CP122 Computer Science I. Chapter 2: Using Objects

Transcription

1 CP122 Computer Science I Chapter 2: Using Objects

2 Tech News! Cyber Monday: $3.3B

3 Tech News! Cyber Monday: $3.3B Small drone warfare

4 Assignment Demos: 1:30pm-3:30pm in Anjali's Office

5 Assignment Demos: 1:30pm-3:30pm in Anjali's Office Class note taker needed $50 gift card for the block Volunteer experience

6 Hacker's Tip of the Day: Use Linux Open Source Software

7 Hacker's Tip of the Day: Use Linux Open Source Software Free

8 Hacker's Tip of the Day: Use Linux Open Source Software Free *cough* Better *cough*

9 Using Eclipse

10 Chapter 2: Using Objects Variables and data types Objects, classes, and methods Java API Graphics

11 Data Types Every value in a Java program has a type

12 Data Types Every value in a Java program has a type That is, what kind of data does the value store?

13 Data Types Every value in a Java program has a type That is, what kind of data does the value store? Binary data: all 1's and 0's

14 Data Types Every value in a Java program has a type That is, what kind of data does the value store? Binary data: all 1's and 0's How are the 1's and 0's interpreted?

15 A Familiar Type: A Literal String public class MyFirstProgram { } public static void main(string[] args) { System.out.println( This is my first program! ); }

16 Other Data Types int (integer) - Any whole number Examples: 15, -50, 0

17 Other Data Types int (integer) - Any whole number Examples: 15, -50, 0 double (floating-point number) A number with the decimal part Examples: , 0.25, 1E6

18 Expressions Expression : a combination of variables, literals, and operators Examples: * * (6-2)

19 Microquiz What are the data types of the following values? I heart CS

20 Variables Variable: a named and typed place to store a value int userage = 19;

21 Variables Variable: a named and typed place to store a value int userage = 19; int is the type of the new variable.

22 Variables Variable: a named and typed place to store a value int userage = 19; userage is the name of the variable.

23 Variables Variable: a named and typed place to store a value int userage = 19; 19 is the initial value for the variable.

24 Variables Variable: a named and typed place to store a value int userage = 19; = is the assignment operator.

25 Variables Variable: a named and typed place to store a value int userage = 19; double hourlypay = 31.50;

26 Variables Variable: a named and typed place to store a value int userage = 19; double hourlypay = 31.50; String avatarname = n00bdestroyer ;

27 Identifier Name Rules - Consist of letters, digits, underscores, and dollar signs (do not use dollar signs) - Cannot use spaces or other symbols - Java keywords are illegal (public, static, void)

28 Identifier Name Style Rules - Start variable names with a lowercase letter - Capitalize subsequent words in the variable (camelcase) userage hourpay - Start class names with an uppercase letter - Use UpperCamelCase

29 Identifier Name Style Rules - Choose descriptive variable names! double x3 = mm2 / i6;

30 Identifier Name Style Rules - Choose descriptive variable names! double x3 = mm2 / i6; double hourlypay = salary / hoursworked;

31 Assignment Operator int userage = 19;... userage = 20; userage = ; userage = userdays / 365;

32 Calculator.java

33 Objects - We can create objects to store data

34 Objects - We can create objects to store data - The structure of an object is defined in a class - The class used to create an object is its data type

35 Objects - We can create objects to store data - The structure of an object is defined in a class - The class used to create an object is its data type - We can call methods on objects to manipulate their data

36 String Object String welcome = Hi! Thanks for visiting! ;

37 String Object String welcome = Hi! Thanks for visiting! ; int stringsize = welcome.length();

38 String Object String welcome = Hi! Thanks for visiting! ; int stringsize = welcome.length(); int firsta = welcome.indexof('a');

39 Object Method int firsta = welcome.indexof('a');

40 Object Method int firsta = welcome.indexof('a'); welcome is our String object.

41 Object Method int firsta = welcome.indexof('a'); The dot operator calls a method using the welcome object.

42 Object Method int firsta = welcome.indexof('a'); indexof is the name of a previously defined method in the String class.

43 Object Method int firsta = welcome.indexof('a'); object dot method

44 Object Method int firsta = welcome.indexof('a'); Inside the () is the input parameter list.

45 Object Method int firsta = welcome.indexof('a'); The output of the indexof method is assigned to the new variable, firsta.

46 Object Method int firsta = welcome.indexof('a'); The output needs to match the data type of the variable, firsta.

47 The Java Application Programmer's Interface (API)

48 Creating Objects Rectangle box = new Rectangle(100, 150, 10, 20);

49 Creating Objects Rectangle box = new Rectangle(100, 150, 10, 20); A new data type: Rectangle

50 Creating Objects Rectangle box = new Rectangle(100, 150, 10, 20); Use the new keyword to create a Rectangle object.

51 Creating Objects Rectangle box = new Rectangle(100, 150, 10, 20); This is called instantiating an object of type Rectangle.

52 Creating Objects Rectangle box = new Rectangle(100, 150, 10, 20); Rectangle is the name of the class.

53 Creating Objects Rectangle box = new Rectangle(100, 150, 10, 20); These are input values to the constructor of the class.

54 Creating Objects Rectangle box = new Rectangle(100, 150, 10, 20); What do they stand for? Java API!

55 Creating Objects Rectangle box = new Rectangle(100, 150, 10, 20); Rectangle box2 = new Rectangle(20, 30, 3, 4);

56 Creating Objects Rectangle box = new Rectangle(100, 150, 10, 20); Rectangle box2 = new Rectangle(20, 30, 3, 4); System.out.println( Height1 is + box.getheight()); System.out.println( Height2 is + box2.getheight());

57 Accessor Methods // Calling getheight does not change the data in // the Rectangle object double height = box.getheight();

58 Mutator Methods // Calling setsize changes the internal values of // the object box.setsize(newwidth, newheight);

59 CreateRectangles.java

60 Import Statements import java.awt.rectangle;

61 Import Statements import java.awt.rectangle; import is a java keyword.

62 Import Statements import java.awt.rectangle; This is the name of the library to import.

63 Graphical Applications import javax.swing.jframe; public class EmptyFrameViewer { public static void main(string[] args) { JFrame frame = new JFrame(); frame.setsize(300, 400); frame.settitle("an Empty Frame"); frame.setdefaultcloseoperation(jframe.exit_on_close); } } frame.setvisible(true);

64 Drawing public class LineDrawer extends JComponent { public void paintcomponent(graphics g) { } public static void main(string[] args) { // Create frame here... LineDrawer mylines = new LineDrawer(); frame.add(mylines); } } frame.setvisible(true);

65 Drawing public class LineDrawer extends Jcomponent { public void paintcomponent(graphics g) { g.drawline(50, 80, 200, 300); } } public static void main(string[] args) {... }

66 Break!