Map is an object that stores key/volume pairs. Given a key, you can find its value. Keys must be unique, but values may be duplicated.
The HashMap class uses a hash table to implementation of the map interface. This allows the execution time of basic operations, such as get() and put() to be constant.
An example is below:
Q. Find the numbers Divisible by an array of numbers ie div[] between a range ie from 1 to N?
Ans. This can be solved using many ways, but we will be solving using HashMap which will help u understand the concept of HashMap in a better way..
HashMap<K,V> : K=key, V=value. K & V can be any thing.
CODE :
1: import java.util.HashMap;
2: public class Main {
3: public static int usingHashmap(int N,int div[]){
4: int count=0;
5: HashMap<Integer, Integer> key=new HashMap<Integer, Integer>();
6: for(int i=1;i<N;i++){
7: key.put(i, i);
8: }
9: for(int i=0;i<div.length;i++){
10: for(int k=1;k<N;k++){
11: if(key.get(k)==null){
12: key.put(k, -1);
13: }
14: if(key.get(k)%div[i]==0){
15: count++;
16: key.remove(k);
17: }
18: }
19: }
20: System.out.println(count);
21: return count;
22: }
23: public static void main(String[] args) {
24: new Main().usingHashmap(50, new int[]{3,2});
25: }
26: }
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It seems as if hashmap was not at all needed(in given example).
It would be nicer if the example is given in which using hashmap decreases time and space complexity.