Object-Oriented Programming (OOP) uses "objects" to model realworld objects. 

Object-Oriented Programming (OOP) consist of some important concepts namely Encapsulation, Polymorphism, Inheritance and Abstraction. These features are generally referred to as the OOPS concepts. 

If you are new to object oriented approach for software development, you can first read about object oriented approach in little more detail @ object-oriented-systems-development. 

An object in OOP has some state and behavior. In Java, the state is the set of values of an object's variables at any particular time and the behaviour of an object is implemented as methods.

Class can be considered as the blueprint or a template for an object and describes the properties and behavior of that object, but without any actual existence. An object is a particular instance of a class which has actual existence and there can be many objects (or instances) for a class.

Static variables and methods are not purely object oriented because they are not specific to instances (objects) but common to all instances. 

Encapsulation

Encapsulation is the process of wrapping up of data (properties) and behavior (methods) of an object into a single unit; and the unit here is a Class (or interface). 

Encapsulate in plain English means to enclose or be enclosed in or as if in a capsule. In Java, everything is enclosed within a class or interface, unlike languages such as C and C++ where we can have global variables outside classes. 

Encapsulation enables data hiding, hiding irrelevant information from the users of a class and exposing only the relevant details required by the user. We can expose our operations hiding the details of what is needed to perform that operation. 

Inheritance

Inheritance describes the parent child relationship between two classes.

A class can get some of its characteristics from a parent class and then add more unique features of its own. For example, consider a Vehicle parent class and a child class Car. Vehicle class will have properties and functionalities common for all vehicles. Car will inherit those common properties from the Vehicle class and then add properties which are specific to a car.

In the above example, Vehicle parent class is known as base class or superclass. Car is known as derived class, Child class or subclass.

Java supports single-parent, multiple-children inheritance and multilevel inheritence (Grandparent-> Parent -> Child) for classes and interfces.

Java supports multiple inheritance (multiple parents, single child) only through interfaces. This is done to avoid some confusions and errors such as diamond problem of inheritance. 

Plymorphism

The ability to change form is known as polymorphism. Java supports different kinds of polymorphism like oveloading and overriding.

Overloading

The same method name (method overloading) or operator symbol (operator overloading) can be used in different contexts.

Java doesn't allow operator overloading except that "+" is overloaded for class String. The "+" operator can be used for addition as well as string concatenation.

Overloading may be also called compile time polymorphism. 

Overriding (or subtype polymorphism)

We can override an instance method of parent class in the child class.

When you refer to a child class object using a Parent reference (e.g.  Parent p = new Child()) and invoke a method, the overriden child class method will be invoked. Here, the actual method called will depend on the object at runtime, not the reference type. 

Overriding is not applicable for static methods or variables (static and non-static). In case of variables (static and non-static) and static methods, when you invoke a method using a reference type variable, the method or variable that belong to the reference type is invoked.

Overriding may be also called runtime polymorphism.

Abstraction

In plain English, abstract is a concept or idea not associated with any specific instance and does not have a concrete existence.

Abstraction in Object Oriented Programming refers to the ability to make a class abstract.

Abstraction captures only those details about an object that are relevant to the current perspective, so that the programmer can focus on a few concepts at a time.

Java provides interfaces and abstract classes for describing abstract types.

• An interface is a contract or specification without any implementation. An interface can't have behavior or state.

• An abstract class is a class that cannot be instantiated, but has all the properties of a class including constructors. Abstract classes can have state and can be used to provide a skeletal implementation. 

The Object class

All classes in Java are actually direct or indirect sub classes of the java.lang.Object class directly or indirectly. Hence methods of the Object class are inherited by all classes in Java. You will read more about the Object class later.