[Algo.Java] I'm getting a null pointer exception when I'm using the get function while implementing the Hash Table

#1

Both of my getAt and removeAt functions are not working when i use them for my Hash Table implementation.

#2

Are you checking for the existence of the element in the table before quering for its value ?

#3

Yes I am, here are the implementation of both the methods

Remove function from the Hash Table

public V remove(K key) throws Exception{
	int bestIndex = hashFunction(key);
	LinkedList<HTPair> bucket = this.bucketArray[bestIndex];
	HTPair removePair = new HTPair(key,null);
	
	if(bucket == null) {
		return null;
	}
	else {
		int findAt = bucket.find(removePair);
		if(findAt == -1) {
			return null;
		}
		else {
			HTPair pair = bucket.removeAt(findAt);
			this.size--;
			return pair.value;
		}
	}
}`

RemoveAt function from the linked list.

`	public T removeAt(int index) throws Exception{
	if(this.size == 0) {
		throw new Exception("Linked List is empty.");
	}
	if(index<0 || index>=this.size) {
		throw new Exception("Index out of bounds.");
	}
	if(index == 0) {
		return removeFirst();
	}
	else if(index == this.size - 1) {
		return removeLast();
	}
	else {
		Node nMinus1 = getNodeAt(index-1);
		Node n = nMinus1.next;
		Node nPlus1 = n.next;
		
		nMinus1.next = nPlus1;
		
		this.size--;
		
		return n.data;
	}
}`
#4

what error is it throwing any kind of exceptions or getting wrong answers with no exceptions thrown ?

#5

When I call for the get function of my hashtable I’m getting a null pointer exception.

#6

Alright, just post your entire code

#7

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#8 
package HashMaps;

public class HashTable<K, V>{
private class HTPair{
K key;
V value;

	HTPair(K key, V value){
		this.key = key;
		this.value = value;
	}
	
	public boolean equals(Object other) {
		HTPair otherPair = (HTPair) other;
		return this.key.equals(otherPair.key);
	}
	
	public String toString() {
		return "{ " + this.key + " - " + this.value + " }";
	}
}

public static final int DEFAULT_CAPACITY = 10;

private LinkedList<HTPair>[] bucketArray;
private int size;

public HashTable() {
	this(DEFAULT_CAPACITY);
}

public HashTable(int capacity) {
	LinkedList<HTPair>[] list = (LinkedList<HTPair>[]) new LinkedList[capacity];
	this.bucketArray = list;
	this.size = 0;
}

public void put(K key, V value) throws Exception {
	int bestIndex = hashFunction(key);
	LinkedList<HTPair> bucket = this.bucketArray[bestIndex];
	HTPair addPair = new HTPair(key, value);
	
	if(bucket == null) {
		bucket = new LinkedList<>();
		bucket.addLast(addPair);
		this.bucketArray[bestIndex] = bucket;
		this.size++;
	}
	else {
		int findAt = bucket.find(addPair);
		if(findAt == -1) {
			bucket.addLast(addPair);
			this.size++;
		}
		else {
			HTPair pair = bucket.getAt(findAt);
			pair.value = value;
		}
	}
	
	double lamda = (this.size*1.0)/this.bucketArray.length;
	if(lamda > 2) {
		this.rehash();
	}
}

private void rehash() throws Exception{
	LinkedList<HTPair>[] oldBucket = this.bucketArray;
	this.bucketArray = (LinkedList<HTPair>[]) new LinkedList[2*oldBucket.length];
	this.size = 0;
	for(LinkedList<HTPair> ob:oldBucket) {
		while(ob!=null && !ob.isEmpty()) {
			HTPair pair = ob.removeFirst();
			this.put(pair.key, pair.value);
		}
	}
}

public V get(K key) throws Exception{
	int bestIndex = hashFunction(key);
	LinkedList<HTPair> bucket = this.bucketArray[bestIndex];
	HTPair findPair = new HTPair(key, null);
	
	if(bucket == null) {
		return null;
	}
	else {
		int findAt = bucket.find(findPair);
		if(findAt == -1) {
			return null;
		}
		else {
			HTPair pair = bucket.getAt(findAt);
			return pair.value;
		}
	}
}

public V remove(K key) throws Exception{
	int bestIndex = hashFunction(key);
	LinkedList<HTPair> bucket = this.bucketArray[bestIndex];
	HTPair removePair = new HTPair(key,null);
	
	if(bucket == null) {
		return null;
	}
	else {
		int findAt = bucket.find(removePair);
		if(findAt == -1) {
			return null;
		}
		else {
			HTPair pair = bucket.removeAt(findAt);
			this.size--;
			return pair.value;
		}
	}
}

private int hashFunction(K key) {
	int hashCode = key.hashCode();
	hashCode = Math.abs(hashCode);
	int bestIndex = hashCode % this.bucketArray.length;
	return bestIndex;
}

public void display() throws Exception{
	for(LinkedList<HTPair> bucket:this.bucketArray) {
		if(bucket != null && !bucket.isEmpty()) {
			bucket.display();
		}
		else {
			continue;
		}
			
		System.out.println();
	}
}

}