Computer Science I Classes and Objects Professor: Evan Korth New York University

1. Computer Science IClasses and ObjectsProfessor: Evan KorthNew York University Evan Korth New York University

2. Road Map • Introduction to object oriented programming. • Classes • Encapsulation • Members • Objects • Constructors • Reading: • Liang 7: chapter 7: 7.1 – 7.4 • Liang 8: chapter 8: 8.1 – 8.5 Evan Korth New York University

3. Object Oriented Programming • Emphasis is placed on nouns or objects. • Nouns (objects) have properties and behaviors. • How do we build these objects? • How do we represent their properties? • How do we define their behaviors? Evan Korth New York University

4. Classes • The main building blocks of Java programs. • Defines objects of the same type. Like a blueprint. Evan Korth New York University

5. Classes (cont) • Every .java file has one or more classes. Only one of the classes can be a public class. • That class must have the same name as the .java file. • If the class has an method called main(), execution can begin in that class. (Therefore, you can test a class by adding a main method to it.) • If there are other classes in the file, they cannot be public classes. Evan Korth New York University

6. Encapsulation • Encapsulation refers to the process of combining elements to create a new entity. • You encapsulate the properties (attributes) and behaviors (activities) of an entity into a class. • Encapsulation also enables us to hide the implementation of a class to other classes (information hiding / abstraction). Evan Korth New York University

7. Designing Classes • A class declaration includes members of the class. • A member can be either a data member or a method member. • A data member (AKA field) is used to define state (attributes or properties) of the entity. • A method member is used to define the behaviors of the entity. Evan Korth New York University

8. Data members • Data members can be a primitive type or a reference to another object*. • Primitive types are integer types, floating point types, characters and booleans. (Note: an int is not the same as an object of type Integer) • The scope of a data member is the entire class, no matter where within the class it is declared. * More on object references in a moment Evan Korth New York University

9. Default values for data members • 0 for all numeric type variables (including both floating point types and all integer types) • \u0000 for char variables • null for reference variables* • false for boolean type variables • Note: No default values for local variables (variables declared inside a method). * More on object references in a moment Evan Korth New York University

10. Objects • An object is an instance of a class. • If we think of a class as a blueprint, an object is one model created from that blueprint. • You can create any number of objects from one class. • An object is distinctly identified by an object reference (except for anonymous objects). Evan Korth New York University

11. Declaring object references • In order to reference an object, we need an object reference variable. • To declare an object reference variable we use the syntax: ClassName objectReferenceName; • The above statement creates a variable objectReferenceName which can reference a ClassName object. It does NOT create an object. Evan Korth New York University

12. Instantiating objects • In order to create an object, we use the new keyword along with a constructor* for the class of the object we wish to create. • To refer to the object, we “point” an object reference variable to the new object. objectReferenceName = new Constructor(); • The declaration and instantiation can be combined as follows: ClassName objectReferenceName = new ClassName(); • Note: the name of a constructor is the same as the name of the class * More on constructors soon Evan Korth New York University

13. Accessing Members of a Class • Within a class you can access a member of the class the same way you would any other variable or method. • Outside the class, a class member is accessed by using the syntax: • Referencing variables: objectReferenceName.varName • Calling methods (sending messages): objectReferenceName.methodName(params) Evan Korth New York University

14. Constructors • Constructors are special methods that instantiate objects. • A constructor is invoked with the new operator. • A constructor should initialize the class variables. If the variables are not initialized, default values are used. • A constructor does not have a return type. • A constructor’s identifier (name) is the same as the class it constructs. Evan Korth New York University

15. Constructors continued • Constructors can be overloaded but each one must have its own signature. • A constructor with fewer arguments can call a constructor with more arguments (we will see how to do this soon). • If no constructor is defined, a default constructor is automatically supplied which accepts no parameters. Variables are initialized to their default values. • If one constructor is explicitly defined, the automatic default constructor is no longer available. In such case, if you want a no parameter constructor, you must define it yourself. Evan Korth New York University