Evan Korth

1
Computer Science IClasses and ObjectsProfessor
Sana OdehNew York University
2
Road Map

• Introduction to object oriented programming.
• Classes
• Encapsulation
• Members
• Objects
• Constructors
• Reading
• Liang 5 chapter 6 6.1 6.4
• Liang 6 chapter 7 7.1 7.4

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?

4
Classes

• The main building blocks of Java programs.
• Defines objects of the same type. Like a
  blueprint.

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.

6
Encapsulation

• Encapsulation refers to the 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).

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.

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
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
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).

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.

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
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
• refVar.varName
• Calling methods (sending messages)
• refVar.methodName(params)

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 constructors identifier (name) is the same as
  the class it constructs.

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.