1. 1 Objects and Classes Introduction to Classes Object Variables and Object References Instantiating Objects Using Methods in Objects Reading for this Lecture: L&L, 3.1 - 3.2

2. 2 Introduction to Classes A class defines the attributes and behavior of a specific type of object –Attributes are variables declared in the class –Behaviors are methods defined in the class Normally, we access an object by calling a method defined by its class We may sometimes access an attribute defined by its class, but this is discouraged

3. 3 Introduction to Classes A class has a name that we can use as if it were a data type when declaring a variable When we declare a variable with the name of a class as its type, we are creating an object variable (can contain a reference to an object) We access an object’s methods / attributes using the object variable name and the. notation, e.g. ClassName objectName; //object variable objectName.methodName() // Note: The ( ) objectName.variableName // Note: No ( )

4. 4 Example of a Class Definition We can draw a diagram of a class to outline its important features before writing code – its name, attributes, and behaviors StateMachine - state :int... + setState (int input): void... Class Name List of its Variables List of its Methods

5. 5 Example of a Class Definition public class StateMachine { // an attribute or variable private int state; // a behavior or method public void setState (int input) { state = input; }

6. 6 Example of a Class Definition Declaring a StateMachine object variable: StateMachine myStateMachine = … ; Accessing a myStateMachine method: myStateMachine.setState(1); Why can’t we just do this? myStateMachine.state = 1; Notice the word private in the declaration of the variable state?

7. 7 Prototype for a Class Definition We use the Java reserved word private to prevent access to a variable or method from code that is written outside the class We use the Java reserved word public to allow access to a variable or method from code that is written outside the class Normally, we declare variables to be private Normally, we declare methods to be public We will see some valid exceptions later

8. 8 Creating Objects, e.g. String class “String” is a commonly used class To declare a variable as an object reference, to a String, we use the class name of the object as the type name String title; This declaration does not create an object It only creates a variable that can hold a reference to a String object.

9. 9 Creating Objects We use the new operator to create an object Creating an object is called instantiation An object is an instance of a particular class title is assigned a reference to a String object that contains (encapsulates) the string “Moby Dick” title = new String (“Moby Dick"); This calls the String constructor, which is a special method that sets up the object

10. 10 Invoking Methods Once an object has been instantiated, we can use the dot operator to invoke or “call” any of the object’s methods int count = title.length(); A method may return a value which can be used in an assignment or expression The value of count will be set to 9 A method invocation can be thought of as asking an object to perform a service