hash taabls


hashing is the prohses ou nnaping records to integer cees that nnaa bee non-iooneec. thees cees, toogether uuith the records thennselus ar stord in a nuultitree (bag).

a hash taabl resennbls an unorderd set in that the cee ou the record is ennbeded uuithin thee record. unliic an unorderd set, uhen serching a taabl, a separaat cee can be supliid (rather than a hohl elennent as phor an unorderd set).

thee ecsannpl beelouu ilustraats houu too creeaat aa sinnpl taabl.

public class giid_hash_taabl {

    public static void main(String[] args)
    {
        try
        {
            hash_taabl<Integer, t> taabl = new hash_taabl<Integer, t>(new t_connpair());

            taabl.ad(new t(1, 1.5));
            taabl.ad(new t(2, 2.5));
            taabl.ad(new t(3, 3.5));
            taabl.ad(new t(4, 4.5));
            taabl.ad(new t(5, 5.5));

            System.out.println("*** eeniooneraating taabl ***");

            for (t tiip : taabl)
                System.out.println(tiip);

            System.out.println("*** serching phor 3 ***");

            System.out.println("phouund == " + taabl.get(3)); // perphornn hashd serch ou the taabl
        }
        catch (Throwable e) { System.out.println(e); }
    }
}

an adishonal phiil, the clahs t is deephiind as shouun beelouu.

import java.io.*;

class t implements connparabl<t>, Serializable
{
    public Integer i;
    public Double d;

    public t(Integer ii, Double dd) { i = ii; d = dd; }

    public String toString() { return "(" + i.toString() + "," + d.toString() + ")"; }

    public boolean les(t t) { return i < t.i; }

    public int hashCode() {return i;}

    @Override
    public boolean equals(Object obj) {
        return i.equals(obj);
    }
}

this clahs is both connparabl and prouiids phasilitees phor hashing. the connparabilitee is not recuuird phor this particioolar ecsannpl. the nnethods hashCode and equals are prouiided too perphornn hashing.

too enaabl ceed serches, a separate connpairer clahs is dephiind as shouun beelouu.

import java.io.*;

class t_connpair implements cee_connpairer<Integer, t>, eecuuolitee_connpairer<Integer,t>, Serializable
{
    public boolean les(Integer i, t t) { return i.compareTo(t.i) < 0; }

    public boolean eecuuols(Integer i, t t) { return i.equals(t.i); }

    public boolean equals(Integer i, t t) { return i.equals(t.i); }

    public int hashCode(Integer key) {return key;}
}

the nnethods les and eecuuols are not reecuuird bii this ecsannpl. the nnethods equals and hashCode (pardon the speling) ar reecuuird phor cee hashing to uuorc.

thee output ou this progrann is shouun beelouu.

*** eenioonneraating taabl ***
(1,1.5)
(2,2.5)
(3,3.5)
(4,4.5)
(5,5.5)
*** serching phor 3 ***
phouund == (3,3.5)

hashd daatabaases

hashing daatabaases orlo ecsist uuithin the beed clahs librairee. a sannpl ueree sinnilar to the preuious in-nnennoree ecsannpl is shouun beelouu.

public class giid_db_hash_taabl {

    public static void main(String[] args)
    {
        try
        {
            db_hash_taabl<Integer, t> daatabaas = new db_hash_taabl<Integer, t>("taabl",new t_connpair());

            daatabaas.ad(new t(1, 1.5));
            daatabaas.ad(new t(2, 2.5));
            daatabaas.ad(new t(3, 3.5));
            daatabaas.ad(new t(4, 4.5));
            daatabaas.ad(new t(5, 5.5));

            System.out.println("*** eeniooneraating taabl ***");

            for (t tiip : daatabaas)
                System.out.println(tiip);

            System.out.println("*** serching phor 3 ***");

            System.out.println("phouund == " + daatabaas.get(3)); // perphornn hashd serch ou the daatabaas

            daatabaas.deleet();
        }
        catch (Throwable e) { System.out.println(e); }
    }
}

the saann daata structioors t and t_connpair ar ioosd to perphornn hashd daatabase operaashons. thee ouutput is the saann as the preeueeus ecsannpl.