89. Custom HashMap Implementation - prabhatrocks07/Core-Java-Programming GitHub Wiki

public class CustomHashMap<K, V> {

private Entry<K, V>[] table; //Array of Entry
private int capacity = 4;    //Initial Capacity of HashMap

static class Entry<K, V> {
	K key;
	V value;
	Entry<K, V> next;
	
	public Entry(K key, V value, Entry<K, V> next) {
		this.key = key;
		this.value = value;
		this.next = next;
	}
}

@SuppressWarnings("unchecked")
public CustomHashMap() {
	table = new Entry[capacity];
}

public void put(K newKey, V data) {
	if(newKey == null) {
		return; // does not allow to store null
	} 
	
	//Calculate hash of Key
	int hash = hash(newKey);
	
	//Create new entry
	Entry<K, V> entry = new Entry<K, V>(newKey, data, null);
	
	//If table location doesn't contain any entry, store entry there.
	if(table[hash] == null) {
		table[hash] = entry;
	} else {
		Entry<K, V> previous = null;
		Entry<K, V> current = table[hash];
		
		while (current != null) { // reached last entry of bucket
			if(current.key.equals(newKey)) {
				if(previous == null) { //node has to be inserted on first of Bucket
					entry.next = current.next;
					table[hash] = entry;
					return;
				} else {
					entry.next = current.next;
					previous.next = entry;
				}
			}
			
			previous = current;
			current = current.next;
		}
		
		previous.next = entry;
		
	}
}

public V get(K key) {
	int hash = hash(key);
	if(table[hash] == null) {
		return null;
	} else {
		Entry<K, V> temp = table[hash];
		
		while(temp != null) {
			if(temp.key.equals(key)){
				return temp.value; // return value corresponding to key
			} 
			temp = temp.next;
		}
		return null;
	}
}

public boolean remove(K deleteKey) {
	int hash = hash(deleteKey);
	if(table[hash] == null){
		return false;
	} else {
		Entry<K, V> previous = null;
		Entry<K, V> current = table[hash];
		
		while(current != null){
			if(current.key.equals(deleteKey)) {
				if(previous == null) {
					table[hash] = table[hash].next; // delete first entry node
					return true;
				} else {
					previous.next = current.next;
					return true;
				}
			}
			previous = current;
			current = current.next;
		}
		return false;
	}
}

public void printMap() {
	for (int i = 0; i < table.length; i++) {
		if(table[i] != null) {
			Entry<K, V> entry = table[i];
			while (entry != null) {
				System.out.println("{" + entry.key + "=" + entry.value + "}");
				entry = entry.next;
			}
		}
	}
}

private int hash(K key) {
	return Math.abs(key.hashCode()) % capacity;
}
}