Packages. 1. Every Java object resides in a package. 2. An object s package is explicitly declared on the first line of code in a.

Transcription

1 Packages

2 Packages 1. Every Java object resides in a package. 2. An object s package is explicitly declared on the first line of code in a.java file: package edu.pcwe.uw.javaintro.misc; public class HelloWorld public static void main( String[] args ) System.out.println( "Hello world!" );

3 Packages 3. The fully qualified nameof a class includes its package name; the fully qualified name of the class in the preceding slide is: edu.pcwe.uw.javaintro.misc.helloworld

4 Packages 3. Packages allow different programming organizations to choose the same class name without fear of confusion: java.awt.list java.util.list

5 Packages 4. A.java file without a package statement is said to reside in the default package. Note: We will not be using explicit packages for the code we write in this class. However you have to know a little bit about packages in order to use pre assembled code.

6 Libraries 1. A library is a collection of packages. 2. A library may reside in a dedicated directory 3. but is more commonly stored in a java archive (.jar) file.

7 Libraries 4. To use an object from a library, you must: a) Import the object from its package into your code: import edu.uweo.javaintro.tools.turtle; b) Tell java where the library lives that contains the package:

8 Objects

9 Definition 1. An object stores information and performs actions. 2. An object has a nameand a type. 3. The name of an object must be unique, and allows you to have many objects of the same type. 4. The type of an object determines what it can store and what it can do. A java Integerobject stores whole numbers; it can perform basic arithmetic, such as adding and subtracting. A Cartographobject might store the coordinates of cities, and be able to compute the distance between them.

10 Declaration 1. A declaration determines the name and type of an object: Person cartman; ^ ^ ^ type name end-declaration 2. Objects need to be created before you can use them: cartman = new Person(); 3. An object can be declared and created in one step: Person cartman = new Person();

11 Methods 1. A method(a.k.a. functionor procedure) is how you usually interact with an object. 2. A method has a nameand a type. 3. A method requires 0 or more argumentsenclosed in parentheses. 4. A method is invokedusing the executor(a period). Person cartmen = new Person(); cartman.takeahike( 3, 2 ); ^ ^ executor ^ arguments name

12 Turtle Objects

13 Introduction See also: Turtle Documentation on the class web site. 1. To work with the Turtle class you must import it from edu.uweo.javaintro.tools. 2. A Turtle is an object that can draw simple figures. 3. A Turtle starts at the center of a blank screen facing east. import edu.uweo.javaintro.tools.turtle; -orimport edu.uweo.javaintro.tools.*;

14 A Turtle s Habitat

15 The paint Method The paintmethod draws a line and changes the position of a Turtle. It takes two arguments: angle and distance. paint( angle, distance ) angle: Causes a turtle to turn a number of degrees; positive numbers turn left; negative numbers turn right. distance: Tells a turtle how many pixels to walk; positive numbers walk forward; negative numbers walk backward.

16 Painting (1)

17 Painting (2)

18 Painting (3)

19 Moving

20 The move Method The move method changes the position of a Turtle without drawing. It takes two arguments: angle and distance. move( angle, distance ) angle: Causes a turtle to turn a number of degrees; positive numbers turn left; negative numbers turn right. distance: Tells a turtle how many pixels to walk; positive numbers walk forward; negative numbers walk backward.

21 The swingaroundmethod The swingaroundmethod tells a turtle to draw a circle; it does not change the position of the turtle. It takes one argument: the radius of the circle. swingaround( radius ) radius: Specifies the radius of the circle.

22 Swinging Around

23 Comments (1) Adding comments to your code can make it more readable; the java compiler ignores comments. The double slash (//) begins a comment that continues to the end of the line: Turtle sue = new Turtle(); sue.paint( 45, 200 ); // start by drawing a diagonal line sue.move( -135, 300 ); // make sue face due south

24 Comments (2) Another kind of comment begins with slash-asterisk (/*) and ends with asterisk-slash (*/): /* This program renders a surrealistic sketch of a monkey eating a banana. Author: Jordan Alispiegal Date: 23-June-2008 */ public class Monkey...

25 Exercises Textbook, Chapter 1, page 1-5: Use an editor (notepad++) to enter the program TwoSquaresfrom listing 1.2. Use javacto compile the program, and javato run it. Textbook, Chapter 1, page 1-7: Exercises 1.3, 1.5 and 1.7 (note: the house in exercise 1.7 should have a flat roof).

26 Parts of a Java Program

27 class+ name matching braces public class HelloWorld /* body of class goes here */ must live in HelloWorld.java A classcreates a new type, something which can contain data and define actions. In java, nothing can happen outside a class. Every class has a name, such as Turtle. class HelloWorld creates a new class named HelloWorld.

28 public + class public class HelloWorld /* body of class goes here */ Declaring a class to be publicmeans that it can be used by anyone. There are other options, such as private,but we won't be talking about those for a while.

29 Braces public class HelloWorld public static void main( String[] args ) System.out.println( "Hello World!" ); In java, braces are used to group things together. In the context of a class the left brace () signals the start of the class, and the right brace () signals the end of the class. Braces are always used in pairs.

30 main public class HelloWorld public static void main( String[] args ) System.out.println( "Hello World!" ); When you start a java program with the javatool, you must designate the class to start executing. This class must have a block of code (called a method) named main, which must be written exactly as shown: public static void main( String[] args )

31 void public class HelloWorld // main does not calculate a value. public static void main( String[] args ) System.out.println( "Hello World!" ); In Java, some methods can be used to calculate values, for example, Math.abs is used to calculate the absolute value of a number: System.out.println( Math.abs( -3 ) ); When a method does notcalculate a value, it must be declared void.

32 public method-name public class HelloWorld public static void main( String[] args ) System.out.println( "Hello World!" ); In Java, some methods can be used by anyone, while access to other methods is restricted. A method declared publiccan be used by anyone. There are other options, such as privatewhich we ll talk about later.

33 static method-name public class HelloWorld public static void main( String[] args ) System.out.println( "Hello World!" ); In Java, some methods can be used anytime, but others can only be used after you create an object (you can t tell a Turtle to draw anything until you create one). staticmethods, on the other hand, can be used whether you have an object or not.

34 String[] args public class HelloWorld public static void main( String[] args ) System.out.println( "Hello World!" ); String[] argsis used to process command line arguments. Don t worry about that right now; just remember that your main method must be declared this way.

35 Spacing public class HelloWorldpublic static void main(string[]args) Hello World ); Spaces and line breaks are (usually) optional in Java code. We use them to make our code more readable.

36 Indentation public class HelloWorld public static void main( String[] args ) System.out.println( "Hello World!" ); Indentation is optional in Java code, but it makes your programs much easier to read. Put the braces that delimit a block of code on lines by themselves, and line them up with the start of the block. Inside the block indent four spaces.

37 Exercises (1 of 2) 1. Go into your HelloWorldprogram and change mainto sam.compile it with the javaccommand. What happens when you try to run it with the java command? (Before you forget, change samback to main.) 2. Add the getgreetingmethod to your HelloWorldprogram, and change the "HelloWorld"line as follows: public class HelloWorld public static void main( String[] args ) System.out.println( getgreeting() ); public static String getgreeting() return "Hi there, world"; (continued on next slide)

38 Exercises (2 of 2) (continued) Compile and run the program. Explain: What happened when you substituted getgreeting() for "HelloWorld"? What is the purpose of Stringin the line public static String getgreeting()? 3. Download and complete TweedleActivity from the class web site.

39 Introduction to Inheritance (1) You can create a new class that inheritsdata and behavior from an existing class by using the extends expression: import edu.uweo.javaintro.tools.turtle; public class Bob extends Turtle public void whoareyou() System.out.println( "I am Bob, the Turtle." ); System.out.println( "Don't mess with me." );

40 Introduction to Inheritance (2) In the above example, an object of type Bobcan do anything a Turtle can. In fact, it is a Turtle, plus a little more: public class Test public static void main( String[] args ) Bob fred = new Bob(); fred.paint( 0, 128 ); fred.whoareyou();

41 The SmartTurtleClass The SmartTurtle class extends Turtle by adding two new methods. public class SmartTurtle extends Turtle public void makesmallsquare() paint( 90, 10 ); paint( 90, 10 ); paint( 90, 10 ); paint( 90, 10 ); public void makebigsquare() paint( 90, 40 ); paint( 90, 40 ); paint( 90, 40 ); paint( 90, 40 ); Questions: 1. Why are the new methods public? 2. Why are they not static? 3. Why do we use the command paintand not something.paint?

42 Instance Methods (1) 1. An instance method can only be executed on a specific object. 2. An instance method is declared without the word static. 3. Inside the method, a command like paintrefers to the object the method was executed on this is called the default executor.

43 Instance Methods (2) When we write smarty.makebigsquare()the paint command in makebigsquareis executed by smarty; when we write iggy.makesmallsquare() the paint command is executed by iggy. public class Test public static void main(string[] args) SmartTurtle smarty = new SmartTurtle(); SmartTurtle iggy = new SmartTurtle(); smarty.move( 0, -100 ); smarty.makebigsquare(); iggy.move( 0, 100 ); iggy.makesmallsquare();

44 The Default Executor In an instance method, the default executor can be explicitly invoked using this. public void makesmallsquare() paint( 90, 10 ); paint( 90, 10 ); paint( 90, 10 ); paint( 90, 10 ); // is equivalent to public void makesmallsquare() this.paint( 90, 10 ); this.paint( 90, 10 ); this.paint( 90, 10 ); this.paint( 90, 10 );

45 Definitions Instance An object that belongs to some class is an instanceof that class. An object can be an instance of more than one class; a SmartTurtleobject is an instance of both the Turtle and SmartTurtleclasses. Subclass When class A inherits from class B, class A is said to be a subclassof class B. Superclass When class A inherits from class B, class B is said to be a superclassof class A. public class A extends B

46 Exercises Textbook, Chapter 1, page 1-8: Create the file SmartTurtle.java, and enter the code from Listing 1.3. Compile the code. Textbook, Chapter 1, page 1-11: Exercises 1.8, 1.10, 1.11, 1.14

47 Turtles: the switchtomethod The switchtomethod takes one argument, a color defined in the Turtle class. switchto( color ) color: a color from the Turtle class. Example: Turtle fred = new Turtle(); fred.switchto( Turtle.RED );

48 Turtles: Constants Data stored in a class can include constant values (they're called constants because they can't be changed). The Turtle class has several constants used to represent colors: Turtle.BLACK Turtle.GRAY Turtle.BLUE Turtle.GREEN Turtle.RED Turtle.YELLOW Turtle.ORANGE Turtle.PINK Turtle.MAGENTA Turtle.WHITE

49 Turtles: the fillcirclemethod The fillcirclemethod takes one argument, a number that designates the radius of a solid circle to draw. fillcircle( radius ) radius: the radius of a circle. Example: Turtle fred = new Turtle(); fred.switchto( Turtle.RED ); fred.fillcircle( 64 );

50 Turtles: the fillboxmethod The fillboxmethod takes two arguments, numbers that designates the width and height of a solid rectangle to draw. fillbox( width, height ) width: the width of a rectangle height: the height of a rectangle Example: Turtle fred = new Turtle(); fred.switchto( Turtle.RED ); fred.fillbox( 128, 32 );

51 Turtles: the say Method The say method takes one argument, a string to draw at the Turtle s current position. say( string ) string: string to draw Example: Turtle fred = new Turtle(); fred.paint( 0, 200 ); fred.say( "making a right turn" ); fred.paint( -90, 200 );

52 Methods Without Objects 1. Methods that can be executed without an object are declared static: public static void sleep( int milliseconds ) 2. To execute a method that doesn t need an object, use the class name: Turtle.sleep( 500 )

53 Turtles: the Sleep Method The sleepmethod takes one argument, the number of millisecondsfor a Turtle to pause between commands. static void sleep( millis ) str: the number of milliseconds to sleep. Example: Turtle fred = new Turtle(); fred.paint( 90, 128 ); Turtle.sleep( 2000 ); //pause for two seconds fred.paint( 90, 64 );

54 Keywords A keywordis a part of the java programming language. Some examples of keywords that you have already seen are: public class static void extends new There are many others, including if, else, for, while and interface.

55 Identifers An identifieris a name that you give to a programming component that you create, including class names (Turtle) method names (makesmallsquare) and objects (Turtle fred = new Turtle()). Rules for creating identifiers are: You may notuse a keyword. The identifier must begin with a letter or underscore. After the first character, you may use letters, numbers and underscores. Capitalization counts; Fred is different from fred. Java conventions that you should follow: Class names should begin with an upper case letter. Non-constant object and method names should begin with a lower case letter. Constant names should not include lower case letters. Identifiers should not begin with an underscore.

56 Exercises Which of the following are keywords that you have encountered in class? public class fred wilma Turtle void paint static Which of the following are valid identifiers? fred _wilma ted_90 9_ted static void 9_5 a 9% Writea program thattells a Turtleto draw a line 100 pixels long, then say 100 ; extend the line by another 100 pixels and say 200. Textbook, Chapter 1, page 1-16: Exercises 1.18, 1.21 OPTIONAL: Textbook, Chapter 1, page 1-30: Write, compile and run the program in Listing 1.11.