An abstract class in Java is a special kind of class that cannot be instantiated on its own. It is used as a base class or blueprint for creating concrete subclasses. Abstract classes are used to provide a common interface and implementation for a set of related classes.
An abstract class is declared using the "abstract" keyword and can contain both abstract and concrete methods. Abstract methods are methods without a body, declared using the "abstract" keyword. Concrete methods are methods with a body, that provide a default implementation.
Abstract classes can also have instance variables, constructor methods, and static methods.
Example:
abstract class Animal {
// instance variables private String name; private int age;
// constructor public Animal
(String name, int age) {
this.name = name; this.age = age;
}
// abstract method abstract void makeSound();
// concrete method public void printInfo() {
System.out.println("Name: " + name); System.out.println("Age: " + age);
}
}
class Dog extends Animal {
// constructor public Dog
(String name, int age) {
super(name, age);
}
// concrete method that implements the abstract method in the superclass void makeSound() {
System.out.println("Woof woof!");
}
}
In this example, the Animal class is an abstract class that provides a common interface for its subclasses. The Dog class is a concrete subclass of Animal that implements the abstract method makeSound. The printInfo method is a concrete method that provides a default implementation for printing the name and age of an Animal.
Advantages of Abstract Classes in Java:
Reusability: Abstract classes provide a way to share common behavior and implementation among related classes, which helps to reduce code duplication and improve code reuse.
Flexibility: Abstract classes allow concrete subclasses to provide their own implementation for abstract methods, which enables each subclass to provide a unique implementation that is appropriate for its particular use case.
Polymorphism: Abstract classes facilitate polymorphism by allowing objects of different concrete subclasses to be treated as objects of the same type (i.e. the type of the abstract class).
Improved Type Safety: By using abstract classes, it becomes possible to define a common interface for a set of related classes, which can improve type safety by catching type errors at compile-time rather than at runtime.
Disadvantages of Abstract Classes in Java:
Limited Instantiability: Since abstract classes cannot be instantiated on their own, they are less flexible than concrete classes, as they can only be used as a base class for creating concrete subclasses.
Increased Complexity: Abstract classes can make code more complex, as they require additional code to implement abstract methods and to handle the inheritance relationship between the abstract class and its subclasses.
Performance Overhead: The use of abstract classes can result in a performance overhead, as the virtual method dispatch required to call an abstract method in a subclass involves additional CPU instructions.
In general, abstract classes should be used when you have a common behavior or implementation that you want to share among a set of related classes. If you don't have a need for shared behavior or implementation, it is probably better to use concrete classes instead.
Summery
An abstract class in Java is a class that cannot be instantiated on its own and is used as a blueprint or base class for creating concrete subclasses. Abstract classes provide a way to share common behavior and implementation among related classes, which can improve code reuse and provide a common interface for a set of related classes. Abstract classes can contain both abstract and concrete methods, and can have instance variables, constructor methods, and static methods. They offer advantages such as improved reusability, flexibility, and polymorphism, but also have disadvantages such as limited instantiability and increased complexity, and can result in a performance overhead. Abstract classes should be used when there is a need for shared behavior or implementation among a set of related classes, and otherwise, concrete classes should be used.