Data Abstraction and Abstract Data Types Tessema M. Mengistu - - PowerPoint PPT Presentation

Data Abstraction and Abstract Data Types

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Tessema M. Mengistu Department of Computer Science Southern Illinois University Carbondale tessema.mengistu@siu.edu Room - 3131

• Outline

–Abstract Data types (ADT) –ADT Design Specification –ADT Java representation

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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

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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

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Data Types

• Reference data type

– 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

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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

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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

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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

• n it or by it.

– Does not indicate any information about the internal representation of the data storage or implementation

• f the operations.

– Independent of any programming language.

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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

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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.

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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.

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Definition: Bag

• A finite collection of objects
• In no particular order
• May contain duplicate objects

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Behaviors

• Determine how many objects in bag

– Full? – Empty?

• Add, remove objects
• Count duplicates
• Look for specific object
• View all objects

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Data

• Collection of Objects
• Number of objects in the bag

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CRC of Bag

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Design Specification of a Bag

• Describe data
• Specify methods for bag’s behaviors

– Name methods – Choose parameters – Decide return types – Write comments

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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

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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?

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UML

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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>

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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

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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

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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 / 100 + "." + totalCost % 100); } }

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• Output

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A Vending Machine

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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

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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()

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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.

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