1. Using Objects

2. Overview In this presentation we will discuss: –Classes and objects –Methods for objects –Printing results

3. Classes and objects A class is the type of an object Just as a variable classSize may have type int, Color.red has type Color Just as 5 is a literal of type int, "Hello" is a literal of type String There are exactly eight primitive types There are thousands of classes, and you can create more

4. Declarations You declare variables to hold primitive values like this: int classSize; double area; You declare variables to hold objects like this: Color uglyBrown; String myName;

5. Assignment statements An assignment statement has the form: variable = expression ; Examples: classSize = 57; area = pi * radius * radius; uglyBrown = new Color(175, 175, 30); myName = "David Matuszek";

6. Combining declaration and assignment Declaration and assignment can be combined into a single statement: int classSize = 57; String myName = "David Matuszek"; Color uglyBrown = new Color(175, 175, 30); You can only declare a variable once, but you can assign to it many times in many places –This rule is “true enough” for now –Exceptions are complicated and left for later

7. Methods Primitives have operations, classes have methods You cannot define new primitives, but you can define new classes You cannot define new operations, but you can define new methods Here we will talk about using methods supplied by Java, not defining new ones

8. Data in classes and objects A class is the type of an object A class describes: –How to make a new object of that class Example: new Color(175, 175, 30); –What kind of data is in an object Example: a Color object contains three numbers representing the amount of red, green, and blue –The methods of an object (the actions it can perform) Example: a Color object can tell you how much red it contains

9. Sending messages to objects We don’t perform operations on objects, we “talk” to them –This is called sending a message to the object We do it like this: –object.message(extra information) Examples: g.setColor(Color.pink); amountOfRed = Color.pink.getRed( );

10. Messages Messages can be used to –Tell an object some information –Tell an object to do something –Ask an object for information (usually about itself) –Any and all combinations of the above

11. Messages to a Graphics If you have a Graphics, and its name is g, here are some things you can do with g: –Tell it to use a particular color: g.setColor(Color.orange); –Ask it what color it is using: Color currentColor = g.getColor(); –Tell it to draw a line: g.drawLine(14, 23, 87, 5);

12. Messages to a Color Once you make a Color, you cannot change it; you can only ask it for information // Make a new purplish color Color myColor = new Color(100, 0, 255); // Ask how much blue is in it int amountOfBlue = myColor.getBlue(); // Ask the color for a brighter version of itself Color brightColor = myColor.brighter();

13. String A String is an object, but......because Strings are used so much, Java gives them some special syntax –There are String literals: "This is a String" (Almost) no other objects have literals –There is an operation, concatenation, on Strings: "Dave" + "Matuszek" gives "DaveMatuszek" In other respects, String s are just objects

14. String methods A String, like a Color, is immutable: once you create it, you cannot ever change it...but you can make new Strings and Colors If s is the name of the string "Hello", then –s.length() tells you the number of chararacters in the string (returns 5 ) –s.toUpperCase() returns the new String "HELLO" ( s itself is not changed)

15. String concatenation + usually means “add,” but if either operand (thing involved in the operation) is a String, then + means concatenation If you concatenate anything with a String, that thing is first turned into a String For example, you can concatenate a String and a number: System.out.println("The price is $" + price);

16. Data in classes A class describes objects. It describes: –How to construct an object of that class, –the kind of data in an object, and –the messages that the object can understand A class can also contain its own data –Constants are often provided this way –Examples: class Color contains the constant Color.red class Math contains the constant Math.PI

17. Printing out results, part 1 In Java, “print” really means “display on the screen” –Actually printing on paper is much harder! System is one of Java’s built-in classes System.out is a data object in the System class that knows how to “print” to your screen We can talk to this mysterious object without knowing very much about it

18. Printing out results, part 2 System.out is a object with useful methods that will let you print anything: –print(x) turns x into a String and displays it –println(x) (pronounced “printline”) turns x into a String and displays it, then goes to the next line Examples: System.out.print("The sum of x and y is "); System.out.println(x + y);

19. New vocabulary class: the type, or description, of an object object: an instance, or member, of a class message: something that you “say” to a class, either telling it something or asking it for information immutable: cannot be changed after it is created operand: one of the inputs to an operation

20. The End