Data Abstraction and Abstract Data Types

Transcription

1 Data Abstraction and Abstract Data Types Tessema M. Mengistu Department of Computer Science Southern Illinois University Carbondale Room

2 Outline Abstract Data types (ADT) ADT Design Specification ADT Java representation 2

3 Data Type A data type is characterized by: a set of values a set of operations, which can be applied uniformly to all these values a data representation, which is common to all these values, and Java has two data types: Primitive data types Reference data types 3

4 Data Type Java provides eight primitive types: boolean char, byte, short, int, long float, double Each primitive type has a set of values a set of operations a data representation These are set in stone there is nothing the programmer can do to change anything about them 4

5 Reference data type Data Types Array All classes built in and user defined Store address of the object that it refers to Has set of values Has set of operations that can be applied Has its own representation 5

6 Data Abstraction The process of hiding the inside detail of a data and providing the essential information It is a programming design technique that relies on the separation of interface and implementation 6

7 Data Abstraction Why abstraction? Easier to design hide what doesn t matter Security prevent access to things that shouldn t be accessed Modifiability can be modified without affecting users 7

8 Abstract Data Type An Abstract Data Type (ADT) can be defined as: Is a specification of a data set and the operations on that data. An ADT is an externally defined data type that holds some kind of data. An ADT has built-in operations that can be performed on it or by it. Does not indicate any information about the internal representation of the data storage or implementation of the operations. Independent of any programming language. 8

9 Abstract Data Type A class can be a data type The possible values of a class are called objects The data representation is a reference (pointer) to a block of storage The structure of this block is defined by the fields (both inherited and immediate) of the class The operations on the objects are called methods Many classes are defined in Java s packages Classes can be defined as needed 9

10 Abstract Data Type ADT doesn t specify how to store the data or how to implement the operations ADTs are independently of any programming language. Data structure An implementation of an ADT within a programming language. 10

11 Abstract Data Type A collection is a general term for an ADT that contains a group of objects. Duplicate Vs non duplicate Ordered Vs non ordered A container is a class that implements a collection. 11

12 Definition: Bag A finite collection of objects In no particular order May contain duplicate objects 12

13 Behaviors Determine how many objects in bag Full? Empty? Add, remove objects Count duplicates Look for specific object View all objects 13

14 Data Collection of Objects Number of objects in the bag 14

15 CRC of Bag 15

16 Design Specification of a Bag Describe data Specify methods for bag s behaviors Name methods Choose parameters Decide return types Write comments 16

17 Design Decisions What should the method add do when it cannot add a new entry? Nothing? Leave bag unchanged, signal client of condition? Throw an Exception 17

18 Design Decisions What should happen when an unusual condition occurs? Assume invalid never happens? Ignore invalid event? Guess at client s intention? Return flag value? Return boolean value success/failure? Throw exception? 18

19 UML 19

20 Interface Bag Can contain only constants (final variables) and abstract method (no implementation) Bag can organize into interface Note items in bag are of same type Generic type <T> 20

21 public interface BagInterface<T> { public int getcurrentsize(); public boolean isfull(); public boolean isempty(); public boolean add(t newentry); public T remove(); public boolean remove(t anentry); public void clear(); public int getfrequencyof(t anentry); public boolean contains(t anentry); public T[] toarray(); } // end BagInterface 21

22 Using ADT Bag Implementation done from specifications User needs know what ADT does, not how Type of object in bag specified by program using the ADT 22

23 public class OnlineShopper { public static void main(string[] args) { Item[] items = {new Item("Bird feeder", 2050), new Item("Squirrel guard", 1547), new Item("Bird bath", 4499), new Item("Sunflower seeds", 1295)}; BagInterface<Item> shoppingcart = new Bag<Item>(); int totalcost = 0; for (int index = 0; index < items.length; index++) { Item nextitem = items[index]; // simulate getting item from shoppingcart.add(nextitem); totalcost = totalcost + nextitem.getprice(); } // end for while (!shoppingcart.isempty()) { System.out.println(shoppingCart.remove()); System.out.println("Total cost: " + "\t$" + totalcost / "." + totalcost % 100); } } 23

24 Output 24

25 A Vending Machine 25

26 Vending Machine Like An ADT Perform only available tasks User must understand the tasks Cannot access inside of mechanism Usable without knowing inside implementation New inside implementation unknown to users 26

27 Class Library The interface Set public boolean add(t newentry) public boolean remove(object anentry) public void clear() public boolean contains(object anentry) public boolean isempty() public int size() public Object[] toarray() 27

28 Summary An abstract data type, or ADT, is a specification of a data set and the operations on that data. This specification does not indicate how to store the data or how to implement the operations, and it is independent of any programming language. A collection is an object that holds a group of other objects. Writing a Java interface is a way to organize a specification for an ADT. 28