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Introduction to Programming. Writing Java Beginning Java Programs. Review of Lab 1. Built-in Types Integer types No decimal point integer math-op integer yields an integer byte, short, int, long Real types Have decimal point real math-op real yields a real float, double .

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Introduction to Programming


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    1. Introduction to Programming Writing Java Beginning Java Programs

    2. Review of Lab 1 • Built-in Types • Integer types • No decimal point • integer math-op integer yields an integer • byte, short, int, long • Real types • Have decimal point • real math-op real yields a real • float, double

    3. Review of lab 1 Continued • Java Variables • Attributes • Name • Type • Value • Specifies where data is stored in memory • Java requires that a variable be declared before it can be used. • Declaration Form: • Type variable-name; • Can only declare once!

    4. Review of Lab 1 Continued • Variable Naming Conventions • Start with letter or _ follow with any number of letters, digits, or _ • Names are case sensitive • Convention – variable names start with lower case letter. • Names should be meaningful. • sales • noItems • qtyOnHand Camel notation

    5. Review of Lab 1 • What is wrong? int firstNum = 0; int firstNum = firstNum + 1; • What does the = operator in Java mean? • What is the type of the result of double total, price; int amt; total = price * amt; • What is wrong with amt = total/price;

    6. The Semicolon Rule • Always end a Java statement with a semicolon! • Be careful! The interaction pane does not require a semicolon after each statement. • In a program typed into the definition pane, Java does. • In the interaction pane, typing a semicolon after a statement does not show the value of the result. Leaving the semicolon off does.

    7. Object Variables • Java is an Object-oriented programming language. • We work mostly with objects. • However, built-in numeric types are not objects. ie. int, float, double, … • Strings are objects. • String myName; • myName is a reference to a string object • Objects must be created before they are assigned. • myName has the initial value of null (it does not reference a string object) • Much more on this later.

    8. Classes and Objects • Classes are templates that define what objects look like and how they behave. • An object is an instantiations of a class • Class names in Java should start with a capital letter. • Turtle is a class • It defines object methods that let you use a Turtle object. • //create a turtle object from the Turtle class Turtle myTurtle = new Turtle(myWorld); Constructor: Method that initializes a new obj. Class name defines the type Makes a new object

    9. Class Methods • Some classes have also define methods. • java.Math • Class that defines math functions to use • Would not want multiple math objects that each implement a abs method. • Class methods are methods that belong to only the class and hence there is only one copy of the method for all objects of the class. • int myVal = Math.abs(-12);

    10. A Basic Java Program // if you have to add import statements put //them here public class CS1Test { public static void main(String [] args) { // put you Java program statements here } } • You will see what this is for in a minute.

    11. Working With TurtlesExample of using classes Parameter that specifies the world to put the turtle in. • Get a world for our turtle to live in • World csExWorld = new World(); • Put a turtle in our world • Turtle ourTurtle = new Turtle(csExWorld); • Check the state of our turtle • System.out.println(ourTurtle); • System.out.println(ourTurtle.toString());

    12. Talking to a Turtle • When we want an object to do something we send it a message. • Make the turtle wander around • ourTurtle.forward(20); • ourTurtle.turnLeft(); • ourTurtle.forward(40); • ourTurtle.turn(45); • ourTurtle.forward(65);

    13. How Do I Know What Turtles Can Do? • The set of methods that define what a class can do is called an API (Application Program Interface). • Java defines a special format called JavaDoc for presenting APIs. • You can find a link to the doc on the desktop of the lab machines and at C:\intro-prog-java\bookClasses\doc\index.html.

    14. Problem: Draw a square • Describe how the turtle will travel in a square. • Program Code: //draw side one ourTurtle.forward(100); // turn 90 deg counterclockwise ourTurtle.turnLeft(); // draw side two ourTurtle.forward(100); // turn 90 deg counterclockwise ourTurtle.turnLeft(); // draw side three ourTurtle.forward(100); //turn 90 degrees counterclockwise ourTurtle.turnLeft(); // draw side four ourTurtle.forward(100);

    15. Reusing Code • What if we wanted to draw a second square? • We would have to retype the code a second time. • Not good! Is not there a better way? • Define a method that draws a square. • We can invoke it every time we need to draw a square.

    16. The drawSquare Method } Parentheses required public void drawSquare() { //draw side one this.forward(100); // turn 90 deg counterclockwise this.turnLeft(); // draw side two this.forward(100); // turn 90 deg counterclockwise this.turnLeft(); // draw side three this.forward(100); //turn 90 degrees counterclockwise this.turnLeft(); // draw side four this.forward(100); } Body of the method • Open the Turtle.java file found in C:\intro-prog-java\bookClasses • Put the method definition at the bottom of the class where the documentation directs. • Create a World, a Turtle, and draw a square. Opening and Closing Brace Required

    17. How do we draw two squares? • Call drawSquare() twice. • What happened? • Cannot call drawSquare again it will draw over top the square we have drawn. • Need to move the Turtle before we draw again. • How? Look at documentation • Pen up • Move to new position • Pen down

    18. Parameters • How do we draw squares of different sizes? • Observation: Each time we draw a square we need to change the 100 to the size we want. • This could generate a lot of almost identical methods. • Use a parameter to specify the width. • public void drawSquare(int width) { … } • Change 100 to width • Call as: • ourTurtle.drawSquare(50);

    19. Draw the squares in Different Colors • Back to the documentation • public void setColor(java.awt.Color color) • Need a Color object found in java.awt package • Must import this package to use it • Find information in Java 1.5 API Documentation • Invoke • ourTurtle.setColor(Color.BLUE); • Draw a blue square.

    20. How do you …? • Save the program? • Put it in a CSTurtleTest class • Hide the turtle? • Draw the squares at an angle to the window?