Abstract classes in Java are used as templates for creating concrete classes and cannot be instantiated directly. They provide a common interface and implementation for subclasses through abstract and concrete methods. For example, an abstract Shape class can define methods like getColor() while making getArea() abstract, requiring subclasses like Circle to implement it. Abstract classes allow for code reuse and polymorphism, serving as a blueprint for related classes to customize as needed.

1. Java abstract Keyword
Java is an object-oriented programming language that provides the “abstract”
keyword to allow programmers to define incomplete classes that cannot be
instantiated. Abstract classes are used as templates for creating concrete classes
and are meant to be extended by subclasses. They provide a blueprint for creating
objects that can be customized to suit specific needs. It is important to understand
the use of abstract classes for Java programming as it allows for code reusability
and saves time.
This article explains the “abstract” keyword in Java, its benefits and limitations. It
also covers how to use it in Java programming. Additionally, the article compares
abstract classes and interfaces in Java. It also illustrates when to use each.
Abstract Keyword in Java: Key
Characteristics
Instantiation of abstract classes:
An abstract class can’t be instantiated directly. Instead, it is designed to be extended
by other classes, which can provide concrete implementations of its abstract
methods.
Method body:
A method without an implementation is said to be abstract. It terminates with a
semicolon rather than a method body and is declared using the abstract keyword.

2. Subclasses of an abstract class must provide a concrete implementation of all
abstract methods defined in the parent class.
Abstract and concrete methods:
Abstract classes can contain both abstract and concrete methods. Concrete
methods are implemented in the abstract class itself and can be used by both the
abstract class and its subclasses.
Constructors:
Abstract classes can have constructors, which are used to initialize instance
variables and perform other initialization tasks. Constructors in concrete subclasses
often invoke the constructors of abstract classes as they cannot be constructed
directly.
Instance variables:
Instance variables contained in the abstract classes may be utilized by both the
abstract class and its subclasses. Subclasses can access these variables directly,
just like any other instance variables.
Interfaces:
Abstract classes can implement interfaces, which define a set of methods that must
be implemented by any class that implements the interface. In this case, the abstract
class must provide concrete implementations of all methods defined in the interface.
Abstract classes
Definition and explanation of abstract classes

3. In Java, an abstract class is a class that cannot be instantiated and is used as a
base class for other classes. It’s designed to be extended by other classes, and it
contains abstract methods, concrete methods, and instance variables. An abstract
class provides a common interface for a set of subclasses, which allows for
polymorphism in Java programming.
Examples of abstract classes and their implementation
in Java
An example of an abstract class in Java is the Shape class. A Shape class can be
extended by various other classes, like Circles, rectangles, Triangles, etc. Each of
these classes can define its methods while still inheriting the properties of the Shape
class. Here’s an example of how to define an abstract class in Java:
public abstract class Shape {
private String color;
public Shape(String color) {
this.color = color;
}
public String getColor() {
return color;
}
public void setColor(String color) {
this.color = color;
}
public abstract double getArea();

4. }
In the above code, the Shape class is declared as abstract using the abstract
keyword. It has a constructor and two methods, getColor() and setColor(), which are
not abstract. It also has an abstract method, getArea().
Advantages of using abstract classes
The advantages of using abstract classes in Java are:
■ It provides a common interface for a set of subclasses, which allows for
polymorphism in Java programming.
■ It can implement common functionality and state shared by all
subclasses.
■ It can enforce a contract that subclasses must implement abstract
methods.
■ It can simplify the design of large and complex software systems by
providing a structure for inheritance.
How to use Abstract Classes and
Methods
To use the abstract class in Java, you need to create a subclass that extends the
abstract class and implements the abstract methods. Here is an example:
public class Circle extends Shape {
private double radius;
public Circle(String color, double radius) {
super(color);
this.radius = radius;
}

5. public double getRadius() {
return radius;
}
public void setRadius(double radius) {
this.radius = radius;
}
public double getArea() {
return Math.PI * radius * radius;
}
}
In the above code, the Circle class extends the Shape abstract class and
implements the getArea() method.
Abstract classes and methods are useful when you want to create a group of related
classes that share some common functionality. They also have their own unique
features. They provide a way to enforce consistency among the related classes
while still allowing for customization.
Abstract keyword rules and best
practices:
There are several rules and best practices to keep in mind when working with the
abstract keyword in Java.
Best Practices:

6. ■ The abstract keyword can only be used with classes and methods.
■ An abstract class can contain constructors and static methods.
■ If a class extends an abstract class, it must implement at least one of the
abstract methods.
■ An abstract class can contain overloaded abstract methods.
■ The local inner class may be declared abstract.
■ The throw clause can be used to declare the abstract method.
Avoid:
■ Do not use the abstract keyword with variables and constructors.
■ An abstract class cannot be instantiated.
■ An abstract method does not contain the body.
■ Do not use the abstract keyword with the final.
■ Do not declare abstract methods as private or static.
■ An abstract method cannot be synchronized.
Abstract Classes vs. Interfaces
In Java, both abstract classes and interfaces are used to define abstractions.
Abstract classes are used to define a common structure and behaviour for a group
of classes that share a common set of features. On the other hand, interfaces are
used to define a common set of methods that can be implemented by multiple
unrelated classes.
Abstract classes can have both abstract and non-abstract methods. At the same
time, interfaces can only have abstract methods. This is the key difference between
abstract classes and interfaces. A class is capable of extending only one abstract
class, but it can implement multiple interfaces.
When must an interface be used over an abstract
class:

7. ■ When you need to define a common behaviour for a group of unrelated
classes
■ When you need to provide multiple inheritances to a class
Examples of when to use an abstract class over an
interface:
■ When you need to define a common structure and behaviour for a group
of related classes
■ When you need to provide default implementations for some of the
methods
Conclusion
In conclusion, understanding the “abstract” keyword is crucial for effective Java
programming. Abstract classes and methods allow developers to define and enforce
a blueprint for the behaviour of child classes. They provide structure and abstraction
to code, making it more efficient and easier to maintain. By using abstract classes
and methods, developers can write more flexible, extensible, and reusable code,
reducing development time and cost.