Introduction to Object-Oriented Programming in Java

Introduction to Object-Oriented Programming in Java
Barb Ericson Georgia Institute of Technology June 2008 Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3
Learning Goals Introduce DrJava Create objects in Java Introduce variables as object references Invoke methods on objects in Java Create a method in Java Pass a parameter to a method in Java Introduce subclasses Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Using Turtles in Java We will work with Turtles in a World in Java We have to define what we mean by a Turtle to the computer We do this by writing a Turtle class definition Turtle.java We compile it to convert it into something the computer can understand Bytes codes for a virtual machine Turtle.class Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

History of Turtles Seymour Papert at MIT in the 60s By teaching the computer to do something the kids are thinking about thinking Develop problem solving skills Learn by constructing and debugging something Learn by making mistakes and fixing them Seymour was also involved in the invention of the Logo programming languages. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Using Turtles The Turtle Class was is part of several classes created at Georgia Tech As part of an undergraduate class Add bookClasses to your classpath to use these classes This set of turtle clip art was created by Robert Savannah, an artist working for the U. S. Fish and Wildlife Service. You can find it at Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Open Preferences in DrJava
Open the preferences window to add the book classes to the classpath. The classpath tells Java where to look for classes. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Adding Book Classes to Classpath
Add bookClasses directory Be sure to add the bookClasses directory and not bookClasses\bookClasses. Also you often need to close DrJava and start it again in order to get the new classpath. Click on Add Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Creating Objects To create objects we ask the object that defines the class to create it Each object is created in memory with space for the fields it needs Each object keeps a reference to the class that created it The class is like a cookie cutter It knows how much space each object needs (shape) Many objects can be created from the class World Object 1 World Object 2 World: Class Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Class as Object Factory
A class is like a factory that creates objects of that class We ask a class to create an object by using the keyword: new ClassName We can also ask the class to initialize the object And pass data to help initialize it This is like ordering a new car from the factory. We have to specify things like the color, types of wheels, etc. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Creating Objects in Java
In Java the syntax for creating an object is: new Class(value, value, …); Our Turtle objects live in a World object We must create a World object first Try typing the following in the interactions pane: new World(); Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Creating Objects If you just do new World(); You will create a new World object and it will display But you will not have any way to refer to it again Once you close the window the object can be garbage collected The memory that the object was using can be reused We need a way to refer to the new object to be able to work with it again Alice named your objects for you bunny, bunny2 The object is created and is in computer memory but we don’t have a way to find where it is. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Naming is Important If you create a new contact in your cell phone you enter a phone number and a name Later you use the name to find the phone number In programming we name things we want to refer to again Gives us a way to work with them Like the World object In programming this is called declaring a variable Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Declaring a Variable To be able to refer to an object again we need to specify what type of thing it is and give it a name This is also called declaring a variable Type name; OR Type name = new Class(value, value, …); The equal sign doesn’t mean equal But assign the value of the variable on the left to the result of the stuff on the right The following creates a variable named earth which refers to a World object created on the right World earth = new World(); A reference to an object is a way to calculate the address of the object in memory. It is not the address of the object. References are not pointers. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Declaring Variables address memory When you declare a variable the computer assigns memory to it The amount of memory depends on the type For each variable the computer stores a map of the name to the memory location and the type When you use the name the computer looks up the memory location And uses the value at that location Object of type World An object reference isn’t really the address of the object in memory, but a way to calculate it. See that the earth variable takes up space in memory and the value of that space has a value of 1 which is the address of the World object. earth variable holds a reference To the World Object above Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Limits on Declaring Variables
You can't declare two variables with the same name! > World earth = new World(); Error: Redefinition of 'earth‘ You can change what an object variable refers to > earth = new World(); The first example would try to name two different boxes with the same name. But, then how would you know which one is the right one when someone refers to that name? Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Cell Phones use Variables
In your cell phone you have names that map to phone numbers When you pick Home it looks up the number and uses it to make the call You can’t have two names that are exactly the same The phone wouldn’t know which number you are referring to You can modify the phone number for a name Barb Home Michael Shannon Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Variables Sizes What value(s) does the memory on the right represent? It could be 4 char values 2 bytes each (16 bits) Unicode Or 2 int values 4 bytes each (32 bits) 2’s compliment Or 1 double value 8 bytes each (64 bits) In IEEE format Or an object reference The size is up to the virtual machine Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Declaring Variables and Creating Objects
You can declare a variable and assign it to refer to a new object in one statement World earth1 = new World(); Turtle tommy = new Turtle(earth1); Declaration of variables After this code executes there are 4 things in memory. Two object variables and two objects. Creating the objects Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Turtle Basics The world starts off with a size of 640 by 480 With no turtles World earth1 = new World(); The turtle starts off facing north and in the center of the world by default You must pass a World object when you create the Turtle object Or you will get an error: java.lang.NoSuchMethodException: Turtle constructor Turtle tommy = new Turtle(earth1); A turtle must be created in a world or you will get an error java.lang.NoSuchMethodException: Turtle constructor. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Java Naming Conventions
Notice that we capitalize the names of the classes, but not the variable names World earth1 = new World(); This is different than English Capitalize proper nouns (the names of things) Not the type of thing Earth is a world. Tommy is a turtle. In Java it is the class names that are the most important Not the variable or method names Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Creating Several Objects
You can create several World objects World mars = new World(); You can create several Turtle objects Turtle shar = new Turtle(mars); Turtle jen = new Turtle(mars); One turtle is on top of the other Turtles cycle through 4 different colors. The 5th turtle you create will have the same color as the first turtle. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Moving a Turtle Turtles can move forward jen.forward(); The default is to move by 100 steps (pixels) You can also tell the turtle how far to move shar.forward(50); Passing the amount to move forward by is also called passing a parameter. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Turning a Turtle Turtles can turn Right jen.turnRight(); jen.forward(); Left shar.turnLeft(); shar.forward(50); Turtles turn by 90 degrees by default. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Turning a Turtle Turtles can turn by a specified amount A positive number turns the turtle to the right jen.turn(90); jen.forward(100); A negative number turns the turtle to the left shar.turn(-90); shar.forward(70); Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

The Pen Each turtle has a pen The default is to have the pen down to leave a trail You can pick it up: turtle1.penUp(); turtle1.turn(-90); turtle1.forward(70); You can put it down again: turtle1.penDown(); turtle1.forward(100); Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Drawing a Letter How would you use a turtle to draw a large letter T? Process Create a World variable and a World object and a Turtle variable and object. Ask the Turtle object to go forward 100 Ask the Turtle object to pick up the pen Ask the Turtle object to turn left Ask the Turtle object to go forward 25 Ask the Turtle object to turn 180 degrees Ask the Turtle object to put down the pen Ask the Turtle object to go forward 50 You can do these in the interactions pane. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Drawing a T World world1 = new World(); Turtle turtle1 = new Turtle(world1); turtle1.forward(100); turtle1.penUp(); turtle1.turnLeft(); turtle1.forward(25); turtle1.turn(180); turtle1.penDown(); turtle1.forward(50); Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Moving to a Location 639 X A turtle can move to a particular location turtle1.penUp(); turtle1.moveTo(500,20); Coordinates are given as x and y values X starts at 0 on the left and increases horizontally to the right Y starts at 0 at the top of the window and increases to the bottom A new turtle starts out at 320,240 by default Y 479 There are 640 pixels in the width (0 to 639) and 480 in the height (0 to 479). Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Challenge Create a World object Don’t forget to declare a variable to hold a reference to it Create a turtle object Use the turtle to draw a Rectangle (but, not a square) Equilateral triangle Hexagon Use the up arrow to reuse previous commands You can either do these in the interactions pane or create methods. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Setting the Pen Width You can change the width of the trail the pen leaves World world1 = new World(); Turtle turtle1 = new Turtle(world1); turtle1.setPenWidth(5); turtle1.forward(100); You will need to reset the interactions pane before you try this. Click on the “Reset” button at the top of the window. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Setting the Pen Color Use setPenColor to set the color of the pen turtle1.setPenColor(java.awt.Color.RED); There are several predefined colors In the package java.awt A package is a group of related classes In the class Color To use them you can use the full name java.awt.Color.RED Other colors are java.awt.Color.BLACK, java.awt.Color.BLUE, java.awt.Color.GREEN, java.awt.Color.GRAY, java.awt.Color.YELLOW, java.awt.Color.ORANGE, java.awt.Color.CYAN, java.awt.Color.MAGENTA, java.awt.Color.WHITE Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Setting Colors You can change the pen color turtle.setPenColor(java.awt.Color.RED); You can change the turtle color turtle1.setColor(java.awt.Color.BLUE); You can change the turtle’s body color turtle1.setBodyColor(java.awt.Color.CYAN); You can change the turtle’s shell color turtle1.setShellColor(java.awt.Color.RED); When you change the pen color all the lines drawn by that turtle will change to the new color. When you change the turtle color you are really changing the body color. The shell color will be a darker shade, if there is no specified shell color. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Objects can Refuse Turtles won’t move completely out of the boundaries of the world World world2 = new World(); Turtle turtle2 = new Turtle(world2); turtle2.forward(600); Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Objects send Messages Objects don’t “tell” each other what to do They “ask” each other to do things Objects can refuse to do what they are asked The object must protect it’s data Not let it get into an incorrect state A bank account object shouldn’t let you withdraw more money that you have in the account Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Creating Methods In Alice you could create a method like teaching a bunny to hop Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Creating a Method We can name a block of Java statements and then execute them again By declaring a method in a class The syntax for declaring a method is visibility returnType name(parameterList) Visibility determines access Usually public or private The return type is the type of thing returned If nothing is returned use the keyword void Name the method starting with a lowercase word and uppercasing the first letter of each additional word Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Example Method public void drawSquare() { this.turnRight(); this.forward(30); } The visibility is public The keyword void means this method doesn’t return a value The method name is drawSquare There are no parameters Notice that the parentheses are still required The keyword this means the object this method was invoked on Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Adding a Method to a Class
3. Compile open files 1. Open file Turtle.java 2. Type the method before the last } To add a method to a class open the file that defines the class. Then add the method before the last curly brace in the file. Then compile the file by clicking on the Compile All button. Then you are ready to try out your method in the interactions pane. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Compile-Time Errors Case matters in Java! turnright isn’t the same as turnRight You can click on each error. The definitions pane will go to that error and highlight the code that is in error. Common errors are wrong case, wrong name, wrong parameters, missing ending semicolon, and that the method is inside the class (before the last } in the file). After you fix any errors you will need to compile again. Keep compiling and fixing errors until there are no more errors. Clicking on the error takes you to the code and highlights it. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Try the New Method Compiling resets the interactions pane Clearing all variables But you can still use the up arrow to pull up previous statements You will need to create a world and turtle again World world1 = new World(); Turtle turtle1 = new Turtle(world1); turtle1.forward(50); turtle1.drawSquare(); turtle1.turn(30); If you are using DrJava you can type the above instructions in the interactions pane to execute them. If you are using another IDE you can put the above statements into a main method. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Saving the Interactions History in DrJava
You can save the interactions history into a script And optionally edit it first before you save it Click on Tools then on Save Interactions History And then latter load and execute the statements in the script Click on Tools and Load Interactions History as Script Use the next button to see the next statement and click on the execute button to execute it Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Adding a Main Method In Alice there is a way to specify what method to execute when the world starts In Java when you execute a class you can have a main method This is where execution will start Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Main Method Syntax The main method is a class (static) method that can take an array of strings It is a class (static) method since no objects of the class exist yet public static void main(String[] args) { // statements to execute } In DrJava you can use use F2 to execute the main method in the current class. Or you can go to the Tools menu and click on Run Document’s Main Method. The main method must be public, must be static, must return void, must be called main, and must take an array of Strings. You don’t have to call the array of strings args. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Better Method to Draw a Square
A method to draw a square public void drawSquare() { int width = 30; this.turnRight(); this.forward(width); } We added a local variable for the width Only known inside the method This makes it easier to change the width of the square But, we still have to recompile to draw a different size square Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Testing the Better Method
Test with: public static void main(String[] args) { World world1 = new World(); Turtle turtle1 = new Turtle(world1); turtle1.forward(50); turtle1.drawSquare(); turtle1.turn(30); } You can put the statements to test with inside of the main method. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Passing a Parameter public void drawSquare(int width) { this.turnRight(); this.forward(width); } Parameter lists specify the type of thing passed and a name to use to refer to the value in the method The type of this parameter is int For integer The name is width Values are passed by making a copy of the passed value Adding parameters to methods makes them more reusable. Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Testing with a Parameter
Test a method with a parameter public static void main(String[] args) { World world1 = new World(); Turtle turtle1 = new Turtle(world1); Turtle turtle2 = new Turtle(world1); turtle1.forward(50); turtle1.drawSquare(30); turtle2.turn(30); turtle2.drawSquare(50); } Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

How Does That Work? When you ask turtle1 to drawSquare(30) turtle1.drawSquare(30); It will ask the Turtle Class if it has a method drawSquare that takes an int value And start executing that method The parameter width will have the value of 30 during the executing of the method The this keyword refers to turtle1 When you ask turtle2 to drawSquare(50) turtle2.drawSquare(50); The width will have a value of 50 The this refers to turtle2 (the object the method was invoked on) Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Challenges Create a method for drawing a rectangle Pass the width and height Create a method for drawing an equilateral triangle all sides have the same length Pass in the length Create a method for drawing a house Using the other methods Create a method for drawing a school Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Subclasses in Java The Turtle class is a subclass of SimpleTurtle public class Turtle extends SimpleTurtle This means it inherits methods and fields from SimpleTurtle See if you can find the forward and turnRight methods in SimpleTurtle Classes can subclass another class in Java and Alice Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Summary You can create objects from classes in Java Each object needs a unique way to refer to it In Java we call this declaring a variable You can create new methods visibility returnType name(Type name, Type name, …) Let’s you reuse a block of statements You can pass parameters to methods To make them more flexible and reusable You can create subclasses of other classes They will inherit fields and methods from the parent class Intro-Objects-Classes-Vars-Methods-Subclasses-Alice-MC3

Introduction to Object-Oriented Programming in Java

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