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CS6308- Java Programming V P Jayachitra Assistant Professor Department of Computer Technology MIT Campus Anna University

• procedural programming: Programs that perform their behavior as a series of steps to be carried out. • object-oriented programming (OOP): Programs that perform their behavior as interactions between objects

About ? • The basic elements of a class • How a class can be used to create objects? • Learn about methods, constructors, and the this keyword.

Class • Any concept to implement in a Java program must be encapsulated within a class. • What is a class? • A class is a structure that defines the data and the methods to work on that data. • A new data type • A class is an encapsulation of attributes and methods. • A class is a logical construct or template.

Class Fundamentals • Class • Defines a new data type • A class is to create an object or instance of the defined type • A class is a template for an object • A class is the blueprint of an object • A class describe object’s properties and behaviors • Example: Blueprint of a house (class) and the house (object) Class Properties : State/variables Behaviors: Methods Class Clock Properties : State/variables int Hour, minute, second; Behaviors: Methods int getMinutes(); int getSeconds(); int getHours(); Blueprint public class Point { int x; int y; } Instance Point p=new Point();

Container class vs Definition class • Container class: • Collection of static methods that are not bound to a specific objects. • Example: Math.sqrt(), Math.pow() • Definition class: • A class that create new objects. • Example: Clock c1;

Object • Object • An object is an instance of a class. • An entity that combines state and behavior • An object is a real world entity. Characteristics of an Object: • State : represents the data (value) of an object. • Behavior : represents the behavior (functionality) of an object. • Identity : An object identity is typically implemented via unique ID. The value of the ID is not visible to the external user. However, JVM can identify each object uniquely.

Class Abstraction • Abstract data type (ADT) • Abstraction refers to the act of representing essential features without including the background details or explanations. • Class is an ADT as it uses the concept of abstraction. • Abstraction: • An abstraction is a process of hiding the implementation details from the user, only the functionality will be provided to the user. • Allow user to understand its external behaviour(interface) but not its internal details. • Example: Buttons • Why? To manage complexity • abstraction is achieved by using Interfaces and Abstract classes.

Encapsulation • State is encapsulated or hidden • The internal state of an object is not directly accessible to other parts of the program • Other parts of the program can only access the object using its interface. • Data-hiding: Data of a class is hidden from any other class and can be accessed only through any member function of it’s own class in which they are declared. Hours Minutes Seconds getMinutes() getHours() getSeconds()

General form of the class • The data, or variables, defined within a class are called instance variables. • Each instance of the class (that is, each object of the class) contains its own copy of these variables. • Thus, the data for one object is separate and unique from the data for another. • The code is contained within .defined within a class are called members of the class. • Classes have a main method(), if that class is the starting point of the program

A simple class Instance variables A class called Box that defines three instance variables: width, height, and depth. Class name A class defines a new type of data. In this case, the new data type is called Box. class name is used to declare objects of type Box. A class declaration only creates a template; It does not create an actual object. Object mybox will refer to an instance of Box. Thus, it will have “physical” reality. Each time on creating an instance of a class means creating an object that contains its own copy of each instance variable defined by the class Every Box object will contain its own copies of the instance variables width, height, and depth. To actually create a Box object, use a statement like the following: Box mybox = new Box(); // create a Box object called mybox

A simple class Dot operator Use the dot operator to access both the instance variables and the methods within an object. The dot operator links the name of the object with the name of an instance variable or methods. To actually create a Box object, use a statement like the following: Box mybox = new Box(); // create a Box object called mybox mybox.width = 100; //assign width variable of mybox the value 100

Save the file that contains this program BoxDemo.java, because the main( ) method is in the class called BoxDemo, not the class called Box. Compilation will create two .class files, one for Box and one for BoxDemo. Each class can also be defined in its own file, called Box.java and BoxDemo.java, Output: Volume is 3000.0

Every Box object will contain its own copies of the instance variables width, height, and depth. Changes to the instance variables of one object have no effect on the instance variables of another. Output : Volume is 3000.0 Volume is 162.0

Box mybox; // declare reference to object mybox = new Box(); // allocate a Box object Declare object The first line declares mybox as a reference to an object of type Box. At this point, mybox does not yet refer to an actual object. The next line allocates an object and assigns a reference to it to mybox. mybox simply holds the memory address of the actual Box object. Declaring Objects Declaring an object of type Box

new operator • The new operator dynamically allocates memory for an object. • The classname is the name of the class that is being instantiated. • The class name followed by parentheses specifies the constructor for the class. • A constructor defines what occurs when an object of a class is created. • Most real-world classes explicitly define their own constructors within their class definition. • However, if no explicit constructor is specified, then Java will automatically supply a default constructor. class-var = new classname ( );

Assigning Object Reference Variables • b1 and b2 will both refer to the same object. • The assignment of b1 to b2 did not allocate any memory or copy any part of the original object. • It simply makes b2 refer to the same object as does b1. • Thus, any changes made to the object through b2 will affect the object to which b1 is referring, since they are the same object. • Assign one object reference variable to another object reference variable, is not creating a copy of the object, only making a copy of the reference. Box b1 = new Box(); Box b2 = b1; Box b1 = new Box(); Box b2 = b1; // ... b1 = null; Here, b1 has been set to null, but b2 still points to the original object.

Introducing Methods • Classes usually consist of two things: • instance variables • methods. • This is the general form of a method: type name(parameter-list) { // body of method return value; } Type Type specifies the type of data returned by the method. This can be any valid type, including class types If the method does not return a value, its return type must be void. Name The name of the method is specified by name. This can be any legal identifier . Parameter-list The parameter-list is a sequence of type and identifier pairs separated by commas. Parameters are essentially variables that receive the value of the arguments passed to the method when it is called. If the method has no parameters, then the parameter list will be empty. Return Methods that have a return type other than void return a value to the calling routine using the return statement:

Adding a Method to the Box Class Use methods to access the instance variables defined by the class. In fact, methods define the interface to most classes. This allows the class implementor to hide the specific layout of internal data structures behind cleaner method abstractions. In addition to defining methods that provide access to data, we can also define methods that are used internally by the class itself. Output: Volume is 3000.0 Volume is 162.0 No dot operator to access instance variable within the class dot operator used to access instance variable outside the class Instance variable that is not part of the class, it must be accessed through an object, by use of the dot operator. Instance variable that is part of the same class, that variable can be accessed directly. The same thing applies to methods.

Adding a Method That Takes Parameters • A parameterized method can operate on a variety of data. • A non parameterized method use is very limited parameter and argument: • A parameter is a variable defined by a method that receives a value when the method is called. • For example, in square( ), i is a parameter. • An argument is a value that is passed to a method when it is invoked. • For example, square(100) passes 100 as an argument. int x, y; x = square(5); // x equals 25 x = square(9); // x equals 81 y = 2; x = square(y); // x equals 4 parameter argument

mybox1.width = 10; mybox1.height = 20; mybox1.depth = 15; setDim( ) method to set the dimensions of each box.

Constructors • A constructor initializes an object immediately upon creation. • Automatic initialization of object is performed through the use of a constructor. • Constructor has the same name as the class in which it resides and is syntactically similar to a method. • Once defined, the constructor is automatically called when the object is created, before the new operator completes. • Constructor’s have no return type, not even void. • The implicit return type of a class’ constructor is the class type itself. Box mybox1 = new Box(); The parentheses are needed after the class name is to invoke the constructor for the class is being called.

Output: Constructing Box Constructing Box Volume is 1000.0 Volume is 1000.0

Default vs defined constructor • Java creates a default constructor for the class if a constructor is not explicitly defined for a class. • The default constructor will initialize all non initialized instance variables to their default values. • zero, null, and false, for numeric types, reference types, and boolean. • Once define your own constructor, the default constructor is no longer used.

Parameterized Constructor Parameterized Constructor Non-parameterized constructor allow all boxes to have the same dimensions. Parameterized constructor is a way to construct Box objects of various dimensions. Output: Volume is 3000.0 Volume is 162.0

The this Keyword • this keyword allows a method to refer to the object that invoked it. • this can be used inside any method to refer to the current object. • this is always a reference to the object on which the method was invoked. • use this anywhere as a reference to an object of the current class’ type.

this keyword:Instance Variable Hiding • It is illegal in Java to declare two local variables with the same name inside the same or enclosing scopes. • There can be Overlap of local variables, including formal parameters to methods with the names of the class’ instance variables. • However, when a local variable has the same name as an instance variable, the local variable hides the instance variable. • this directly refers to the object, • use this to resolve any namespace collisions that might occur between instance variables and local variables.

Garbage collections • In java, since objects are dynamically allocated by using the new operator, such objects are destroyed and their memory released for later reallocation automatically. • In some languages, such as traditional C++, dynamically allocated objects must be manually released by use of a delete operator. • It works like this: when no references to an object exist, that object is assumed to be no longer needed, and the memory occupied by the object can be reclaimed. • There is no need to explicitly destroy objects. • Garbage collection only occurs sporadically (if at all) during the execution of your program.

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