Introduction to Classes, Objects, Methods and Attributes Lecture # 5

1-2 Object-Oriented Programming Object-oriented programming is centered on creating objects rather than procedures. Objects are a melding of data and procedures that manipulate that data. Data in an object are known as attributes. Procedures in an object are known as methods.

1-3 Object-Oriented Programming Data Hiding Data hiding is important for several reasons. – It protects attributes from accidental corruption by outside objects. – It hides the details of how an object works, so the programmer can concentrate on using it. – It allows the maintainer of the object to have the ability to modify the internal functioning of the object without “breaking” someone else’s code.

1-4 Object-Oriented Programming Code Reusability Object-Oriented Programming (OOP) has encouraged component reusability. A component is a software object contains data and methods that represents a specific concept or service. Components typically are not stand-alone programs. Components can be used by programs that need the component’s service. Reuse of code promotes the rapid development of larger software projects.

3-5 Encapsulation Classes should be as limited in scope as needed to accomplish the goal. Each class should contain all that is needed for it to operate. Enclosing the proper attributes and methods inside a single class is called encapsulation. Encapsulation ensures that the class is self- contained.

1-6 Classes and Objects Components are objects. The programmer determines the attributes and methods needed, and then creates a class. A class is a collection of programming statements that define the required object A class as a “blueprint” that objects may be created from. An object is the realization (instantiation) of a class in memory.

1-7 Object-Oriented Programming Object Attributes (data) Methods (behaviors / procedures)

1-8 Classes and Objects Classes can be used to instantiate as many objects as are needed. Each object that is created from a class is called an instance of the class. A program is simply a collection of objects that interact with each other to accomplish a goal.

1-9 Classes and Objects Insect class The Insect class defines the attributes and methods that will exist in all objects that are an instances of the Insect class. housefly object The housefly object is an instance of the Insect class. mosquito object The mosquito object is an instance of the Insect class.

3-10 Designing a Class When designing a class, decisions about the following must be made. – what data must be accounted for, – what actions need to be performed, – what data can be modified, – what data needs to be accessible, and – any rules as to how data should be modified. Class design typically is done with the aid of a Unified Modeling Language (UML) diagram.

3-11 UML Class Diagram A UML class diagram is a graphical tool that can aid in the design of a class. The diagram has three main sections. Class Name Attributes Methods UML diagrams are easily converted to Java class files. There will be more about UML diagrams a little later. The class name should concisely reflect what the class represents.

3-12 Attributes The data elements of a class define the object to be instantiated from the class. The attributes must be specific to the class and define it completely. Example: A rectangle is defined by – length – width. The attributes are then accessed by methods within the class.

3-13 Methods The class’ methods define the actions that an instance of the class can perform Methods headers have a format: AccessModifier ReturnType MethodName(Parameters) { //Method body. } Methods that need to be used by other classes should be made public.

3-14 Methods The attributes of a class might need to be: – changed, – accessed, and – calculated. The methods that change and access attributes are called accessors and mutators.