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Java abstract Keyword
The abstract keyword is used to achieve abstraction in Java. It is a non-access modifier which is used to create abstract class and method.
The role of an abstract class is to contain abstract methods. However, it may also contain non-abstract methods. The method which is declared with abstract keyword and doesn’t have any implementation is known as an abstract method.
Syntax:-
Note – We cannot declare abstract methods in non abstract class.
Rules of abstract keyword
Don’ts
- An abstract keyword cannot be used with variables and constructors.
- If a class is abstract, it cannot be instantiated.
- If a method is abstract, it doesn’t contain the body.
- We cannot use the abstract keyword with the final.
- We cannot declare abstract methods as private.
- We cannot declare abstract methods as static.
- An abstract method can’t be synchronized.
Do’s
- An abstract keyword can only be used with class and method.
- An abstract class can contain constructors and static methods.
- If a class extends the abstract class, it must also implement at least one of the abstract method.
- An abstract class can contain the main method and the final method.
- An abstract class can contain overloaded abstract methods.
- We can declare the local inner class as abstract.
- We can declare the abstract method with a throw clause.
Examples of abstract Keyword
Example 1: Abstract class containing the abstract method
Output:
Bike is running
Example 2: Abstract class containing the abstract and non-abstract method
Output:
Bike is running Car is running
Example 3: Abstract class containing the constructor
Output:
Constructor is invoked
Example 4: Abstract class containing overloaded abstract methods
Output:
abstract method is invoked overloaded abstract method is invoked
Example 5: Inner abstract class
Output:
inner abstract class is invoked
Example 6: Nested abstract class
Output:
nested abstract class is invoked
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