Abstract Method in Java with Examples

An abstract method in Java is a method that is declared without a body and is marked using the abstract keyword. It specifies what a method should do but leaves the implementation to subclasses. Abstract methods are used to achieve abstraction by defining a common contract that derived classes must follow.

• Must be implemented by subclasses.
• Helps achieve abstraction in Java.
• Supports runtime polymorphism.

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Explanation: In this example, the abstract class ArithmeticOperation declares an abstract method printInfo(). The Add and Sub classes extend the abstract class and provide their own implementations of printInfo(). When the method is called using the abstract class reference, the corresponding subclass implementation executes, demonstrating runtime polymorphism.

Syntax

abstract returnType methodName(parameterList);

• abstract + returnType: Defines a method with no body and specifies what type of value it will return (e.g., int, void, String)
• methodName: The name used to identify and call the method
• parameterList: Inputs given to the method inside () (can be empty, single, or multiple values)

Example of Java Abstract Method

Example 1: Abstract Class with Abstract and Concrete Methods

B's implementation of m1.
This is a concrete method.

Explanation: In this example, class A contains both an abstract method m1() and a concrete method m2(). Class B extends A and implements the abstract method m1(). The object of class B can access both the implemented abstract method and the inherited concrete method.

Note: Abstract classes cannot be instantiated, but their references can point to subclass objects, enabling runtime polymorphism.

Example 2: Abstract class with an abstract method.

Area of rectangle=156
Area of square=144
Area of circle=15.197601

Explanation: In this example, the abstract class Geometry defines three abstract methods for calculating rectangle, square, and circle areas. The Easy class implements all these methods and provides the actual area calculation logic. This ensures a common structure while allowing specific implementations.

Java Abstract Method in Interface

All the methods of an interface are public abstract by default because of which we can declare abstract methods inside an interface.

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Explanation: In this example, the interface Multiply declares abstract methods for multiplication operations. The Demo class implements the interface and provides method implementations. This demonstrates how interfaces use abstract methods to define behavior contracts.

Final Method in Abstract Class

As we have mentioned that we cannot use final with abstract Method as a modifier but we can create a Final method in abstract class.

Area = 156
Perimeter = 50

Explanation: In this example, the abstract class Geometry contains an abstract method rectangleArea() and a final concrete method rectanglePerimeter(). The abstract method must be implemented by subclasses, while the final method cannot be overridden, ensuring consistent behavior.