Object-Oriented Programming: Classes and Objects

• The lecture introduces classes and objects in object-oriented programming.
• The course code BMED-2111 focuses on Object-Oriented Programming.

Objectives

• Understand classes, objects, methods, and instance variables.
• Learn to declare a class and create objects from it.
• Implement class behaviors by declaring methods within a class.
• Implement class attributes by declaring instance variables.
• Call an object's methods to execute tasks.
• Differentiate between instance variables and local method variables
• Use constructors to initialize object data upon creation.
• Understand the differences between primitive and reference types.

Introduction

• Classes are a core concept.
• Objects are instances of classes
• Methods define actions or behaviors.
• Parameters are inputs to methods.
• The double primitive type is covered.

Classes, Objects, Methods, and Instance Variables

• Analogy: Relate classes to car design and operation.
• Imagine pressing an accelerator pedal to make a car go faster.
• Cars need to be designed before they can be driven.
• Engineering drawings are to cars like blueprints are to houses.
• Car designs include an accelerator pedal design.
• Pedals hide complex mechanisms, just as a brake pedal hides the slowing mechanism.
• This allows people to drive cars easily without engine knowledge.
• Cars must be built from engineering drawings.
• Drivers press an accelerator pedal to send commands to the car.
• Methods are required to perform tasks in a program.
• Methods describe how to perform the task.
• A method hides complex tasks from the user.
• In Java, a class contains methods, as a car's engineering drawings include the accelerator pedal design.
• A class provides methods to perform tasks.
• Programs perform tasks by using objects of a class.
• Java is object-oriented.
• Pressing a gas pedal in a car sends a message to make the car go faster.
• Sending messages to an object is a method call.
• The method call directs the object to perform its task.
• A car has attributes like color, number of doors, gas in the tank, speed, and miles driven.
• A car's attributes are in its engineering diagrams.
• Each car maintains its own attributes.
• Cars know how much gas they have, without knowing how much other cars have.
• Objects have attributes carried with them.
• Attributes are specified as part of the object's class.
• Bank accounts have balances which are the amount of money in the account.
• Each knows its balance, but not the balances of other accounts.
• Instance variables specify attributes.

Declaring a Class with a Method and Instantiating an Object of a Class

• You can create a new class such as GradeBook.
• You can use a class to create an object.
• Public classes must be stored in files named after the class with a .java extension.
• The "public" keyword is an access modifier.
• The public keyword signifies that the class is available to the public.
• Declaring more than one public class within a single file results in a compilation error.
• main is called automatically by the Java Virtual Machine (JVM).
• Usually methods must be called to perform any tasks.
• A public method is available to the public.
• Return type is the data type the method returns after being run.
• Return type void signifies that the method will perform a task but not return information to its caller after it has completed.
• A method name goes after the return type.
• Method names by convention begin with a lowercase letter with subsequent words are capitalized.
• Empty parentheses indicate it does not require additional information.
• The method header has the first line of the method code.
• The braces delimit the method’s body.
• The method body tells what task is performed by the program
• To use a class you must declare one within the program.
• The GradeBook class is shown as an example.
• main is essential if a class is to be run.
• Error message will be thrown without a main method: NoSuchMethodError:main .
• A static method is special.
• Static methods are called without creating objects in the class in which the class is declared.
• You can’t call a method unless you create an object.
• Each class becomes a new type to declare variables and create objects.
• Classes are extensible, as new class types can be declared.
• new keyword creates a new object with the class specified to the right of the keyboard.
• The new keyword is used to initialize a variable.
• Putting parentheses at the right of a class name is calling a constructor.
• The class-type variable dictates how you will call the method.
• Variables are named, followed by a period or "dot separator" then the named
• Calls make the method preform its task.
• Main is contained in all classes, the JVM will call in the class used to execute the application.
• A compile command for the class is javac Gradebook.java GradeBookTest.java.
• You can compile within a directory by running javac *.java.

Declaring a Method with a Parameter

• The gas pedal makes a car go faster.
• Pressing the pedal further accelerates the car more.
• A message can include the task to preform and other information that helps the car perform the task, just as a parameter provides additional information for a method to preform it’s task.
• Methods can require one or more parameters, separated by commas and listed in the parenthesis that follow the method name.
• Each parameter must state a type and identifier.
• A method call supplies values (arguments) for each of the methods parameters.
• nextLine reads the inputted characters until they are ended, and returns a String.
• Enter submits the string, and inserts a new line at the end.
• The newline is discarded by nextLine.
• next reads individual words.
• next reads inputted characters until it encounters a whitespace character, and returns a String, discarding the whitespace.
• If there is information after the first whitespace, other statements can be use scan it.
• When calling arguments, match the parameters.
• Argument type must be the name as the parameters of the call, ensuring consistency.
• System and String are contained within java.lang.
• java.lang is imported into every Java program implicitly.
• User-defined classes must be imported with the import command.
• If classes are compiled to the same directory , they are in the same package called the default package, and exist under one namespace.
• If you use fully qualified class names, you don't have to import.
• import declarations are more convenient as most Java programmers prefer use of those rather than long namespace addressing.

Instance Variables, set Methods and get Methods

• Local variables: Variables declared for each method and cannot be used by other methods after they loose them.
• Java carries objects, and attributes that existed after they execute
• Classes typically consist of manipulated by a class methods.
• Attributes and variables in class declarations.
• Called fields, declared that class but not its methods.
• When an object of a class exists its called an instance variable
• Each has a variable within their memory.
• Instances declared typically exist as private.
• Private is an access modifier that can only be accessed by methods within the class.
• Data hiding is when an instance is labeled as private.
• Variables that are private and can be only accessed by the methods within the object.
• This prevents classes in other programs being modified.
• A "set" method is typically used as a modifier to the values of the variable, while a "get" method retrieves the values of a variable.
• Instance variables should be listed with access modifiers.
• "Get" and "set" are just suggested conventions, the prefix is not essential.
• Return types should be labeled for methods with a void return type.
• setCourseName and getCourseName help the class in each of the class's methods.
• Static methods do no follow the rule, more information can be found in Chapter 8.
• Methods not defined does not matter with respect to order of execution but only when they are called.
• Methods can call each other within a class just with their first name.
• Variables are initialized using the default field value of Java.
• Variables that are defined within class is required to have values, local however do not need explicit values.
• The default value for a field is for a String to be null.

Primitive Types vs. Reference Types

• Every field has default values even null strings as defined in each Java implementation.
• There are primitive types and reference types.
• Primitive types include boolean, byte, char, short, int, long, float, and double.
• Non-primitive types are reference types.
• A primitive-type variable can store exactly one value of its declared type at a time.
• Primitive-type instance variables are initialized by default to 0 for byte, char, short, int, long, float and double, otherwise null.
• Booleans will default to false.
• Specificity is given by the variable within the declaration.
• Attempting to use a uninitialized variable will return a compiler error.
• References can be referenced and contain many methods.
• Objects are contained as a null value, or "nothing", reserved for the memory.
• The object needs to be invoked to work.

Initializing Objects with Constructors

• When an object is created its instance variables are set to default values.
• Every class provides a constructor to the specific object during declaration.
• Java calls new to assign a constructor in memory.
• Constructors will have the same name as the class
• By default compilers assigns with no parameters in any, but the values have to be customized.
• Parameters list out the information that the constructor needs to take and perform.
• You typically can't return types of data.
• They are typically labelled as public.
• You should initialize your variables otherwise Java can cause problems by assigning default values.
• These functions are specified inline.

Floating-Point Numbers and Type double

• Floating numbers are variables such as 2.3, 0.242.
• Float and doubles are primitive types.
• Doubles can hold larger numbers and finer detail.
• float represents single-precision to 7 digits.
• 15 digits is roughly twice the numbers for double.
• Double values are what Java uses by default for floating values.
• Refer to the appendix to check ranges for variables.
• Floating point numbers are hard to represent precisely which may result in improper usage.
• Use System.out.printf("%.2f\n"); in which %.2f denotes two digits to the name,
• If you do not enter the decimal it rounds down to the nearest placement.
• System scanner double is scanner.nextDouble.

GUI and Graphics

• Can display text.
• The function JOptionPane can display windows, such as message dialogs.
• javax.swing can create GUIs.
• GUIs makes the data entry and display available to the user.
• JOptionPane.showMessageDialog shows the users a message
  • Requires 2 arguments
  • The first positions dialogue based on location in which null makes its center
  • The second states the message and string to display.
• It uses static methods.
• They are always available, called name with ClassName.methodName.
• It allows uses to enter the data.
• Prompts user and field in text.
• When the function is entered it returns a strings of characters.
• Canceled values can be null.
• String format works like System.out.printf except output is a string.