Create custom hashmap in Java


One of the common interview question, is to build own CustomHashMap. Following code sample demonstrate generic map with put(), get() and remove() operations.

/**
 * The CustomHashMap uses an array of KeyValuePair.
 * KeyValuePair class  where K is the key and V value and  next is the element appended to it. The KeyValuePair acts 
 * as a list
 * 
 *  MapList is used to store elements. the getHash() method is used to find the index of the array. 
 * 
 * @param <K>
 * @param <V>
 */
public class CustomHashMap<K, V> {

	KeyValuePair<K, V> mapList[] = new KeyValuePair[100];

	public V get(K key) {
		int index = getHash(key);
		KeyValuePair<K,V> list = mapList[index];
		return getMatchValue(list, key);
	}

	public void put(K key, V value) {
		int index = getHash(key);
		storeValue(index, key, value);
	}
	
	public void remove(K key) {
		int index = getHash(key);
		KeyValuePair<K,V> list = mapList[index];
		if (list == null)
			return;
		// if only one element is present in the list ,set the index to null
		if(list.getKey().equals(key)){
			if (list.next == null){
				mapList[index] = null;
				return;
			}
		}
		KeyValuePair<K,V> prev = null;
		do{
			if(list.key.equals(key)){
				if (prev == null){
					list = list.getNext();
				}else{
					prev.next = list.getNext();
				}
				break;
			}
			list = list.next;
		}while(list != null);
		
		mapList[index] = list;
	}

	/*
	 * find the match value and return , if not found either throw exception or return null.
	 */
	private V getMatchValue(KeyValuePair<K, V> list, K key) {
		while (list != null) {
			if (list.getKey().equals(key))
				return list.getValue();
			list = list.next;
		}
		return null;
	}

	private void storeValue(int index, K key, V value) {
		KeyValuePair<K, V> list = mapList[index];
		
		// if list is empty , enter as first element
		if (list == null) {
			mapList[index] = new KeyValuePair<K, V>(key, value);
		} else {
			boolean done = false;
			// traverse through list , if a key is found ,replace the value or add it at the end of the list
			while(list.next != null) {
				if (list.getKey().equals(key)) {
					list.setValue(value);
					done = true;
					break;
				} 
				list = list.next;
			}
			// add at the end of the list
			if (!done)
				list.next = new KeyValuePair<K, V>(key, value);
		}

	}
	
	private int getHash(K key) {
		int hash = key.hashCode();
		return hash % 100;
	}
	
	public static void main(String args[]) {
		CustomHashMap<Integer, Integer> map = new CustomHashMap<Integer, Integer>();
		map.put(1, 1);
		map.put(2, 2);
		map.put(201,201);
		System.out.println("get value is " + map.get(1));
		System.out.println("get value is " + map.get(201));
		System.out.println("get value is " + map.get(2));
		map.remove(1);
		System.out.println("After deletion " + map.get(1));
		System.out.println("get value is " + map.get(201));
	}

}

class KeyValuePair<K, V> {
	K key;
	V value;
	KeyValuePair<K, V> next = null;

	public KeyValuePair<K, V> getNext() {
		return next;
	}

	public void setNext(KeyValuePair<K, V> next) {
		this.next = next;
	}

	public KeyValuePair(K key, V value) {
		super();
		this.key = key;
		this.value = value;
	}

	public K getKey() {
		return key;
	}

	public void setKey(K key) {
		this.key = key;
	}

	public V getValue() {
		return value;
	}

	public void setValue(V value) {
		this.value = value;
	}

}
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3 thoughts on “Create custom hashmap in Java

  1. It is quite strange decision to take 100 for the size of array. To reduce the number of collisions it is worth to use prime numbers (see Donald Knuth). The java implementation uses length of array equal to 2^k – so instead of mod operation ( index = hashCode % size ) it uses AND bitwise operation ( index = hashCode & (k – 1) ) it is much much faster and profitable in terms of all modern cpu ( mod operation involves math pipeline that blocks other internal cpu pipeline for that).
    More over – the case is only one case of conflict solving with chains – there are many others – one of the well known is open-addressing.

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