Contents of page :
1) Methods used in custom HashMap >
public void add(E value)
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Add objects in setCustom
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public boolean contains(E obj)
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Method returns true if setCustom contains the object.
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public boolean remove(E obj)
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Method removes object from setCustom.
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public void display()
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-Method displays all objects in setCustom.
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2) Let’s find out answer of few very important questions before proceeding >
Q1. How HashSet implements hashing?
A. Method internally uses HashMap’s hash method for hasihng.
Q2. How add method works internally?
A. public void add(E value){
hashMapCustom.put(value, null);
}
Method internally uses HashMap’s put method for storing object.
Q3. How contains method works internally?
A. public boolean contains(E obj){
return hashMapCustom.contains(obj) !=null ? true :false;
}
Q4. How remove method works internally?
A. public boolean remove(E obj){
return hashMapCustom.remove(obj);
}
Method internally uses HashMap’s put remove for storing object.
3) Full Program/SourceCode for implementing custom HashSet >
package com.ankit;
/** Copyright (c), AnkitMittal JavaMadeSoEasy.com */
/**
* @author AnkitMittal
* Copyright (c), AnkitMittal . All Contents are copyrighted and must not be reproduced in any form.
* This class provides custom implementation of HashSet(without using java api's- we will be using HashMapCustom)- which allows does not allow you to store duplicate values.
* Note- implementation does not allow you to store null values.
* does not maintain insertion order.
* @param <K>
* @param <V>
*/
class HashSetCustom<E>{
private HashMapCustom<E, Object> hashMapCustom;
public HashSetCustom(){
hashMapCustom=new HashMapCustom<>();
}
/**
* add objects in SetCustom.
*/
public void add(E value){
hashMapCustom.put(value, null);
}
/**
* Method returns true if set contains the object.
* @param key
*/
public boolean contains(E obj){
return hashMapCustom.contains(obj) !=null ? true :false;
}
/**
* Method displays all objects in setCustom.
* insertion order is not guaranteed, for maintaining insertion order refer LinkedHashSet.
*/
public void display(){
hashMapCustom.displaySet();
}
/**
* Method removes object from setCustom.
* @param obj
*/
public boolean remove(E obj){
return hashMapCustom.remove(obj);
}
}
/**
* @author AnkitMittal
* Copyright (c), AnkitMittal . All Contents are copyrighted and must not be reproduced in any form.
* This class provides custom implementation of HashMap(without using java api's)- which allows us to store data in key-value pair form..
* @param <K>
* @param <V>
*/
class HashMapCustom<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 HashMapCustom(){
table = new Entry[capacity];
}
/**
* Method allows you put key-value pair in HashMapCustom.
* If the map already contains a mapping for the key, the old value is replaced.
* Note: method does not allows you to put null key thought it allows null values.
* Implementation allows you to put custom objects as a key as well.
* Key Features: implementation provides you with following features:-
* >provide complete functionality how to override equals method.
* >provide complete functionality how to override hashCode method.
* @param newKey
* @param data
*/
public void put(K newKey, V data){
if(newKey==null)
return; //does not allow to store null.
int hash=hash(newKey);
Entry<K,V> newEntry = new Entry<K,V>(newKey, data, null);
if(table[hash] == null){
table[hash] = newEntry;
}else{
Entry<K,V> previous = null;
Entry<K,V> current = table[hash];
while(current != null){ //we have reached last entry of bucket.
if(current.key.equals(newKey)){
if(previous==null){ //node has to be insert on first of bucket.
newEntry.next=current.next;
table[hash]=newEntry;
return;
}
else{
newEntry.next=current.next;
previous.next=newEntry;
return;
}
}
previous=current;
current = current.next;
}
previous.next = newEntry;
}
}
/**
* Method returns value corresponding to key.
* @param key
*/
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;
temp = temp.next; //return value corresponding to key.
}
return null; //returns null if key is not found.
}
}
/**
* Method removes key-value pair from HashMapCustom.
* @param key
*/
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){ //we have reached last entry node of bucket.
if(current.key.equals(deleteKey)){
if(previous==null){ //delete first entry node.
table[hash]=table[hash].next;
return true;
}
else{
previous.next=current.next;
return true;
}
}
previous=current;
current = current.next;
}
return false;
}
}
/**
* Method displays all key-value pairs present in HashMapCustom.,
* insertion order is not guaranteed, for maintaining insertion order refer LinkedHashMapCustom.
* @param key
*/
public void display(){
for(int i=0;i<capacity;i++){
if(table[i]!=null){
Entry<K, V> entry=table[i];
while(entry!=null){
System.out.print("{"+entry.key+"="+entry.value+"}" +" ");
entry=entry.next;
}
}
}
}
/**
* Method returns null if set does not contain object.
* @param key
*/
public K contains(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 key;
temp = temp.next; //return value corresponding to key.
}
return null; //returns null if key is not found.
}
}
/**
* Method displays all objects in setCustom.
* insertion order is not guaranteed, for maintaining insertion order refer LinkedHashSet.
*/
public void displaySet(){
for(int i=0;i<capacity;i++){
if(table[i]!=null){
Entry<K, V> entry=table[i];
while(entry!=null){
System.out.print(entry.key+" ");
entry=entry.next;
}
}
}
}
/**
* Method implements hashing functionality, which helps in finding the appropriate bucket location to store our data.
* This is very important method, as performance of HashMapCustom is very much dependent on this method's implementation.
* @param key
*/
private int hash(K key){
return Math.abs(key.hashCode()) % capacity;
}
}
/**
* Main class- to test HashMap functionality.
*/
public class HashSetCustomApp {
public static void main(String[] args) {
HashSetCustom<Integer> hashSetCustom = new HashSetCustom<Integer>();
hashSetCustom.add(21);
hashSetCustom.add(25);
hashSetCustom.add(30);
hashSetCustom.add(33);
hashSetCustom.add(35);
System.out.println("HashSetCustom contains 21 ="+hashSetCustom.contains(21));
System.out.println("HashSetCustom contains 51 ="+hashSetCustom.contains(51));
System.out.print("Displaying HashSetCustom: ");
hashSetCustom.display();
System.out.println("\n\n21 removed: "+hashSetCustom.remove(21));
System.out.println("22 removed: "+hashSetCustom.remove(22));
System.out.print("Displaying HashSetCustom: ");
hashSetCustom.display();
}
}
/*Output
HashSetCustom contains 21 =true
HashSetCustom contains 51 =false
Displaying HashSetCustom: 21 25 33 30 35
21 removed: true
22 removed: false
Displaying HashSetCustom: 25 33 30 35
*/
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Summary of article >
In this tutorial we learned how to create and implement own/custom Set in java with full program, diagram and examples to insert and retrieve values in it.
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