CS 5JA Introduction to Java Graphics One of the powerful things about Java is that there is a large library of code that already exists. Not only can you use it, but you can extend the library with your own custom code. We’ll talk more about his in detail in a later class, but I just want to give you enough to use the built in user- interface widgets (called “Swing”) so that you can make some simple 2D graphics. The basic “widget” in Java is called a “JFrame” (J for java...). A frame is the square object that contains an application. It usually has a title bar, a button for minimizing and closing the application, and a panel that contains the content of the application. This panel is called a JPanel (J for java...). You can create your own JPanel and do anything you want to it. For our next homework assignment we’re going to learn how to make digital art.

CS 5JA Introduction to Java Graphics The JPanel contains lots of built-in methods to handle all sorts of things, like recognizing mouse focus, knowing how to minimize itself, knowing when to be behind another application, etc. So we can utilize all of this functionality, but customize it for our purposes. Here’s how we do it: public class MyArtwork extends JPanel The JPanel object has a built-in method called paintComponent which describes how to draw itself. By default it just draws a big gray square. You can override this method and tell your JPanel to draw itself in a more interesting way.

CS 5JA Introduction to Java Graphics //overriding! public void paintComponent(Graphics artist) { //dip paintbrush into new color artist.setColor(Color.BLACK); //paint a rectangle artist.fillRect(30, 30, 100, 100); } Each JPanel has a built-in palette and a built-in artist. The artist is prized for his graphics abilities, so he is of data type “Graphics”. Here we name him “artist”. In this method, we are telling the artist inside JPanel to paint the screen. In the first line we are telling artist to change colors. In the second line we are telling him to draw a rectangle.

CS 5JA Introduction to Java Graphics You can set the color using built-in colors, or by defining your own color using its RGB values. Let’s make a new color and tell the artist to use it: Color newColor = new Color( 255, 0, 0 ); //Red Green Blue artist.setColor(newColor); The range of the canvas is defined by pixel values. You can think of a graph, except the origin (0, 0) is in the top-left corner, and the y value is reversed. The y-value increases as you go down. The x-value increases as you go toward the right. Code example...

CS 5JA Introduction to Java Math Last week we talked about the difference between static and non-static methods. A normal instance variable exists only after an object has been instantiated (using the new keyword). Likewise, a normal method can be invoked only after the object is instantiated. The method can refer to any of the instance variables, static or non-static. The static variables are also available to normal methods because the definition of a static variable is that it is created before the object is instantiated, and it is shared with all instances of the object. That is, if you change the value of a static variable, you change it for all the objects of that class. For this reason, static variables are sometimes called class variables, and static methods are sometimes called class methods.

CS 5JA Introduction to Java Math So when would you want to use the static keyword? Because sometimes you want a particular variable to apply to all instances of a class. For example, in the Math package (built into the Java language) there are constants which never change, like the number “pi” and the number “e”. In fact, all methods inside Math are static. Math isn’t really a normal object. It’s just a collection of methods that immediately return an answer. No variables are created or manipulated or stored. For example: double number =.5001; double roundedNumber = Math.round(number);

CS 5JA Introduction to Java Math For example: double number =.5001; long roundedNumber = Math.round(number); the round method looks at the number that is passed in and returns a number (a long whole number) that is closest to the decimal. That is, it rounds it up or down. Since the round method static, you don’t need to instantiate a Math object. You can refer to the method using dot notation on the class itself.

CS 5JA Introduction to Java Math Here are some other common Math methods: double number =.999; long flooredNumber = Math.floor(double number); double number =.001; long roofedNumber = Math.ceiling(double number); double number = 9.0; double squareRoot = Math.sqrt(number); double squared = Math.pow(number, 2); double cubed = Math.pow(number, 3); double angle = 90.0; double sine = Math.sin(Math.toRadians(angle)); double number1through10 = Math.random() * 10.0;

CS 5JA Introduction to Java Math What if you wanted to make a random number 1 through 10 that was an integer? It takes a few steps: a. Make a random number between 0.0 and 1.0 b. Multiply it by 9 – now your number is between 0.0 and 9.0 c. Round it using the Math.round method -- now your number is 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9 d. Add one to it – now your number is between 1 and 10. You could also do this in one line: long num = Math.round(Math.random() * 9) + 1;

CS 5JA Introduction to Java Math What if you wanted to make a random number between -5 and 5?

CS 5JA Introduction to Java Math What if you wanted to make a random whole number between -5 and 5? How many possible numbers are there? 11 -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5 So, Math.random() * 10 Then round: between 0 and 10 (11 total numbers, including the 0) Then subtract 5: between -5 and 5 (11 total numbers, including the 0) long num = Math.round(Math.random() * 11) - 5;

CS 5JA Introduction to Java Math What if you were building an audio application and you wanted to display your sound waves on the screen? Sound waves can be mathematically modeled with a bunch of sine waves: so let’s make a small application which draws a sine wave. What do we do first? What elements do we need– not for a complicated version, just for a prototype that draws a single sinewave on the screen...

CS 5JA Introduction to Java Math We need something to draw on. We need a sinewave. [see SinewaveTest.java]

CS 5JA Introduction to Java Math What if you wanted to make some random polka dots? [see PolkadotTest.java]

CS 5JA Introduction to Java Math What if you wanted to make sure that the colors didn’t change... [see PolkadotTest.java]

CS 5JA Introduction to Java Arrays An array is simply an ordered list of elements. All the elements are of the same data type. Thus you can talk about an array of ints, or an array of Strings, or an array of Accounts, etc. Arrays are used to keep track of a group of similar information. For instance, if I am a bank and I have a list of Accounts I can store these Accounts in an array. This way, when I want to check the balance of all of them, I don’t need to get each Account one at a time. Instead I can iterate through the list more easily.

CS 5JA Introduction to Java Arrays It’s kind of like taking a scattered pile of papers and putting them neatly in a filing cabinent. Here’s how you declare an array and initialize an array int[] numberArray; numberArray = new int[100]; In the declaration, the brackets next to the data type indicate that you are creating an array of that type. In the initialization of the array, the number in the brackets describe how many elements are in the array. Right now, you have an empty filing cabinet with room for 100 numbers. But there is nothing in the filing cabinent.

CS 5JA Introduction to Java Arrays We can interate through the array (the filing cabinet) and put number in each position: for (int i = 0; i < numberArray.length; i++) { numberArray[i] = (int) (Math.random() * 10); } Likewise, later on we can access the array and iterate through every element in the array: for (int i = 0; i < numberArray.length; i++) { System.out.println(“number at position “ + i + “ is “ + numberArray[i]); }

CS 5JA Introduction to Java Arrays We get at the data in the array using the indexing notation: int checkNumber = myNumbers[0]; int checkNumber = myNumbers[55]; etc...