01-introductionto Object ooriented Programming in JAVA CS.ppt

1
CSE 331
Introduction;
Review of Java and OOP
slides created by Marty Stepp
based on materials by M. Ernst, S. Reges, D. Notkin, R. Mercer, Wikipedia
http://www.cs.washington.edu/331/

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What is this course about?
• specification and documentation
• object-oriented design
 taking a problem and turning it into a set of well-designed classes
• testing, debugging, and correctness
• learning to use existing software libraries and APIs
• using software tools and development environments (IDEs)
• working in small groups to solve programming projects
• things that are "sort of" course topics:
 Java language features
 graphical user interfaces (GUIs)

3
Building Good Software is
Hard
• large software systems are enormously complex
 millions of "moving parts"
• people expect software to be malleable
 software mitigates the deficiencies of other components
• we are always trying to do new things with software
 relevant experience is often missing
• software engineering is about:
 managing complexity and change
 coping with potential defects
• customers, developers, environment, software

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Managing Complexity
• abstraction and specification
 procedural, data, control flow
 why they are useful and how to use them
• writing, understanding, and reasoning about code
 the examples are in Java, but the issues are more general
• program design and documentation
 the process of design; design tools
• pragmatic considerations
 testing
 debugging and defensive programming

5
Prerequisite knowledge
To do well in this course, you should know (or quickly review):
 basic Java syntax (loops, if/else, variables, arrays, parameters/return)
 primitive vs. object types; value vs. reference semantics
 creating classes of objects (syntax and semantics)
• fields, encapsulation, public/private, instance methods, constructors
• client (external) vs. implementation (internal) views of an object
• static vs. non-static
 inheritance and interfaces (basic syntax and semantics)
 Java Collections Framework (List, Set, Map, Stack, Queue, PriorityQueue)
• using generics; primitive "wrapper" classes
 exceptions (throwing and catching)
 recursion
see Review slides on course web site, or Core Java Ch. 1-6, for review material

6
OOP and OOD
• object-oriented programming: A programming paradigm where a
software system is represented as a collection of objects that
interact with each other to solve the overall task.
 most CSE 142 assignments are not object-oriented (why not?)
 many CSE 143 assignments are object-oriented
• but not all are well-designed (seen later)
 most software you will write after CSE 143 is object-oriented
• exceptions: functional code; systems programming; web programming

7
Major OO concepts
• Object-oriented programming is founded on these ideas:
 object/class: An object is an entity that combines data with behavior
that acts on that data. A class is a type or category of objects.
 information hiding (encapsulation): The ability to protect some
components of the object from external entities ("private").
 inheritance: The ability for a class ("subclass") to extend or override
functionality of another class ("superclass").
 polymorphism: The ability to replace an object with its sub-objects to
achieve different behavior from the same piece of code.
 interface: A specification of method signatures without supplying
implementations, as a mechanism for enabling polymorphism.

8
Object-oriented design
• object-oriented design: The process of planning a system of
interacting objects and classes to solve a software problem.
 (looking at a problem and deducing what classes will help to solve it)
 one of several styles of software design
• What are the benefits of OO design?
 How do classes and objects help improve the style of a program?
 What benefits have you received by using objects created by others?

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Inputs to OO design
• OO design is not the start of the software development process.
First the dev team may create some or all of the following:
 requirements specification: Documents that describe the desired
implementation-independent functionality of the system as a whole.
 conceptual model: Implementation-independent diagram that
captures concepts in the problem domain.
 use cases: Descriptions of sequences of events that, taken together,
lead to a system doing something useful to achieve a specific goal.
 user interface prototype: Shows and describes the look and feel of the
product's user interface.
 data model: An abstract description of how data is represented and
used in the system (databases, files, network connections, etc.).

10
Classic OO design exercise
• A classic type of object-oriented design question is as follows:
 Look at a description of a particular problem domain or software
system and its necessary features in high-level general terms.
 From the description, try to identify items that might be good to
represent as classes if the system were to be implemented.
 Hints:
• Classes and objects often correspond to nouns in the problem description.
 Some nouns are too trivial to represent as entire classes; maybe they are
simply data (fields) within other classes or objects.
• Behaviors of objects are often verbs in the problem description.
• Look for related classes that might make candidates for inheritance.

11
OO design exercise
What classes are in this Texas Hold 'Em poker system?
 2 to 8 human or computer players
 Computer players with skill setting: easy, medium, hard
 Each player has a name and stack of chips
 Summary of each hand:
• Dealer collects ante from appropriate players, shuffles the deck, and deals
each player a hand of 2 cards from the deck.
• A betting round occurs, followed by dealing 3 shared cards from the deck.
• As shared cards are dealt, more betting rounds occur, where each player
can fold, check, or raise.
• At the end of a round, if more than one player is remaining, players' hands
are compared, and the best hand wins the pot of all chips bet.

12
OO design exercise
What classes are in this video store kiosk system?
 The software is for a video kiosk that replaces human clerks.
 A customer with an account can use their membership and credit
card at the kiosk to check out a video.
 The software can look up movies and actors by keywords.
 A customer can check out up to 3 movies, for 5 days each.
 Late fees can be paid at the time of return or at next checkout.

13
Java's object-oriented
features (overview)

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Fields
• field: A variable inside an object that is part of its state.
– Each object has its own copy of each field.
• Declaration syntax:
private type name;
– Example:
public class Point {
private int x;
private int y;
...
}

15
Instance methods
• instance method (or object method): Exists inside each object of a
class and gives behavior to each object.
public type name(parameters) {
statements;
}
 same syntax as static methods, but without static keyword
Example:
public void tranlate(int dx, int dy) {
x += dx;
y += dy;
}

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Categories of methods
• accessor: A method that lets clients examine object state.
 Examples: distance, distanceFromOrigin
 often has a non-void return type
• mutator: A method that modifies an object's state.
 Examples: setLocation, translate
• helper: Assists some other method in performing its task.
 often declared as private so outside clients cannot call it

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The toString method
tells Java how to convert an object into a String for printing
public String toString() {
code that returns a String representing this object;
}
 Method name, return, and parameters must match exactly.
 Example:
// Returns a String representing this Point.
public String toString() {
return "(" + x + ", " + y + ")";
}

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Constructors
• constructor: Initializes the state of new objects.
public type(parameters) {
statements;
}
– runs when the client uses the new keyword
– no return type is specified; implicitly "returns" the new object
private int x;
private int y;
public Point(int initialX, int
initialY) {
x = initialX;
y = initialY;
}

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The keyword this
• this : Refers to the implicit parameter inside your class.
(a variable that stores the object on which a method is called)
 Refer to a field: this.field
 Call a method: this.method(parameters);
 One constructor this(parameters);
can call another:

20
Calling another constructor
private int x;
private int y;
public Point() {
this(0, 0);
}
public Point(int x, int y) {
this.x = x;
this.y = y;
}
...
}
• Avoids redundancy between constructors
• Only a constructor (not a method) can call another constructor

21
Inheritance
• inheritance: Forming new classes based on existing ones.
 a way to share/reuse code between two or more classes
 superclass: Parent class being extended.
 subclass: Child class that inherits behavior from superclass.
• gets a copy of every field and method from superclass
 is-a relationship: Each object of the subclass also "is a(n)" object of the
superclass and can be treated as one.

22
Inheritance syntax
public class name extends superclass {
 Example:
public class Lawyer extends Employee {
...
}
• By extending Employee, each Lawyer object now:
 receives a copy of each method from Employee automatically
 can be treated as an Employee by client code
• Lawyer can also replace ("override") behavior from Employee.

23
The super keyword
• A subclass can call its parent's method/constructor:
super.method(parameters) // method
super(parameters); // constructor
public class Lawyer extends Employee {
public Lawyer(String name) {
super(name);
}
// give Lawyers a $5K raise (better)
public double getSalary() {
double baseSalary = super.getSalary();
return baseSalary + 5000.00;
}
}

24
Shapes example
• Consider the task of writing classes to represent 2D shapes such as
Circle, Rectangle, and Triangle.
• Certain attributes or operations are common to all shapes:
 perimeter: distance around the outside of the shape
 area: amount of 2D space occupied by the shape
 Every shape has these, but each computes them differently.

25
Interfaces
• interface: A list of methods that a class can promise to implement.
 Inheritance gives you an is-a relationship and code sharing.
• A Lawyer can be treated as an Employee and inherits its code.
 Interfaces give you an is-a relationship without code sharing.
• A Rectangle object can be treated as a Shape but inherits no code.
 Analogous to non-programming idea of roles or certifications:
• "I'm certified as a CPA accountant.
This assures you I know how to do taxes, audits, and consulting."
• "I'm 'certified' as a Shape, because I implement the Shape interface.
This assures you I know how to compute my area and perimeter."

26
Interface syntax
public interface name {
public type name(type name, ..., type name);
...
}
Example:
public interface Shape {
public double area();
public double perimeter();
}

27
Implementing an interface
public class name implements interface {
...
}
• A class can declare that it "implements" an interface.
 The class promises to contain each method in that interface.
(Otherwise it will fail to compile.)
 Example:
public class Rectangle implements Shape {
...
public double area() { ... }
public double perimeter() { ... }
}

28
Interfaces + polymorphism
• Interfaces benefit the client code author the most.
 they allow polymorphism
(the same code can work with different types of objects)
public static void printInfo(Shape s) {
System.out.println("The shape: " + s);
System.out.println("area : " + s.area());
System.out.println("perim: " + s.perimeter());
System.out.println();
}
...
Circle circ = new Circle(12.0);
Triangle tri = new Triangle(5, 12, 13);
printInfo(circ);
printInfo(tri);

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