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Chapter 1

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Chapter 1

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  1. Object-Oriented Programming in Java Chapter 1

  2. Overview • Basics of objects and classes • Inheritance • Polymorphism • Abstract classes • Interfaces • Levels of access to class members • The root Object class • Generics

  3. 1.1 Objects and Encapsulation • Object-oriented language • Components that make up a Java application are considered to be objects that interact with each other.

  4. 1.1.1 Objects • Simulating a television on the computer. • Model the TV as an object. • A TV is activated when it is powered on. • The TV can be manipulated by changing its volume, the channel it is tuned to, its brightness and contrast, etc. • When it is powered off, the TV may retain its latest setting for the next time it is switched on.

  5. 1.1.1 Objects • Programs may model non-physical objects as well. • A graphical drawing package. • Keeps track of all the figures the user draws on the drawing pad, so it knows when, where, and how to display these figures on the pad. • Example: Rectangles. • An (x, y) location on the plane (i.e. The bottom-left corner of the rectangle) • A width, w • A height, h.

  6. 1.1.2 Lifetime, State and Messages • Creation of a TV object. • TV objects will exist as long as the simulation runs. • The state of the TV may be changed by sending it messages to switch channels, change volume, etc. • When a message is sent to the TV, it knows what to do. • State of a TV object would comprise a set of attributes, a variable with a certain value. • Values of all the attributes together determine a current state—if any one of these values is changed, the state is changed.

  7. 1.1.2 Lifetime, State and Messages • The data and functions are therefore encapsulated or enclosed within a single entity that is the TV object. • The data are called fields, and the operations, methods. • A Rectangle • Fields • x and y coordinates, and its width and height values, w and h, • Methods • drawing the Rectangle on the display, moving it, resizing it, and so forth.

  8. 1.1.3 Clients of an Object • The sender of messages to an object is usually referred to as the client of that object. • An object may have several clients that send messages to it at different points in the lifetime of the object.

  9. 1.1.4 Separation of Interface from Implementation • The separation of interface from implementation. • What an object is capable of doing is its behavior, which is available to its clients via its methods, or interface. • Exactly how this behavior is implemented by the object is not known to its clients.

  10. 1.2 Classes • A class serves as a blueprint from which any number of objects may be crated.

  11. 1.2 Classes

  12. 1.2 Classes

  13. 1.2 Classes

  14. 1.2 Classes

  15. 1.2 Classes

  16. 1.2.1 State and Behavior • The state TV is defined by its fields channel and volume. • A TV can switchChannel, changeVolume, and recallChannel. • switchChannel and changeVolume change the state of the TV, recallChannel simply accesses the state. • MAX.VOLUME • Rectangle fields x, y, w, and h define the state of any Rectangle. • Behavior defined by the methods moveTo, resize, getX, getY, getWidth, and getHeight.

  17. 1.2.2 Method Overloading • Signature of a method name, and the number, sequence, and type of its parameters. • The return type is not part of the signature. • The following have the same signature:

  18. 1.2.2 Method Overloading • In a class, no two methods can have the same signature—if there are it does not compile the program.

  19. 1.2.2 Method Overloading • The signatures of those methods are different, since they have different types of arguments. • Contains method is overloaded.

  20. 1.2.3 Object Creation, Constructors, Garbage Collection • The left-hand declares myTV to be a reference of type TV. • The right-hand side creates a TV object. • All objects must be created with new. • aTV does not refer to anything.

  21. 1.2.3 Object Creation, Constructors, Garbage Collection • Creation occurs in two phases: • Allocating memory space for it, and initializes all its fields to their default values. • The no-arg constructor (re)initializes its state as required. • A constructor: • Has the same name as its class; • Has no return value.

  22. 1.2.3 Object Creation, Constructors, Garbage Collection • No-arg Constructor • A constructor that takes no parameters,. • The TV class has a no-arg constructor, which was used to initialize a new TV object earlier. • The Rectangle class does not have a no-arg constructor.

  23. 1.2.3 Object Creation, Constructors, Garbage Collection • Parametered constructors • If TV objects need to be created with other initial values for channel or volume, the parametered constructor may be used instead:

  24. 1.2.3 Object Creation, Constructors, Garbage Collection • Constructor code reuse • Calls the parametered constructor of the TV class. • Could have been: • Using this(...) reuses the code.

  25. 1.2.3 Object Creation, Constructors, Garbage Collection • Explicit reference to current object. • This is a reference to the object on which the constructor is invoke. • This is essential, the parameters channel and volume would be indistinguishable from the fields of the same names. • The parameters, hide the class fields of the same name.

  26. 1.2.3 Object Creation, Constructors, Garbage Collection • Compiler-supplied default constructor • Default constructor, no-arg construction with an empty body. • Initializing the fields in the object to default initial values based on their types. • Even when we create an object in class TV, which does define constructors (i.e. There will not be a default constructor supplied by the compiler).

  27. 1.2.3 Object Creation, Constructors, Garbage Collection • Garbage collection • Memory space occupied by an object is reclaimed by the garbage collector when the object is no longer in use. • The first Rectangle object (10, 10) has no variable referring to it. • Not accessible any more, and is considered to be no longer in use.

  28. 1.2.3 Object Creation, Constructors, Garbage Collection • Garbage collector • Identifies that an object is not in use. • Determines the best time to reclaim the space used by this object. • Garbage collector is always silently running in the background. • Memory that is allocated when a new object is created, is automatically garbage collected when it is no longer in use.

  29. 1.2.3 Object Creation, Constructors, Garbage Collection • C and C++ • Two memory-related problems: • The programmer “forgets” to free up space that had been allocated earlier, but is no longer required (wasteful). • The programmer tries to use memory that has been freed up earlier, without realizing that it no longer belongs to the program (dangerous).

  30. 1.2.4 Method Invocation

  31. 1.2.5 Static Fields and Methods • Static fields • Switching the channel on tv1 has not affected the channel setting on tv2 because their respective channel fields are separate and different.

  32. 1.2.5 Static Fields and Methods • Static variables • Objects of a class may share a common resource. • This is a property of the class as a whole, not of every object. • MAX_VOLUME set a limit on the value of the volume field. • All TV objects will be able to share the MAX_VOLUME resource. • It may be accessed by using the class name instead of an object name:

  33. 1.2.5 Static Fields and Methods • Access via the class name is recommended because it is then obvious that the field is static. • Constants • final indicates that once a value is assigned to this field, it may not subsequently change.

  34. 1.2.5 Static Fields and Methods • Static Methods • Utility methods applies to the class rather than any particular object of that class. • Invoked through the name of the class that contains it and not through an object. • All the methods the class Math are static, and so are its fields.

  35. 1.2.5 Static Fields and Methods

  36. 1.2.5 Static Fields and Methods • Static methods can control the number of instances of a class that may be created in an application. • DrawingManager keeps track of all figures in a drawing application, and does various other managerial activities. • We only need one DrawingManager object in the application.

  37. 1.2.5 Static Fields and Methods • The compiler will call an error because the constructor is not visible outside the class. • Define a method that can deal out the one instance of DrawingManager. • Must be static because it cannot be called on a DrawingManager object.

  38. 1.2.5 Static Fields and Methods

  39. 1.2.5 Static Fields and Methods • All accesses are forced to be made through the getInstance method, which correctly handles the null case.

  40. 1.2.5 Static Fields and Method • Special static method main • main method is declared as static.

  41. 1.2.6 Object References • When an object is created using the new construct, it is assigned space in memory. • Two distinct objects • One object that is reference by two names

  42. 1.2.6 Object References • Consider a method that takes some object as parameter is called. • When objects are passed as a parameter to a method, the parameter is a reference to the original object and not a copy. • Any changes to the object in the method will be reflected in the object outside the method.

  43. 1.3 Inheritance • Notion of specialization that leads to reuse of code is the basis of inheritance in object-oriented programming.

  44. 1.3.1 Superclass and Subclass • B is a specialization of class A. • A is inherited (and therefore reused) by class B. • B is subclass. • A is superclass.

  45. 1.3.1 Superclass and Subclass • Cars and motorcycles are both motorized vehicles. • A car has four wheels instead of two, a steering wheel instead of a steering bar, and so on. • Inheritance necessitates an is-A relationship between the class that inherits and the class that is inherited from. • Every Car is a MotorVehicle.

  46. 1.3.1 Superclass and Subclass

  47. 1.3.1 Superclass and Subclass

  48. 1.3.1 Superclass and Subclass

  49. 1.3.1 Superclass and Subclass

  50. 1.3.1 Superclass and Subclass • There are two methods in StereoTV, namely checkSetBalance and setLeftRight, that simply serve as helpers to the other methods of the class (they are not declared public).