . It's the easiest . result 2 An hour passed , There's no sign of an end yet .

Think about it , The new set is getting bigger and bigger , There are more and more comparisons , It's like rice on a chessboard , There is twice as much rice in each grid as in the previous one ; In the end, even the whole country's grain depots are filled with rice , I can't fill the whole chessboard .

take 25 Ten thousand records should be sorted before processing ? Sorting alone is too busy , No way .

take 25 Ten thousand records are saved in the database first , Another group ? It should work , But I always feel stupid , And the speed should also be in minutes .

Finally decided to use HashMap To make this new collection .
As mentioned above ,HashMap according to Key To write or read values . The point is this Key How can I get . The above example , It's the person who wrote the code who gave some characters as Key. And in our project , You can use the hash value of the sum of longitude and latitude as Key. With the same hash value , It's the same latitude and longitude , You just need to determine what's in the new set , Is there this Key The corresponding element is OK , There's no need to compare at all .

Because there are two different longitudes and latitudes together , The result is the same possibility , So let's start with longitude multiply 1000, Plus latitude , In this way, we can basically eliminate the chance of conflict .

The code is as follows :

private HashMap<Long,SimpleItem> recGeo(HashMap<Long, SimpleItem> map,String geo,int j){

Combine records of the same coordinates into one
HashMap<Long, SimpleItem> map, New collection
String geo, Coordinate string
int j Record ID
try {

Point p = (Point);
Calculate the hash value
Because if you use loops to compare , Too much data , It's too slow
To avoid longitude in different coordinates + In the case of the same latitude result , Change longitude * 1000 Add it up
// Calculation Key
long k = Long.valueOf(Double.doubleToLongBits(p.getX() * 1000 + p.getY())).hashCode();
SimpleItem si = map.get(k);
if(si != null){
// In the new set Key The corresponding element already exists , It should be a record of the same coordinates 
si.getPointers().add(j);// Merger 
} else {
// Otherwise insert 
si = new SimpleItem();
List<Integer> pointers = new ArrayList();
} catch (ParseException e) {

return map;
private static GeometryFactory geometryFactory = JTSFactoryFinder.getGeometryFactory( null );
private static WKTReader reader = new WKTReader( geometryFactory );
class SimpleItem{

private Point geo;
private List<Integer> pointers;
public Point getGeo() {

return geo;
public void setGeo(String geo) {

try {

this.geo = (Point);
} catch (ParseException e) {

public List<Integer> getPointers() {

return pointers;
public void setPointers(List<Integer> pointers) {

this.pointers = pointers;

In a few seconds , The new set gives 5 Ten thousand elements .

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