Anatomy of a Class

Creating a Class: 5.1.1

• A class defines an abstract data class
• An object is an instance of a class
  • A variable object reference
• A class is a blueprint which creates objects

Writing classes

• start a class declaration with public class nameOfClass
  • The body of a class is defined in curly braces
• You can create objects of a type using ClassName objectName = new ClassName()

public class House {
    // Define attributes and methods here
}

House myHouse = new House();
House friendsHouse = new House();

• Objects have attributes and behaviors
  • Correspond with instance variables and methods
  • instance variables hold data
  • methods hold behaviors
• constructors initialize the instance variables
• a main method can run code

Instance variables: 5.1.2

• instance variables hold the data for an object
  • also known as attributes, fields, or properties
• In general these should be declared private
  • only code in the class can directly access the variable
• They are usually declared right after the class declaration

import java.awt.Color; 

public class Person {
    private String name;
    private String email;
    private String phoneNumber;
}

• Once we have declared a class, we can create many instances of it with different attribute values
• Object-oriented programming stresses data encapsulation
  • Data (instance variables) and code acting on it (methods) are wrapped together in a single unit
  • Implementation details are hidden
  • This protects the data from harm

• Anything outside can only interact with the object through public methods

• When designing a class, make conscious decisions about what data to make accessible from the outside
  • The private access modifier protects data from external access

Methods: 5.1.3

• Methods define what an object can do
  • Most are public
    • Accessible outside the class
  • Some are private if they’re only meant to be used internally
• Methods are defined as public returnType methodName(type param0)
• Methods can use an object’s instance variables
• void return type means the method returns nothing

public class House {
    private Color paintColor = Color.red;

    public boolean isRed() {
        return paintColor.equals(Color.red);
    }

    public void printDescription() {
        System.out.println("This house is " + paintColor.toString());
    }
}

Object-Oriented Design: 5.1.4

• in Object Oriented Design programmers decide what classes are needed to solve a problem
  • Also what data are needed in each class
  • What methods each class needs

Summary: 5.1.6

• programmers use code to represent physical objects or nonphysical concepts using classes
  • Based on attributes or behaviors of the object/concept
  • In robotics, we use Java to model our robot into “subsystems” which each have their own class!
• Instance variables define attributes (data) of objects
• methods define behaviors and functions
• data encapsulation is a technique which hides the implementation details from the user
  • data is private; accessible through public methods which act on the data
• keywords public and private affect the access of classes, data, constructors, and methods
  • private restricts access outside of the class
    • instance variables are usually private
  • public allows access from outside the class
    • most methods should be public, unless they aren’t meant to be externally accessed