Lecture Java methods: Object-oriented programming and data structures (2nd AP edition): Chapter 25 - Maria Litvin, Gary Litvin

Chapter 25 - Lookup tables and hashing. After you have mastered the material in this chapter, you will be able to: Learn about lookup tables, learn about hashing, review java.util.HashSet and java.util.HashMap.

Lookup Tables and Hashing

Hash Function

Danger  Keep Out 

25 Chapter

Copyright © 2011 by Maria Litvin, Gary Litvin, and Skylight Publishing All

rights reserved.

Java Methods

Object-Oriented Programming

and Data Structures

Maria Litvin ● Gary Litvin

2nd AP edition with GridWorld

Objectives:

java.util.HashMap

Lookup Tables

data very quickly

(or some values)

an array index using a simple formula

Lookup Tables (cont’d)

particular index (no collisions).

Lookup Tables: Example 2

private static final int [ ] n_thPowerOf3 =

{ 1, 3, 9, 27, 81, 243, 729, 2187, 6561, 19683 }; .

Lookup Tables: Example 3

256 colors used

in a particular image; each of the palette entries corresponds to a triplet of RGB values

Hash Tables

the valid range

onto the same array index — this situation is

Hash Tables (cont’d)

the array indices randomly and uniformly

minimize the number of collisions

resolving collisions: chaining and probing.

Danger  Keep Out 

Danger  Keep Out 

computed using a certain probing formula

using hash tables (with chaining)

small

Load factor =  Total number of items

Number of buckets

iterations over the whole set

the table is automatically rehashed into a

larger table; if possible this should be

avoided

helps calculate the hashing function

range of indices in a particular hash table

should agree with each other:

 x.equals (y) x.compareTo (y) == 0

 x.equals (y) x.hashCode( ) == y.hashCode( )

Never mind

25­20

Review:

table and a hash table?

Review (cont’d):

the load factor is too high? Too low?

when the load factor exceeds the specified

limit?

capacity to 16 and the load factor limit to

0.75 How many values can be stored in this table before it is rehashed?

Review (cont’d):

method?