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Hashing is an elegant technique which powerfully illustrates how computation can facilitate search. Hashing is introduced conceptually. Linear probing and variations are discussed. Chaining is discussed. An assignment is posed to implementing a Hashing ADT in terms of chaining. Another assignment, a "color viewer" application, is posed which makes use of a hash table to store color values.
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/*
* HashTest.java
*/
package testers;
import hashing.*;
import specialobject.*;
import java.awt.*;
public class HashTest
{
static public void main(String args[])
{
System.out.println("\nCREATE AND DISPLAY INITIAL HASH TABLE.");
HashTable ht = new HashTable(10);
ht.display();
System.out.println("\nADD OBJECTS TO THE HASH TABLE.");
SpecialObject color;
color = new SpecialObject("aliceblue",new Color(240,248,255));
ht.add(color);
color = new SpecialObject("antiquewhite",new Color(250,235,215));
ht.add(color);
color = new SpecialObject("aqua",new Color(0,255,255));
ht.add(color);
color = new SpecialObject("aquamarine",new Color(127,255,212));
ht.add(color);
color = new SpecialObject("azure",new Color(240,255,255));
ht.add(color);
color = new SpecialObject("beige",new Color(245,245,220));
ht.add(color);
color = new SpecialObject("bisque",new Color(255,228,196));
ht.add(color);
color = new SpecialObject("black",new Color(0,0,0));
ht.add(color);
color = new SpecialObject("blanchedalmond",new Color(255,235,205));
ht.add(color);
color = new SpecialObject("blue",new Color(0,0,255));
ht.add(color);
color = new SpecialObject("blueviolet",new Color(138,43,226));
ht.add(color);
color = new SpecialObject("brown",new Color(165,42,42));
ht.add(color);
color = new SpecialObject("burlywood",new Color(222,184,135));
ht.add(color);
color = new SpecialObject("cadetblue",new Color(95,158,160));
ht.add(color);
color = new SpecialObject("chartreuse",new Color(127,255,0));
ht.add(color);
color = new SpecialObject("chocolate",new Color(210,105,30));
ht.add(color);
System.out.println("\nDISPLAY THE HASH TABLE.");
ht.display();
}
}
init:
deps-jar:
compile-single:
run-single:
CREATE AND DISPLAY INITIAL HASH TABLE.
Bucket 0
[]
Bucket 1
[]
Bucket 2
[]
Bucket 3
[]
Bucket 4
[]
Bucket 5
[]
Bucket 6
[]
Bucket 7
[]
Bucket 8
[]
Bucket 9
[]
ADD OBJECTS TO THE HASH TABLE.
id = aliceblue
sum of squares of letter positions = 874
hash value of aliceblue = 4
id = antiquewhite
sum of squares of letter positions = 2532
hash value of antiquewhite = 2
id = aqua
sum of squares of letter positions = 732
hash value of aqua = 2
id = aquamarine
sum of squares of letter positions = 1528
hash value of aquamarine = 8
id = azure
sum of squares of letter positions = 1467
hash value of azure = 7
id = beige
sum of squares of letter positions = 184
hash value of beige = 4
id = bisque
sum of squares of letter positions = 1201
hash value of bisque = 1
id = black
sum of squares of letter positions = 279
hash value of black = 9
id = blanchedalmond
sum of squares of letter positions = 1210
hash value of blanchedalmond = 0
id = blue
sum of squares of letter positions = 614
hash value of blue = 4
id = blueviolet
sum of squares of letter positions = 1973
hash value of blueviolet = 3
id = brown
sum of squares of letter positions = 1278
hash value of brown = 8
id = burlywood
sum of squares of letter positions = 2533
hash value of burlywood = 3
id = cadetblue
sum of squares of letter positions = 1065
hash value of cadetblue = 5
id = chartreuse
sum of squares of letter positions = 1974
hash value of chartreuse = 4
id = chocolate
sum of squares of letter positions = 1102
hash value of chocolate = 2
DISPLAY THE HASH TABLE.
Bucket 0
[[blanchedalmond:java.awt.Color[r=255,g=235,b=205]]]
Bucket 1
[[bisque:java.awt.Color[r=255,g=228,b=196]]]
Bucket 2
[[antiquewhite:java.awt.Color[r=250,g=235,b=215]],
[aqua:java.awt.Color[r=0,g=255,b=255]],
[chocolate:java.awt.Color[r=210,g=105,b=30]]]
Bucket 3
[[blueviolet:java.awt.Color[r=138,g=43,b=226]],
[burlywood:java.awt.Color[r=222,g=184,b=135]]]
Bucket 4
[[aliceblue:java.awt.Color[r=240,g=248,b=255]],
[beige:java.awt.Color[r=245,g=245,b=220]],
[blue:java.awt.Color[r=0,g=0,b=255]],
[chartreuse:java.awt.Color[r=127,g=255,b=0]]]
Bucket 5
[[cadetblue:java.awt.Color[r=95,g=158,b=160]]]
Bucket 6
[]
Bucket 7
[[azure:java.awt.Color[r=240,g=255,b=255]]]
Bucket 8
[[aquamarine:java.awt.Color[r=127,g=255,b=212]],
[brown:java.awt.Color[r=165,g=42,b=42]]]
Bucket 9
[[black:java.awt.Color[r=0,g=0,b=0]]]
BUILD SUCCESSFUL (total time: 1 second)
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/*
* SpecialObjectADT.java
*/
package specialobject;
public interface SpecialObjectADT
{
public String key();
public Object object();
public String toString();
}
/*
* SpecialObject.java
*/
package specialobject;
public class SpecialObject implements SpecialObjectADT
{
String key;
Object object;
public SpecialObject(String k, Object o)
{
key = k;
object = o;
}
public String key()
{
return key;
}
public Object object()
{
return object;
}
public String toString()
{
return "["+key+":"+object+"]";
}
}
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/*
* HashTableADT.java
*/
package hashing;
import specialobject.*;
public interface HashTableADT
{
public void add(SpecialObject object);
public void delete(String key);
public SpecialObject lookup(String key);
public boolean memberp(String key);
public void display();
}
/*
* HashTable.java
*/
package hashing;
import specialobject.*;
import hashing.HashTableADT;
import java.util.*;
public class HashTable implements HashTableADT
{
Vector[] hashtable;
public HashTable(int n)
{
hashtable = new Vector[n];
for ( int x = 0; x < n; x++ ) {
hashtable[x] = new Vector();
}
}
public void add(SpecialObject object)
{
String key = object.key();
int a = hash(key);
hashtable[a].addElement(object);
}
public boolean memberp(String key)
{
int a = hash(key);
return memberp(hashtable[a],key);
}
private boolean memberp(Vector v,String k)
{
for ( int i = 0; i < v.size(); i++ ) {
SpecialObject so = (SpecialObject)v.elementAt(i);
if ( so.key().equals(k) ) return true;
}
return false;
}
public SpecialObject lookup(String key)
{
int a = hash(key);
return lookup(hashtable[a],key);
}
private SpecialObject lookup(Vector v,String k)
{
for ( int i = 0; i < v.size(); i++ ) {
SpecialObject so = (SpecialObject)v.elementAt(i);
if ( so.key().equals(k) ) return so;
}
return null;
}
public void delete(String key)
{
int a = hash(key);
delete(hashtable[a],key);
}
private void delete(Vector v,String k)
{
for ( int i = 0; i < v.size(); i++ ) {
SpecialObject so = (SpecialObject)v.elementAt(i);
if ( so.key().equals(k) ) {
v.removeElementAt(i);
break;
}
}
}
public void display()
{
for ( int i = 0; i < hashtable.length; i++ ) {
System.out.println("Bucket " + i );
System.out.println(hashtable[i].toString());
}
}
private int hash(String s)
{
System.out.println("id = " + s);
int sum = 0;
for ( int i = 0; i < s.length(); i++ ) {
String c = s.substring(i,i+1);
int x = alphaPos(c);
sum = sum + ( x * x );
}
System.out.println("sum of squares of letter positions = " + sum);
int h = modulo(sum,hashtable.length);
System.out.println("hash value of " + s + " = " + h);
return h;
}
private int alphaPos(String c)
{
int result = "abcdefghigklmnopqrstuvwxyz".indexOf(c.toLowerCase()) + 1;
return result;
}
private int modulo(int a, int b)
{
if ( a < b ) {
return a;
} else {
return modulo(a-b,b);
}
}
}