OOP'S In Java

Java is a Object Oriented Programming Language. Object Oriented programming works on object instead of functions.
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OOP'S has many advantages over procedural programming. Procedural programming uses functions whereas OOP'S uses objects. OOP'S allow code reusability but not by procedural programming.
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1.3.1  Encapsulation
Binding "Data and Method" into a single unit.
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1.3.2   Abstraction
Working on the essential things and to forget the things which are not essential for us.
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1.3.3   Inheritance
Used to show parent - child relationship like one class inheriting the features of the other classes. It provides code reusablility if used wisely.
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1.3.4   Polymorphism
one thing having several forms.
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Class is a blueprint for providing state and behavior to instances.
Object is an instance of a class. When JVM encounters the new keyword, it will use the same class to make an object.
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Instance Members are the variables that are globally define in the class
Class Methods are the methods which are define inside the class.
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Here You can see how class and objects are created.
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Here you can see how the Heap size is occupied in the memory.
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Since JDK 1.5 Java have a new feature to get size of the Object.
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Here are some examples for Class, Object and Instance Members.