Class Fundamentals BCIS 3680 Enterprise Programming

Overview 2  Building Your Own Classes  Members of a class  Variables (instance and static)  Methods (instance and static)  Storing and accessing values in an object  Constructor methods  Getter and setter methods

Class 3  A class defines a number of properties (variables) and actions/behaviors (methods).  An object created from this class will have those variables and methods.  Variables are used to store data.  Methods are used to specify action.

Object vs. Class 4  Object reference  Identifier (name) of the object  Holds the address of the object  A class is a template for making objects.  An object is an instance of a class.  The process of creating an object out of a template (class) is called instantiation, “creating an instance of the class”, etc.  Remember the naming convention:  The name of a class starts with an uppercase letter.  The name of a variable starts with a lowercase letter.

Creating Objects 5  Two things required to create an object:  Declaring an object reference for the new object, ClassName objRef; e.g., Student cobStudent;  Using the new keyword to actually create the object, objRef = new ClassName([arguments]); e.g., cobStudent = new Student("ITDS", "Undergrad");  The two steps may be combined into one: ClassName objRef = new ClassName([arguments]); Student cobaStudent = new Student("ITDS", "Undergrad"); // One step

Using Objects 6  To use an object’s variables and methods, you use the “.” operator (dot notation). objectName.variableName cobStudent.dept = "Marketing"; objectName.methodName([arguments]); cobStudent.regClass("BCIS3680");

Instance Variable 7  Instance variables are used to store data specific to a particular object.  An instance variable doesn’t exist (no memory location is assigned to it) until an object of that class is created.  All objects of the same class have the same instance variables.  However, for each object, the variables store values that are unique to that object.  Example:  The Student class has an instance variable called untId.  alice and bob are two instances of the Student class.  In the alice object, the untId variable contains the value  In the bob object, the untId variable contains the value

Static Variable 8  A class may also contain static variables.  Once a static variable is defined in the class, it comes into being immediately. No instantiation is needed.  Static variables can be accessed from outside of the class by following the ClassName.staticVarName syntax.  Values stored in static variables are not tied to any particular instances of the class. Rather, they are relevant at the class level.  Example:  The Student class has a studentCount static variable.  The number of all Student objects ever created can be stored in the studentCount variable.  It is not related to any particular Student object. If it were, we could have a “the-chicken-or-the-egg” problem.

Instance Method 9  Instance methods are used to define actions that an object of the class is capable of performing.  An instance method cannot be called until an object of that class is created.  An instance method is defined the same way for objects of the same class. However, even if the action is the same, in each object, the action works on data unique to that object (stored in the respective instance variables).  Example:  The Student class has an instance method called showStudentId().  alice and bob are two instances of the Student class.  When called on the alice object, the showStudentId() method displays the value  When called on the bob object, the showStudentId() method displays the value

Static Method 10  A class may also contain static methods.  Once a static method is defined in the class, it comes into being immediately. No instantiation is needed.  Static methods can be accessed from outside of the class by following the ClassName.staticMethod() syntax.  It can be used to perform action that is relevant to all instances of the class.  Example:  The Student class has an addToCount() static method and a getStudentCount() static method.  The addToCount() method increments the student count whenever a new student object is instantiated.  The getStudentCount() method displays the current count of student objects.

Members of A Class 11

The OOP “PIE” 12

Encapsulation  A fundamental concept in OOP.  The values of instance variables inside a class shouldn’t be changed arbitrarily by other classes.  They should be declared as “private”.  Manipulation of their values are done only by special-purpose methods inside the class.  These methods are public and “exposed”.  Setting values is possible only through calling these methods.  If program logic requires that these values to be changed, regardless of where the program execution is done, these special-purpose methods must be called. 13

Encapsulation  For example, suppose the main method is inside the Driver class and it needs to set the value of studentID of a Student object called alice,  It shouldn’t do: alice.studentID = "12345";  Instead, the setter method of that property should be run: alice.setStudentID("12345");  Whether a field or method is accessible (visible) to other classes is controlled by access modifiers.  public (UML sign + ) – All other classes can access the member.  private (UML sign - ) – Other classes must access the member through special methods defined in this class. 14

UML Representation of a Class + public - private 15

Access Modifiers Visibility PublicProtected Private Within the same class Yes From any class in the same package Yes No From a subclass in the same package Yes No From a subclass outside the package YesYes*No From any non-subclass class outside the package YesNo * Through inheritance 16

Storing Values to Instance Variables  Constructor Methods  A constructor is a special method that is run when an object is created. It is typically used to set initial (default) values for instance variables.  A class may have more than one constructor, i.e., constructors often are overloaded.  Setter (mutator) Methods  A setter method is a void method that performs an action (set the value of an instance variable).  It takes a parameter – the value to be stored in the variable.  It does not return a value. 17

Accessing Values of Instance Variables 18  Getter (Accessor) Methods  A setter method is a value-returning method that returns a value (the value of an instance variable).  It does not have parameters.

Constructor  When an object is instantiated, the instantiation process runs the class’ constructor method.  If the business rules of your application require other initial values for (some of all) variables in your class, then you need to write a constructor and assign initial values to those that need them.  A constructor may also perform some initial actions in addition to or instead of setting initial values for variables.  Besides initializing instance variables, a constructor may also work with static variables/methods of the class.  A constructor must use the same name and capitalization as the name of the class. 19

Default Initial Values  If you haven’t defined a constructor for the class, Java will create a default constructor for you Data TypeDefault Value byte, short, int, long 0 float, double 0.0 charspace boolean false Any object reference (e.g., String) null 20

Overloaded Constructor  Often, even though objects are instantiated from the same class, you want their variables to be initialized differently or different sets of variables to be initialized.  To achieve that, you write a few different constructors accordingly.  Constructors often are overloaded. 21

Setter Method 22  The program logic may need to change the value of an instance variable after it is initialized by the constructor. To do that, the setter method of that variable needs to be called.  The design pattern of a setter:  Access modifier: public  Return value: void  Naming: setVarName  Parameter list: one and only one parameter – (dataType paramName)  Method body: although more complicated assignment may be needed sometimes, typically the method contains only one assignment statement that assigns the parameter value to the variable – varName = paramName;

Getter Method 23  A getter method retrieves the value of an instance variable on behalf of methods in external classes.  The design pattern of a getter:  Access modifier: public  Return value:  Naming: getVarName  Parameter list: ()  Method body: although more complicated actions may be needed sometimes, typically the method contains only one statement that returns the value of the variable – return varName;

Access Method Examples  Example: // Declare the property private double unitPrice; … // The setter method public void setUnitPrice (double price) { unitPrice = price; } // The getter method public double getUnitPrice { return unitPrice; } 24

The this Keyword  If inside a constructor or setter method, the parameter name is the same as the name of the instance variable that will be assigned the parameter’s value, there should be a way to differentiate the “namesakes.”  The this keyword is used to refer to the object itself.  Variable name preceded by this and a dot refers to the instance variable itself.  Variable name standing by itself refers to the parameter. public void setUnitPrice (double unitPrice) { this.unitPrice = unitPrice; } 25