1.abstract class in java

Abstract. This keyword is used on classes. It does not indicate a style of art. It indicates a "template-only" class, one that cannot be created directly.

Class details. Most classes can be instantiated with "new." An abstract class cannot be. Instead we must create an instance of a derived class.

Abstract:
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We decorate the Page class with the abstract keyword. We also use an abstract method, one with no body.
Class
Extends:
Article and Post are derived from the Page class. They are not abstract. Neither are their open methods.
Here:
We create instances of Article and Post, but use them with a Page reference. We call open(). The derived open methods are used.

fields, member variables or instance variables
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Fields are variables that hold data specific to each object. For example, an employee might have an ID number. There is one field for each object of a class.
member methods or instance methods
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Member methods perform operations on an object. For example, an employee might have a method to issue his paycheck or to access his name.
static fields or class fields
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Static fields are common to any object of the same class. For example, a static field within the Employee class could keep track of the last ID number issued. Each static field exists only once in the class, regardless of how many objects are created for that class.
static methods or class methods
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Static methods are methods that do not affect a specific object.
inner classes
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Sometimes it is useful to contain a class within another one if it is useless outside of the class or should not be accessed outside the class.
Constructors
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A special method that generates a new object.
Parameterized types — Since 1.5, parameterized types can be assigned to a class during definition. The parameterized types will be substituted with the types specified at the class's instantiation. It is done by the compiler. It is similar to the C language macro '#define' statement, where a preprocessor evaluates the macros.