1 / 33

Computer Science 1 (Studio) 06A-Objects and Classes

Computer Science 1 (Studio) 06A-Objects and Classes. What is Object Oriented Programming?. OOP is a practice which gained mainstream acceptance in the 1990’s thanks to languages like C++ and Java The goal behind OOP is to organize a program in a way which mirrors the real world

icundiff
Download Presentation

Computer Science 1 (Studio) 06A-Objects and Classes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Computer Science 1 (Studio) 06A-Objects and Classes CS1s - 06A-Objects and Classes (v1.00)

  2. What is Object Oriented Programming? • OOP is a practice which gained mainstream acceptance in the 1990’s thanks to languages like C++ and Java • The goal behind OOP is to organize a program in a way which mirrors the real world • The real world is composed entirely of objects which have personal attributes and activities • A Java program can be viewed as a series of objects which interact with each other CS1s - 06A-Objects and Classes (v1.00)

  3. What is an Object? • The state defines the current object, and the behavior defines what the object can do • Consider a Circle object: the state is the current value/s of the data fields radius = 5 r=5 find Area the behavior is the set of supported methods find Circumference The real world OOP CS1s - 06A-Objects and Classes (v1.00)

  4. What is an Object? • So an object encapsulates both its state and behaviors into one entity • The object is the singular, unifying construct radius = 5.0 find Area find Circumference CS1s - 06A-Objects and Classes (v1.00)

  5. What is a Class? • A class is a blueprint for creating objects of the same type • A class defines the complete collection of data (properties) and methods (actions) • Consider a Circle class in Java: class Circle { double radius = 5.0; double findArea() { return radius * radius * Math.PI; } double findCircumference() { return 2 * Math.PI * radius; } } class name data methods CS1s - 06A-Objects and Classes (v1.00)

  6. Objects vs Classes • In class activity: • Consider your Java textbook, “Introduction to Java Programming”. • Based on how this book is represented in the real world, draw a picture of the object • What is the state of this object? Identify the nouns you come up with when describing this textbook • What are its behaviors? Identify the verbs which describe what you can do with a book • Refer back to the Circle object for help • Now, define a Java class for this object • What would the name of the class be? • What would the class look like as Java code (data/methods)? • Refer back to the Circle class for help CS1s - 06A-Objects and Classes (v1.00)

  7. Objects vs Classes • Identify each of the following as being an object or a class • Movie...Star Wars • Rose...Flower • Band...Pink Floyd • Acura NSX...Car • Programming Language...Java • College...RIT • Guinness...Beer CS1s - 06A-Objects and Classes (v1.00)

  8. Instantiation • When you create an object from a class, we call that object an instance of that class • “Object” and “instance” are used interchangeably • You can create many instances from the same class • Creating an instance is referred to as instantiation radius = 5 radius = 5 radius = 5 find Area find Area find Area find Circumference find Circumference find Circumference CS1s - 06A-Objects and Classes (v1.00)

  9. Instantiation • One class has the ability of generate any number of objects whose type is a Circle class Circle { double radius = 5.0; double findArea() { return radius * radius * Math.PI; } double findCircumference() { return 2 * Math.PI * radius; } } class instantiates radius = 5 radius = 5 radius = 5 objects find... find... find... find... find... find... CS1s - 06A-Objects and Classes (v1.00)

  10. Instantiation • Each Circle object shares the same collection of behaviors and data • However, each Circle object maintains its own individual values for the data, and it’s own address in memory radius = 5 radius = 2 radius = 3 find... find... find... find... find... find... CS1s - 06A-Objects and Classes (v1.00)

  11. Creating Objects • Objects are created using the new operator, followed by the class name new ClassName(); • To create a circle object: new Circle(); radius = 5 find... find... CS1s - 06A-Objects and Classes (v1.00)

  12. Object Reference Variables • In order to use an object, we must have some way to get to it (usually by name) • A name that refers to an object is a reference variable • To create a reference variable, use the class name following by the reference variable name ClassNameobjectReference; • To create a reference to a Circle called myCircle: CirclemyCircle; myCircle reference name null reference value reference type Circle CS1s - 06A-Objects and Classes (v1.00)

  13. Putting It All Together • To combine the entire process into one statement: Circle myCircle = new Circle(); • We now have a Circle reference called myCircle. It refers to the Circle object created by new on the right. myCircle radius = 5 address find... find... Circle CS1s - 06A-Objects and Classes (v1.00)

  14. Primitives vs. Objects • With primitive types, like int, the variable holds the actual value (i.e. an integer) int myInt = 10; • Arrays are objects and the array name is a reference variable . The value of the reference variable is a reference to the object int myIntArray[] = new int [ 5 ]; myInt 10 int myIntArray 0 0 0 0 0 address1 int[] CS1s - 06A-Objects and Classes (v1.00)

  15. Primitives vs. Objects • Draw the memory diagram which represents each declaration statement: double myDouble = 5.5; Generator myGen = new Generator(); Circle myCircle1 = new Circle(); char myChar = ’A’; String str = new String(“Hello”); Circle myCircle2 = new Circle(); String str2 = new String(“Goodbye”); CS1s - 06A-Objects and Classes (v1.00)

  16. Primitive Assignment • Regardless of whether you are dealing with a primitive or a reference, the behavior with assignment is to copy the value • Primitive assignment: int i = 10; int j; j = i; i j 10 ??? int int i j 10 10 int int value 10 is copied CS1s - 06A-Objects and Classes (v1.00)

  17. Reference Assignment • Reference/object creation: Circle circle1 = new Circle(); Circle circle2 = new Circle(); circle1 radius = 5 address1 find... find... Circle different values for address circle2 radius = 5 address2 find... find... Circle CS1s - 06A-Objects and Classes (v1.00)

  18. Reference Assignment • Reference assignment: circle2 = circle1; circle1 radius = 5 address1 find... find... Circle address in circle1 is copied into circle2 circle2 radius = 5 address1 find... find... Circle CS1s - 06A-Objects and Classes (v1.00)

  19. Primitives vs. Objects • Draw the memory diagram which represents each assignment statement: double myDouble = 5.5; Generator myGen = new Generator(); Circle myCircle1 = new Circle(); char myChar = ’A’; String str = new String(“Hello”); Circle myCircle2 = new Circle(); String str2 = new String(“Goodbye”); myDouble = 10; myChar = ’B’; str2 = str; myCircle2 = myCircle1; CS1s - 06A-Objects and Classes (v1.00)

  20. Garbage Collection • What happens to the bottom object? • Objects occupy memory, so there must be a way to free the space this object occupies if it is not used anymore circle1 radius = 5 address1 find... find... Circle circle2 radius = 5 address1 find... find... Circle CS1s - 06A-Objects and Classes (v1.00)

  21. Garbage Collection • When there are no references to an object, that object is free to be destroyed • The Java runtime system detects garbage and automatically reclaims the space the object occupies circle1 radius = 5 address1 find... find... Circle circle2 address1 POOF! Circle CS1s - 06A-Objects and Classes (v1.00)

  22. Java vs C/C++ • Garbage collection is a huge reason why Java is the preferred introductory language to C or C++ • There is no automated garbage collection in C/C++ • It is up to the programmer to manually handle the deallocation of any unused objects • Failure to manage memory in C/C++ will result in a memory leak, which can cause the program to crash (or behave strangely) CS1s - 06A-Objects and Classes (v1.00)

  23. Garbage Collection • In Java, if you don’t want a reference variable to refer to an object, set the value of the reference to null Circle circle1 = new Circle(); circle1 = null; circle1 radius = 5 address1 find... find... Circle circle1 POOF! null Circle CS1s - 06A-Objects and Classes (v1.00)

  24. Accessing an Object’s Data and Methods • To access the data and methods in an object, use the dot notation • To access the instance data in an object: referenceVariable.data • For example: Circle circle1 = new Circle(); double radius = circle1.radius; circle1 radius radius = 5 address1 5.0 find... find... Circle int CS1s - 06A-Objects and Classes (v1.00)

  25. Accessing an Object’s Data and Methods • To invoke an object’s instance method: referenceVariable.method(arguments); • For example: double area = circle1.findArea(); circle1 area radius = 5 address1 78.648.. find... find... Circle int CS1s - 06A-Objects and Classes (v1.00)

  26. TestCircle.java • In class activity • Write a program, TestCircle.java • There will be two classes in this file. The first class is the Circle class presented earlier in the notes. Put it at the very bottom of your file • The second class is called TestCircle which should go at the top of your file • The TestCircle class must have a main method inside it (Circle does not!) • Inside the main method for TestCircle, you should create a reference variable called myCircle and assign it to a newly created Circle object • Using the instance, myCircle, change its radius to 12 • Print out myCircle’s radius, area and the circumference CS1s - 06A-Objects and Classes (v1.00)

  27. What’s Wrong? • What’s wrong with the following code snippets? Circle myCircle; System.out.println(myCircle.findArea()); Circle myCircle = new Circle(); System.out.println(myCircle.radius()); System.out.println(myCircle.findArea); System.out.println(Circle.findArea()); int x; System.out.println(x); new Circle().findArea(); CS1s - 06A-Objects and Classes (v1.00)

  28. TestFrame.java • In class activity • Copy the following program over to your local directory: /usr/local/pub/sps/courses/cs1/TestFrame/TestFrame.java • Compile the program and run it • Now look at the source code and answer these questions: • How many objects are created? • How many references are created and what are their names? • How many methods are invoked on each instance and what are their names? • Do any of the instance methods take parameters? CS1s - 06A-Objects and Classes (v1.00)

  29. Constructors • One problem with our Circle class is that all objects created have a radius of 5 • A constructor is a special method which can be used to initialize an object’s data • The constructor has the same name as the class • Like all methods, the constructor can be overloaded to receive different parameters • The constructor does not have a return type (not even void) CS1s - 06A-Objects and Classes (v1.00)

  30. Constructors class Circle { double radius; Circle(double r) { radius = r; System.out.println(“Circle(” + radius + “)”); } Circle() { radius = 5.0; System.out.println(“Circle(” + radius + “)”; } double findArea() { return radius * radius * Math.PI; } double findCircumference() { return 2 * Math.PI * radius; } } CS1s - 06A-Objects and Classes (v1.00)

  31. Constructors • An example of overloaded constructors Circle circle1 = new Circle(10); // “Circle(10)” Circle circle2 = new Circle(); // “Circle()” • Calling new invokes a class constructor • If a class does not define any constructors, a default constructor is defined implicitly • Common programming blunder: public void Circle() { //no no } CS1s - 06A-Objects and Classes (v1.00)

  32. TestCircle.java • In class activity • Modify the TestCircle.java program so it adds two constructors for the Circle class • A default constructor which sets the radius to 1.0 • An overloaded constructor which takes a double and sets the radius to that parameter • In the main method of TestCircle, create two Circle instances • The first instance, circle1, should be created with the default constructors • The second instance, circle2, should be created with a radius of 10.0 • Print out the radius, area and circumference of both instances CS1s - 06A-Objects and Classes (v1.00)

  33. Revision History • Revision History • v1.00, 10/11/2004 10:11 PM, sps Initial revision. CS1s - 06A-Objects and Classes (v1.00)

More Related