I am writing a hash table which uses generic class, but this line shows a null pointer exception. Is my way of calling the element wrong?
table[hashVal].tableKey = key;
Here is the entire code
public class LinearProbingHashTable<K, V> {
// initialize size of table and entry counter
int size = 13;
int entryCounter = 0;
// create an entry
private static class Entry<K, V> {
K tableKey;
V tableValue;
String status;
}
Entry<K, V> table[] = new Entry[size];
// insert new entry into table
public boolean insert(K key, V value) {
// throw exception if key is null
if ( key == null ) {
throw new IllegalArgumentException("Invalid key");
}
// rehash if the table is half full
if ( entryCounter >= ( size / 2) ) {
rehash();
}
// return false if duplicate
for ( int i = 0; i < table.length; i++ ) {
if ( table[i] != null) {
if ( table[i].tableValue.equals(value) == true )
return false;
}
}
// calculate hash value
int hashVal = 0;
String keyToString = key.toString();
for ( int i = 0; i < keyToString.length(); i++ ) {
hashVal = 37 * hashVal + keyToString.charAt(i);
}
hashVal %= size;
if ( hashVal < 0 )
hashVal += size;
// if entry is empty, insert key and value into the table and return true
if ( table[hashVal] == null) {
table[hashVal].tableKey = key;
table[hashVal].tableValue = value;
entryCounter++;
return true;
}
// element collision
else {
// linear probing
for ( int i = hashVal + 1; i < table.length; i++ ) {
// if probe reaches end, restart from 0
if ( table[i] == null )
i = 0;
// if entry is empty, insert key and value and return true
if ( table[i].tableKey == null ) {
table[i].tableKey = key;
table[i].tableValue = value;
entryCounter++;
return true;
}
}
}
return false;
}
public V find(K key) {
// look for the key
for ( int i = 0; i < table.length; i++ ) {
// if key is found, return its value
if ( table[i].tableKey.equals(key) == true )
return table[i].tableValue;
}
// if key is not found, return null
return null;
}
public boolean delete(K key) {
// look for the key
for ( int i = 0; i < table.length; i++) {
// if key is found, change entry's status and return true
if ( table[i].tableKey.equals(key) == true) {
table[i].status = "deleted";
return true;
}
}
// if key is not found, return false
return false;
}
private void rehash() {
size = size * 2;
}
public int getHashValue(K key) {
// search the table
for ( int i = 0; i == table.length; i++) {
// if the key exists, exit the loop
if ( table[i].tableKey.equals(key) == true )
break;
// if key is not in the table, return -1
else if ( table[i].tableKey.equals(key) == false && i == table.length )
return -1;
}
// calculate
int hashVal = 0;
String keyToString = key.toString();
for ( int i = 0; i < keyToString.length(); i++ ) {
hashVal = 37 * hashVal + keyToString.charAt(i);
}
hashVal %= size;
if ( hashVal < 0 )
hashVal += size;
return hashVal;
}
public int getLocation(K key) {
// search for the key
for ( int i = 0; i < table.length; i++) {
// return its position if found
if ( table[i].tableKey.equals(key) == true)
return i;
}
// if not found, return -1
return -1;
}
public String toString() {
int i = 0;
// call recursive function
return toString(i);
}
// print string recursively
private String toString(int i) {
// return blank at the end of the table
if ( i == table.length )
return "";
// print null when entry is empty
if ( table[i].tableValue.equals(null) == true)
System.out.println(i + " " + "null");
// print value
else
System.out.println(i + " " + table[i].tableValue);
i++;
return toString(i);
}
public static void main(String[] args) {
LinearProbingHashTable<Object, Object> hashTable = new LinearProbingHashTable<>();
hashTable.insert("test", 234);
}
}
