Unit 2 - Classes and Objects

Unit 2 - Classes and Objects

• Classes and objects are fundamental concepts in object-oriented programming (OOP).
• A class defines a blueprint or template for creating objects.
• An object is an instance of a class, embodying the data (attributes) and methods (actions) specified by the class.
• Collectively, the methods and variables defined within a class are called members of the class.
• Instance variables are the variables defined within a class. Each instance of the class contains its own copy of these variables.
• The code contained within a class is called member methods; these embody the operations of an object.
• Data hiding (or encapsulation) is one of the four fundamental OOP concepts (the other three are inheritance, polymorphism, and abstraction). Encapsulation is accomplished by declaring class variables as private and providing public getter and setter methods to access and modify the values.
• A class, in Java, is a blueprint that defines a new data type.
• An object, in Java, is an instance of a class. It is an actual implementation of the class's blueprint.
• Java classes have several components such as: access controls (public, private, etc.), data or variables, methods, constructors, and inheritance.
• Constructors are special methods automatically called at the moment an object of the declared class is created.
• Constructors do not have explicit return types like void, etc.
• There are two types of constructors, default constructor and parameterized constructor.
• A default constructor is a constructor that does not take any arguments.
• A parameterized constructor is a constructor that accepts one or more arguments.
• Method overloading is a technique in Java where a class can have multiple methods with the same name but different parameter lists (number or type of parameters). The compiler differentiates between these methods based on the method parameters.
• Method overriding is a feature in object-oriented programming where a subclass method overrides or redefines a method from its superclass. The method names and the method signatures (including parameters) must be identical to what exists in the superclass.
• The this keyword can resolve name collisions between local variables and instance variables in a method or constructor. The keyword is useful when formal parameters in a method or constructor have the same name as instance variables within the same class.
• The super keyword is used to refer to a parent class either to call a parent class method or to refer to a parent class variable. It should be used when calling a parent class method or variable in a subclass when there is a name clash between members in the parent and child class.
• Static members (variables and methods in a class) are associated with the class rather than any object instances. A static member can be accessed by using only the class name. These members are useful for storing data accessed by all objects or when an operation is performed that does not depend on the specifics of a particular object.
• A class can be declared as final; this means that that class cannot be subclassed. Similarly a method or variable in a class can also be declared as final—this prevents overriding of methods and prevents the modification of the value.
• Polymorphism refers to a class' ability to have multiple forms. One way is through method overloading and a second is through method overriding. Method overloading is when methods have the same name but different signatures. Method overriding is when subclass methods redefine a method that exists in a superclass. The choice of method to be executed is resolved at run time based on the arguments used to call the method.
• An interface defines a contract for a set of methods. It can only declare methods, without specific implementations. Methods in an interface are implicit abstract and public methods. Multiple interfaces can be implemented by a class, but only one class can be extended by another.
• There are various ways to access class members depending on the access specifier (private, protected, default (package-private), and public) associated with the member. The scope or visibility of variables or methods depends on the access specifier that modifies them when they exist within a class.
• The scope of a class member depends on the modifiers (access specifiers) that modify it. These can include private, protected, default (package private) and public.