INTRODUCTION TO CREATING CLASSES

INTRODUCTION TO CREATING CLASSES AND OBJECTS

Object-oriented programming languages group program variables with procedures to create program components called objects.

Objects are created in programs to model concrete objects that exist in the real world
Objects have states, also called attributes or properties, that describe the characteristics of the object

Objects also have methods, or procedures, that are encapsulated with the attributes of an object and
can set or modify the state of the object

In Java, object-oriented programming resolves around a few key concepts:

A class is template from which you create objects.
An object is an instance of an class, much as a variable is an instance of data type.
Data members are those variables that are part of a class, and they store the data the object uses. Objects support both instance variables, whose value are specific to the object, and class variables, whose values are shared among the objects of that class.
A method is a function built into a class. You can have instance methods and class methods. You use the instance methods with objects, but you can use a class method just by referring to the class by name - no object required.
Inheritance is a process of deriving one class, called the derived class, from another, called the base class, and being able to make use of the base class's methods in the derived class.

OBJECTS

An object is an instance of an class, much as a variable is an instance of data type. Objects escapsulate methods and instance variabes
Objects inherit attributes from classes and all objects have predictable behaviors because they are members
of certain classes
- The data components of a class are referred to as instance variables
- Objects have methods associated with them. All objects instantiated from a class will
  have the same methods, and therefore have predictable behaviors
- Using a method of an object it is the equivalent of sending a message to that object
- An object may also return information to the source of the message

There are two steps to object creation

Create the class from which objects will be instantiated
Create other classes that will use the objects, attributes and methods that are
specified in the class
These steps may be performed by different programmers

CREATING A CLASS

The general form of creating a class

access class classname [extends ...] [implements ...]
{ [access] [static] data type instance-variable(s);

[access] [static] data type method(s) (parameter list)
{

// Method body

}

To create a class, assign a name to it and decide what attributes (data) and methods (behaviors) it will have.
A class header has three parts

An optional access modifier, (public, final, abstract)
The keyword class
Any legal identifier for the class name

Public classes are accessible by all objects and may be extended, or used as the basis for another class. The public modifier is used for most classes

Instance variables, or fields, are placed between the curly brackets in the body of the class

Instance variables are usually private but may use private, public, friendly, protected, static, or final modifiers
Private access means that the variables are accessible only by methods of the class, and no other classes have access to them
If you do not specify an access modifier for a variable its access is friendly

Instance methods

In addition to data, classes also contain methods which provide an interface to the class data
The static method modifier is used to declare a method of a class that may be used without instantiating an object of that class. These are class methods.
Instance methods are non-static, and require an instantiated object of the class in order to be invoked
A class will normally have a method to modify the value of private data, and also to return the value of that data

Example
public class Student
{ private int stuNum;
private String stuName;
//Create two methods to get the data members
public int getStuNum( )
{ return stuNum;
}
public String getStuName( )
{ return stuName;
}
//Create two methods to set the values of two data members
public void setStuNum(int n)
{ stuNum = n;
}
public void setStuName(String Name)
{ stuName = Name;
}
}

ORGANIZING CLASSES

Normally the data fields are placed in some logical order at the beginning of the class
declaration, although there is no requirement to do so
Methods are declared following the data fields
Method prototypes are not required in Java
Methods which will modify a data field should have the prefix set, and methods which return
a data field's value should have the prefix get

CREATING OBJECTS

Declaring a class does not create any objects. A class is just an abstract description of what an object will be like when created. There are two steps to object creation.
- Specify a type (class name) for the object and give it a name
- Allocate memory for that object with the new operator
General format: Classname object name = new Classname( );
The object name becomes a reference to the location of the object in memory.
Creating an object automatically causes the object's constructor method to be invoked
After an object has been instantiated, its methods may be accessed using the object's identifier, a dot, and the method call

Example Assume the class Student has been created and compiledt. Now create a class called TestStudent which will use the class Student.
public class TestStudent
{ public static void main(String[ ] args)
   { Student stu1= new Student( );
      stu1.setStuNum(123);
      stu1.setStuName("John Nguyen");
      System.out.println("Student Number: " + stu1.getStuNum( ) );
      System.out.println("Student Name: " + stu1.getStuName( ) );
    }
}

Or two classes, Student and TestStudent, can be put in the same file called TestStudent. Only one class can be declared public in one file, and that's TestStudent in this case.
class Student
{ private int stuNum;
private String stuName;
//Create two methods to get the data members
public int getStuNum( )
{ return stuNum;
}
public String getStuName( )
{ return stuName;
   }
//Create two methods to set the values of two data members
public void setStuNum(int n)
{ stuNum = n;
}
public void setStuName(String Name)
{ stuName = Name;
   }
}
public class TestStudent
{ public static void main(String[ ] args)
   { Student stu1= new Student( );
      stu1.setStuNum(123);
      stu1.setStuName("John Nguyen");
      System.out.println("Student Number: " + stu1.getStuNum( ) );
      System.out.println("Student Name: " + stu1.getStuName( ) );
    }
}

USING CONSTRUCTOR

The constructor is a method which establishes the object
The constructor method must have the same name as the class
Constructors do not have a return type
Constructor can receive data as other methods can.
Any statement may appear in a constructor, but they are generally used to initialize the data
fields of an object

Example

public class Student
{ private int stuNum;
private String stuName;
//Create a constructor to initialize data members
   Student( )
{ stuNum = 0;
     stuName = "zzzz";
}
//Create a constructor to receive data
   Student(int n, String Name)
   { stuNum = n;
      stuName = Name;
   }
//Create two methods to get the data members
public int getStuNum( )
{ return stuNum;
}
public String getStuName( )
{ return stuName;
   }
//Create two methods to set the values of two data members
public void setStuNum(int n)
{ stuNum = n;
}
public void setStuName(String Name)
{ stuName = Name;
   }
}

public class TestStudent
{ public static void main(String[ ] args)
   { Student stu1= new Student( );
      Student stu2 = new Student(100, "Linda");
      Student stu3 = new Student( );
      stu1.setStuNum(123);
      stu1.setStuName("John Nguyen");
      System.out.println("Student Number: " + stu1.getStuNum( ) );
      System.out.println("Student Name: " + stu1.getStuName( ) );
      System.out.println("Student Number: " + stu2getStuNum( ) );
      System.out.println("Student Name: " + stu2.getStuName( ) );
      System.out.println("Student Number: " + stu3.getStuNum( ) );
      System.out.println("Student Name: " + stu3.getStuName( ) );
    }
}

Output
Student Number: 123
Student Name: John Nguyen
Student Number: 100
Student Name: Linda
Student Number: 0
Student Name: zzzz