Changing an Object s Properties

Transcription

1 Go ahead and PULL Lecture Materials & Sign-in on PollEv Right Click Lecture > Team > Pull Poll Everywhere: pollev.com/comp110 Lecture 4 Changing an Object s Properties Fall 2016

2 Announcements Review Session - Tomorrow at 4pm in Sitterson 014 Worksheet 0 Practice with method definitions and calling. Due Friday. Start today! HackNC - hacknc.com - October 28th - 30th Office Hours Queuing App We ll be debuting new functionality for office hours later this week! Allows you to see how busy office hours is right now and grab your spot in line if it s busy

3 Office Hours Ticket System COMP 110 Office Hours OPEN SN008 until 9pm See how busy office hours are before walking over Waiting Being Helped 110 Staff To reserve a spot in line, describe the issue you are facing and what you have tried: You ll need to describe what you need help with and what you have tried so far. Request Help

4 Office Hours Ticket System COMP 110 Office Hours OPEN SN008 until 9pm You ll see your position in line count down in real-time. Your Status 5 Ahead of You Your request: I am having trouble understanding the way expressions work with double values. I have tried adding.0 after each number but it still isn t working. Cancel

5 Office Hours Ticket System COMP 110 Office Hours When a UTA is ready for you, you ll see which one to go find in SN008. OPEN Your request: SN008 until 9pm Dorian is ready for you. I am having trouble understanding the way expressions work with double values. I have tried adding.0 after each number but it still isn t working. Warning: You must be in SN008 within 2 minutes of your ticket being called or else lose your spot. Cancel

6 Office Hours Ticket System Excited to give more predictability to office hours! Please wait in the lobby area outside SN008 until your reservation is called. When other students are waiting, appointments will be limited to 10 minutes. When released, I ll everyone with instructions! Secret to best office hours experience: start early.

7 Last Week Methods with return types and values The main Method Variable Declarations Instantiating new Objects Variable Assignment

8 Method Calling in Detail

9 Parameters vs Arguments // within the Robot class void moveforward(int numberofsteps) { // code that moves robot forward } Parameter // within the RobotMover class void moverobot(robot bb8) { bb8.moveforward(3); bb8.turnright(); bb8.moveforward(1); bb8.turnright(); bb8.moveforward(3); } Arguments In declaring a method, the parameter is the name by which a method refers to its piece of information passed to it e.g. x and y in the function f(x,y) = x + y - it is a dummy name determined by the definer In calling a method, an argument is the actual value passed in e.g. 2 and 3 in add(2, 3)

10 Calling Methods That Have Parameters Each of Calculator s methods takes in two ints, which it refers to as x and y (no need or ability to know those names, also called identifiers) Whenever we call these methods on ti110, we must provide two ints first our desired value for the first parameter, then our desired value for the second class Calculator { } int multiply(int x, int y) { return x * y; } // Some code elided

11 Calling Methods That Have Parameters // Somewhere else in code mycalculator.multiply(2,3); Java does parameter passing by first checking that the one-to-one correspondence is honored, // within the Calculator class int multiply(int x, int y) { return x * y; }

12 Calling Methods That Have Parameters // Somewhere else in code mycalculator.multiply(2,3); Java does parameter passing by first checking that the one-to-one correspondence is honored, then assigning arguments to parameters, (Java does this behind the scenes) // within the Calculator class int multiply(int x, int y) { // Begin hidden step x = 2; y = 3; // End hidden step return x * y; }

13 Calling Methods That Have Parameters // Somewhere else in code mycalculator.multiply(2,3); Java does parameter passing by first checking that the one-to-one correspondence is honored, then assigning arguments to parameters, and finally executing the method body using the arguments // within the Calculator class int multiply(int x, int y) { // Begin hidden step x = 2; y = 3; // End hidden step return 2 x * 3; y; } 6 is returned

14 Calling Methods That Have Parameters Let s say we have an instance of Calculator named ti110 When we call a method on ti110, we provide a commaseparated list of arguments (in this case, ints) between the parenthesis These arguments are the values we want the method to use for the first and second parameters when it runs. Note divide (8, 4) isn t divide (4, 8) - order matters! // Somewhere else in code ti110.add(2, 3); ti110.multiply(6, 2); ti110.divide(8, 4); ti110.subtract(5, 3); Arguments

15 Calling Methods That Have Parameters When calling a method that takes in parameters, you must provide a valid argument for every parameter That means that the number and type of arguments must match the number and type of parameters with a one-to-one correspondence This is why we couldn t write bb8.turnright(3)! Order matters! The first argument you provide will correspond to the first parameter, the second to the second, etc.

16 Hands-on #1 Pull the latest lecture materials Right click Lecture > Team > Pull Under the comment TODO #1. **Using ONLY method calls on ti110, can you: 1. Call add with arguments 100 and 7. Assign the result to sum. 2. Use System.out.println to print the value of sum. Try running! 3. Check-in on PollEv.com/comp110 when you are done or stuck.

17 TBT to Lecture 2 How did the runner result in this GUI (graphical user interface)? We didn t quite get there on Thursday

18 Lecture 3 s CalculatorRunner????????????? Class Declaration main Method Declaration Variable Declaration Instantiation & Assignment Method Calling package comp110.lecture02; public class CalculatorRunner { public static void main(string[] args) { Calculator ti110; ti110 = new Calculator(); System.out.println(ti110.multiply(2, 3)); } }

19 Packages A package is a group of related classes We ve been using them without acknowledging them: package comp110; package comp110.lecture04; Packages provide organization for: large programs sharing classes between programs

20 Package Organization A package is just like a folder on your computer In fact, that s how we organize them!

21 Package Declaration Required at the top of every Java file Form: package <package name>; The package name can contain periods. The names between periods correspond to folders, i.e.: Package: comp110.lecture04 Folder: src/comp110/lecture04

22 Fully Qualified Class Names When we ve used classes in our code we ve used unqualified class names: Calculator ti110 = new Calculator(); We could use a fully qualified class name : comp110.lecture02.calculator ti110 = new comp110.lecture02.calculator(); Form of a fully qualified class name: <package>.<class> Classes in the same package can use one another with unqualified class names Classes in the different packages must be fully qualified unless you import them. More on this in a couple slides.

23 Follow Along At TODO #2: Instantiate a new comp110.autogui and give it ti110 as an argument: new comp110.autogui(ti110); Notice this is a fully qualified class name. Try running and we should see the COMP110 AutoGUI

24 Importing Classes AutoGUI is a class defined in the comp110 package. To use a class declared in another package, we have two options: 1. Use its Fully Qualified Class Name 2. import the class from the other package We ll try doing this with AutoGUI: import comp110.autogui; import statements are placed before class declarations Now when Java sees AutoGUI, it knows it is referring to comp110.autogui

25 Back to Graphics!

26 Classes and Instances (1/3) We ve been saying bb8 is a Robot. We ll now refer to it as an instance of class Robot. This means bb8 is a particular Robot built using the Robot class as a blueprint All Robots (all instances of the class Robot) have the exact same capabilities: the methods defined in the Robot class

27 Classes and Instances (2/4) The Robot class is like a blueprint.

28 Classes and Instances (3/4) We can use the Robot class to build actual Robots instances of the class Robot, using instantiation via: new Robot() Each instance has its own properties that can vary from other instances (this week). bb8 bb9 lightbot darkbot

29 Classes and Instances (4/4) Method calls are received by instances of a class. Instance Instance Instance Instance bb8 bb9 lightbot darkbot

30 Objects and Properties In Java, we model objects by their properties In Address Book, someone you know is modeled with a name, number, and address. Instances of a class have the same properties available. Each instance has its own values, though. In Address Book, the fields of each contact are the same but the values you type in are different.

31 If you made a Facebook for circles, what fields would you have for their profiles?

32 Introducing Circle Our graphics projects will use JavaFX this semester Modern library for building apps, drawing scenes, playing audio, etc. The Pokemon maze was made with JavaFX. Today we ll use the javafx.scene.shapes.circle class You don t need to memorize the package! Just note we re using a built-in Java class and will import it from a package.

33 Working with Circle s Properties Most classes have methods allowing us to get and set each instance s properties These are called getter and setter methods Thursday we ll write our own getter and setter methods Today, we ll get some practice working with Circle s A setter method allows us to say to a Circle set your radius to 5.0 A getter method allows us to ask the Circle hey, what s your radius?

34 Disable Code Folding Type Folding in the Quick Access bar of Eclipse Select Folding - Java/Editor Uncheck the Enable Folding box Press OK

35 Hands-on #2 Open comp110.lecture04.circlerunner and run After the TODO comment: 1. Call acircle s setter method for radius: void setradius(double radius) 2. Run it! The acircle instance should now be visible. 3. Call acircle s setter methods for its X and Y coordinates: void setcenterx(double x) void setcentery(double y) 4. Run it again. Check-in on PollEv.com/comp110

36 Hands-on #2 Example package comp110.lecture04; import comp110.autogui; import javafx.scene.shape.circle; import javafx.scene.paint.color; import javafx.scene.group; public class CircleRunner { public static void main(string[] args) { Circle acircle = new Circle(); acircle.setradius(10.0); acircle.setcenterx(25.0); acircle.setcentery(50.0); new AutoGUI(aCircle); } }

37 Follow Along How do we draw more than one Circle if AutoGUI only works with a single argument? We need to create a Group of shapes, and add each Circle to the Group JavaFX s Group class is in the javafx.scene package and already imported in today s hands-on. We ll declare a new Group variable, assign a new Group instance to it, and add Circles to it. Adding shapes to a Group named agroup looks funny: agroup.getchildren().add(acircle);

38 package comp110.lecture04; import comp110.autogui; import javafx.scene.shape.circle; import javafx.scene.paint.color; import javafx.scene.group; public class CircleRunner { public static void main(string[] args) { Hands-on #2 Example Group agroup = new Group(); Circle acircle = new Circle(); acircle.setradius(10.0); acircle.setcenterx(25.0); acircle.setcentery(50.0); agroup.getchildren().add(acircle); Circle anothercircle = new Circle(); anothercircle.setradius(5.0); anothercircle.setfill(color.brown); agroup.getchildren().add(anothercircle); new AutoGUI(aGroup); } }

39 Today s Important Notes When a method call expression is encountered in code: 1. arguments must match parameters 2. arguments are assigned to parameters 3. the body of the method runs line-by-line 4. if the method returns, the return value is what the original method call expression evaluates to Packages are used to organize classes To use a class from another package you must import it first Objects have properties in addition to capabilities Setter methods allow us to change an object s properties Getter methods allow us to ask an object for its properties values Thursday we ll look at how to declare properties in Classes