How HashMap Works in Java ?

  • HashMap is one of the most widly used implementation of Map to store key-value pairs. To implement HashMap with ArrayList.
  • It is provides two basic HashMap functions they are: get(key) and put(key, value).
  • While storing code to be checking the duplicate values. While duplicate values are presents to be removed.
  • Implementation should not be used as a replacement of HashMap. Also while testing the code, make sure that the Object used in the KEY has proper implementation of equals() method.

Example:

package com.journaldev.util;

import java.util.ArrayList;
import java.util.List;

public class MyHashMap {

	class Container{
		Object key;
		Object value;
		public void insert(Object k, Object v){
			this.key=k;
			this.value=v;
		}
	}
	
	private Container c;
	private List<Container> recordList;
	
	public MyHashMap(){
		
		this.recordList=new ArrayList<Container>();
	}
	
	public void put(Object k, Object v){
		this.c=new Container();
		c.insert(k, v);
		//check for the same key before adding
		for(int i=0; i<recordList.size(); i++){
			Container c1=recordList.get(i);
			if(c1.key.equals(k)){
				//remove the existing object
				recordList.remove(i);
				break;
			}
		}
		recordList.add(c);
	}
	
	public Object get(Object k){
		for(int i=0; i<this.recordList.size(); i++){
			Container con = recordList.get(i);
			//System.out.println("k.toString():"+k.toString()+"con.key.toString()"+con.key.toString());
			if (k.toString()==con.key.toString()) {
				
				return con.value;
			}
			
		}
		return null;
	}
	
	public static void main(String[] args) {
		MyHashMap hm = new MyHashMap();
		hm.put("1", "1");
		hm.put("2", "2");
		hm.put("3", "3");
		System.out.println(hm.get("3"));
		hm.put("3", "4");
		
		System.out.println(hm.get("1"));
		System.out.println(hm.get("3"));
		System.out.println(hm.get("8"));
	}

}

Output:

3
1
4
null

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