OOP Review

1. OOP Review

2. Key OOP Concepts • Object, Class • Instantiation, Constructors • Encapsulation • Inheritance and Subclasses • Abstraction • Reuse • Polymorphism, Dynamic Binding

3. Object Definition: a thing that has identity, state, and behavior • identity: a distinguished instance of a class • state: collection of values for its variables • behavior: capability to execute methods * variables and methods are defined in a class

4. Class Definition: a collection of data (fields/ variables) and methods that operate on that data • data/methods define the contents/capabilities of the instances (objects) of the class • a class can be viewed as a factory for objects • a class defines a recipe for its objects

5. Instantiation • Object creation • Memory is allocated for the object’s fields as defined in the class • Initialization is specified through a constructor • a special method invoked when objects are created

6. Encapsulation • A key OO concept: “Information Hiding” • Key points • The user of an object should have access only to those methods (or data) that are essential • Unnecessary implementation details should be hidden from the user • In Java, use classes and access modifiers (public, private, protected)

7. Inheritance • Inheritance: • Programming language feature that allows for the implicit definition of variables/methods for a class through an existing class • Subclass relationship • B is a subclass of A • B inherits all definitions (variables/methods) in A

8. Abstraction • OOP is about abstraction • Encapsulation and Inheritance are examples of abstraction • What does the verb “abstract” mean?

9. Reuse • Inheritance encourages software reuse • Existing code need not be rewritten • Successful reuse occurs only through careful planning and design • when defining classes, anticipate future modifications and extensions

10. Polymorphism • “Many forms” • allow several definitions under a single method name • Example: • “move” means something for a person object but means something else for a car object • Dynamic binding: • capability of an implementation to distinguish between the different forms during run-time

11. Visual and Event-driven Programming • Fits very well with the OO Paradigm • Visual Programming and GUIs • windows, icons, buttons, etc. are objects created from ready-made classes • Event-driven Programming • execution associated with user interaction with visual objects that causes the invocation of other objects’ methods

12. OOP and Object Interaction • Objects pass messages to each other • An object responds to a message by executing an associated method defined in its class • Causes a “chain reaction” • The user could be viewed as an object • Depicted in an Interaction Diagram

13. Java Review

14. What is Java? Java is a general purpose programming language that is: • object-oriented • interpreted, architecture-neutral, portable • distributed (network-aware), secure • simple, robust • multi-threaded • high-performance (?)

15. Two Types of Java Programs • Applications • general-purpose programs • standalone • executed through the operating system • Applets • programs meant for the WWW • embedded in a Web page • normally executed through a browser

16. Fundamental Language Issues • Compilation and Execution • Data Types • Variables • Statements • Arrays • Others

17. Java ProgramCompilation and Execution • Prog.java is compiled to Prog.class • javac Prog.java • Execution • for applets, the browser loads Prog.class (specified in html file) and UI events can then be processed • for applications, a Java interpreter loads Prog.class and causes program execution to begin in the “main()” method of this class

18. The Java Virtual Machine • Browsers and the Java interpreters have to simulate this “standard” machine • “Compile once, run anywhere” • Class Loader • The JVM facilitates the loading and execution of classes • Several classes, not just one class, are loaded • Java class library

19. Data Types • Primitive types • int, double, char, float, long, boolean, byte • Data type sizes • In Java, the size of a data type type is strictly specified

20. Sizes and Rangesfor some Java Types • int: 4 bytes • -2,147,483,648 to 2,147,483,647 • float: 4 bytes • 1.01e-45 to 3.40e+38 • double 8 bytes • 4.94e-324 to 1.80e+308

21. Sizes and Rangesfor some Java Types • boolean: true, false • size: 1 bit • Not compatible with integer types as in C • char: Unicode character set • size: 2 bytes • Superset of ASCII • Internationalization • Still compatible with integer types

22. Two Kinds of Variables • Variables of a primitive type e.g., int x; char c; • Variables of a reference type (class) e.g., Button b; String s; • Conventions • Primitive types are reserved words in Java and are indicated in all-lower-case letters • Class names: first letter usually capitalized

23. Variables and Values • Primitive type variables int x; … x = 5; X X 5

24. Variables and References • Reference type variables Button x; … x = new Button(“copy”); X Button Object “copy” X

25. The new Keyword • new Button(“copy”) creates a Button object and returns a reference (an address) to that object that a Button variable could hold Button Object “copy” X 1023 1023: 1023 is some address in memory

26. The null Keyword • Use null to indicate (or test) that the variable does not currently refer to an object x = null; if (x == null) ... X null

27. Statements • Expression statements • Operations, assignment, function calls • Control structures • if, switch, while, do-while, for, try-catch (for exception handling) • In a compound statement or block ({…}), variable declarations and statements may intersperse

28. Arrays • Declaration: double nums[]; • Creation: nums = new double[8]; • Use: nums[3] = 6.6; * Note: starting index is 0 (0 to 7, above)

29. Visualizing an Array nums nums = new double[8]; 6.6 nums[3] = 6.6; double nums[];

30. Array of Objects TextField object slots TextField object TextField object TextField object TextField object

31. Classes and Objectsin Java

32. Variables and Objects Let Circle be a class with: variable r that indicates its radius method area() that computes its area • Declaration: Circle c; • Instantiation: c = new Circle(); • Usage: c.r = 5.5; System.out.println(c.area());

33. The complete Circle class public class Circle { public double x,y; // center coordinates public double r; // radius // the methods public double circumference() { return 2*3.14*r; } public double area() { return 3.14*r*r; } }

34. Using the Circle class public class TestCircle { public static void main(String args[]) { Circle c; c = new Circle(); c.x = 2.0; c.y = 2.0; c.r = 5.5; System.out.println(c.area()); } }

35. The Dot (“.”) Operator • Allows access to variables (primitive and reference types) and methods of reference type variables. • Example: TextField t = new TextField(10); t.setText(“hi”); // accessing the setText method

36. Method Invocation • Syntax for method invocation object.methodName(arguments) • Method may return a value or simply produce an effect on the object • To find out what methods are available for a given class • javap package.name.NameOfClass • e.g., javap java.awt.Button, or javap Circle

37. The this keyword • this refers to the current object • In the Circle class, the following definitions for area() are equivalent: public double area() { return 3.14 * r * r; } public double area() { return 3.14 * this.r * this.r; } • Using the keyword clarifies that you are referring to a variable inside an object • dot operator used consistently

38. Constructors • A constructor is a special type of method • has the same name as the class • It is called when an object is created • new Circle(); // “calls” the Circle() method • If no constructor is defined, a default constructor that does nothing is implemented

39. A constructor for the Circle class public class Circle { public double x,y; // center coordinates public double r; // radius public Circle() { // sets default values for x, y, and r this.x = 0.0; this.y = 0.0; this.r = 1.0; } ... }

40. A constructor with parameters public class Circle { … public Circle(double x, double y, double z) { this.x = x; this.y = y; this.r = z; // using this is now a necessity } ... }

41. Using the different constructors Circle c, d; c = new Circle(); // radius of circle has been set to 1.0 System.out.println(c.area()); d = new Circle(1.0,1.0,5.0); // radius of circle has been set to 5.0 System.out.println(d.area());

42. Method Overloading • In Java, it is possible to have several method definitions under the same name • but the signatures should be different • Signature: • the name of the method • the number of parameters • the types of the parameters

43. Encapsulation in Java • Access modifiers • public • a public variable/method is available for use outside the class it is defined in • private • a private variable/method may be used only within the class it is defined in

44. The Circle class Revisited public class Circle { private double x,y; // center coordinates private double r; // radius // ... } // when using the Circle class ... Circle c; c.r = 1.0; // this statement is not allowed

45. Outside accessto private data • No direct access • Define (public) set and get methods instead or initialize the data through constructors • Why? • If you change your mind about the names and even the types of these private data, the code using the class need not be changed

46. Set and Get Methods • Variables/attributes in a class are often not declared public • Instead: • define use a (public) set method to assign a value to a variable • define a get method to retrieve that value • Still consistent with encapsulation

47. Set and Get Methods for Radius public class Circle { // ... private double r; // radius // … public void setRadius(double r) { this.r = r; } public double getRadius() { return this.r; } // ... }

48. Inheritance

49. Inheritance and the extends Keyword • In Java, public class B extends A { … } • means B is a subclass of A • objects of class B now have access* to variables and methods defined in A

50. The EnhancedCircle class public class EnhancedCircle extends Circle { // as if area(), circumference(), setRadius() and getRadius() // automatically defined; x,y,r are also present (but are private // to the the Circle class) private int color; public void setColor(int c) { this.color = c; } public void draw() { … } public double diameter() { return this.getRadius()*2; } }