Примерно внедряване на Hashtable с използване на Generics и Linear Probing за разрешаване на сблъсъци. Има някои предположения, направени по време на изпълнението и те са документирани в javadoc над класа и методите.
Тази реализация няма всички методи на Hashtable като keySet, putAll и т.н., но обхваща най-често използваните методи като get, put, remove, size и т.н.
Има повторение на кода в get, put и remove за намиране на индекса и може да се подобри, за да има нов метод за намиране на индекс.
class HashEntry<K, V> {
private K key;
private V value;
public HashEntry(K key, V value) {
this.key = key;
this.value = value;
}
public void setKey(K key) { this.key = key; }
public K getKey() { return this.key; }
public void setValue(V value) { this.value = value; }
public V getValue() { return this.value; }
}
/**
* Hashtable implementation ...
* - with linear probing
* - without loadfactor & without rehash implementation.
* - throws exception when table is full
* - returns null when trying to remove non existent key
*
* @param <K>
* @param <V>
*/
public class Hashtable<K, V> {
private final static int DEFAULT_CAPACITY = 16;
private int count;
private int capacity;
private HashEntry<K, V>[] table;
public Hashtable() {
this(DEFAULT_CAPACITY);
}
public Hashtable(int capacity) {
super();
this.capacity = capacity;
table = new HashEntry[capacity];
}
public boolean isEmpty() { return (count == 0); }
public int size() { return count; }
public void clear() { table = new HashEntry[this.capacity]; count = 0; }
/**
* Returns null if either probe count is higher than capacity else couldn't find the element.
*
* @param key
* @return
*/
public V get(K key) {
V value = null;
int probeCount = 0;
int hash = this.hashCode(key);
while (table[hash] != null && !table[hash].getKey().equals(key) && probeCount <= this.capacity) {
hash = (hash + 1) % this.capacity;
probeCount++;
}
if (table[hash] != null && probeCount <= this.capacity) {
value = table[hash].getValue();
}
return value;
}
/**
* Check on the no of probes done and terminate if probe count reaches to its capacity.
*
* Throw Exception if table is full.
*
* @param key
* @param value
* @return
* @throws Exception
*/
public V put(K key, V value) throws Exception {
int probeCount = 0;
int hash = this.hashCode(key);
while (table[hash] != null && !table[hash].getKey().equals(key) && probeCount <= this.capacity) {
hash = (hash + 1) % this.capacity;
probeCount++;
}
if (probeCount <= this.capacity) {
if (table[hash] != null) {
table[hash].setValue(value);
} else {
table[hash] = new HashEntry(key, value);
count++;
}
return table[hash].getValue();
} else {
throw new Exception("Table Full!!");
}
}
/**
* If key present then mark table[hash] = null & return value, else return null.
*
* @param key
* @return
*/
public V remove(K key) {
V value = null;
int probeCount = 0;
int hash = this.hashCode(key);
while (table[hash] != null && !table[hash].getKey().equals(key) && probeCount <= this.capacity) {
hash = (hash + 1) % this.capacity;
probeCount++;
}
if (table[hash] != null && probeCount <= this.capacity) {
value = table[hash].getValue();
table[hash] = null;
count--;
}
return value;
}
public boolean contains(Object value) {
return this.containsValue(value);
}
public boolean containsKey(Object key) {
for (HashEntry<K, V> entry : table) {
if (entry != null && entry.getKey().equals(key)) {
return true;
}
}
return false;
}
public boolean containsValue(Object value) {
for (HashEntry<K, V> entry : table) {
if (entry != null && entry.getValue().equals(value)) {
return true;
}
}
return false;
}
@Override
public String toString() {
StringBuilder data = new StringBuilder();
data.append("{");
for (HashEntry<K, V> entry : table) {
if (entry != null) {
data.append(entry.getKey()).append("=").append(entry.getValue()).append(", ");
}
}
if (data.toString().endsWith(", ")) {
data.delete(data.length() - 2, data.length());
}
data.append("}");
return data.toString();
}
private int hashCode(K key) { return (key.hashCode() % this.capacity); }
public static void main(String[] args) throws Exception {
Hashtable<Integer, String> table = new Hashtable<Integer, String>(2);
table.put(1, "1");
table.put(2, "2");
System.out.println(table);
table.put(1, "3");
table.put(2, "4");
System.out.println(table);
table.remove(1);
System.out.println(table);
table.put(1, "1");
System.out.println(table);
System.out.println(table.get(1));
System.out.println(table.get(3));
// table is full so below line
// will throw an exception
table.put(3, "2");
}
}
Примерно изпълнение на горния код.
{2=2, 1=1}
{2=4, 1=3}
{2=4}
{2=4, 1=1}
1
null
Exception in thread "main" java.lang.Exception: Table Full!!
at Hashtable.put(Hashtable.java:95)
at Hashtable.main(Hashtable.java:177)
person
JRG
schedule
28.06.2017
