How is the performance for HashMap.get()

Question

hi all
i just want to ask you guys how quick the hashmap.get() runs for a given key, if there are thousand of entries in the hashmap. thanks

Answer 1

If you want to know how something is going to perform, write a tiny program and test it. Either put it on a profiler, or just put time markers around a big loop. Here's something that somebody (UJ, I think) came up with recently for comparing casting vs. toString after extracting from a Map. It should be easy to adapt to just the Map.get() timing. In fact, I think I added a chunk to do just that, as a control. Knock yourself out. import java.util.*;
public class MapTiming {
    Random rnd = new Random(13L);
    public static void main(String[] args) {
        int numEntries = Integer.parseInt(args[0]);
        Integer key = Integer.valueOf(args[1]);
        new MapTiming().zillion2(numEntries, key);
    void zillion2(int numEntries, Integer key) {
        HashMap map = new HashMap();
        int ix;
        for (ix = 0; ix < numEntries; ix++) {
            int entry = rnd.nextInt();
            map.put(new Integer(entry), String.valueOf(ix));
        map.put(key, String.valueOf(ix));
        final int LOOPS = 10000000;
        final int TIMES = 5;
        long time;
        for (int j=0; j<TIMES; j++) {
            time = System.currentTimeMillis();
            for (int i=0;i<LOOPS; i++) {
                String item = map.get(key).toString();
            System.out.println("toString= " + (System.currentTimeMillis()-time));
        for (int j=0; j<TIMES; j++) {
            time = System.currentTimeMillis();
            for (int i=0;i<LOOPS; i++) {
                String item = (String)map.get(key);
            System.out.println("cast= " + (System.currentTimeMillis()-time));
        for (int j=0; j<TIMES; j++) {
            time = System.currentTimeMillis();
            for (int i=0;i<LOOPS; i++) {
                Object obj = map.get(key);
            System.out.println("get= " + (System.currentTimeMillis()-time));
        for (int j=0; j<TIMES; j++) {
            time = System.currentTimeMillis();
            for (int i=0;i<LOOPS; i++) {
                String item = map.get(key).toString();
            System.out.println("toString= " + (System.currentTimeMillis()-time));
        for (int j=0; j<TIMES; j++) {
            time = System.currentTimeMillis();
            for (int i=0;i<LOOPS; i++) {
                String item = (String)map.get(key);
            System.out.println("cast= " + (System.currentTimeMillis()-time));
        for (int j=0; j<TIMES; j++) {
            time = System.currentTimeMillis();
            for (int i=0;i<LOOPS; i++) {
                Object obj = map.get(key);
            System.out.println("get= " + (System.currentTimeMillis()-time));
}