Abstract Class in Java: Definition, Rules, and Examples

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Abstract Class in Java

An abstract class is a class that is declared using the abstract keyword. It is used to achieve partial abstraction in Java.

Key Points

  • Any class declared with the abstract keyword is called an abstract class.
  • An abstract class can contain:
            ✓Abstract methods
            ✓Concrete methods
  • An abstract class cannot be instantiated (object creation is not allowed).
  • If a class contains at least one abstract method, then the class must be declared abstract.
  • An abstract class does not support multiple inheritance.
  • An abstract class supports partial abstraction.

Abstract Method

  • An abstract method is a method that:
                ✓Does not have a method body
                 ✓Ends with a semicolon (;)
  • Abstract methods only provide method declaration, not implementation.

            abstract void display();

Concrete Method

  • A concrete method is a method that:
                 ✓Has a method body
                 ✓Provides complete implementation

void show() {
    System.out.println("Concrete method");
}

Types of Abstraction in Abstract Class
1. Partial Abstraction

An abstract class is called partial abstraction when it contains both abstract methods and concrete methods.

abstract class Demo {
    abstract void test(); // abstract method
    void show() { // concrete method
        System.out.println("Hello");
    }
}

2. No Abstraction

If an abstract class contains only concrete methods, then it is considered as no abstraction.

abstract class Sample {
    void display() {
        System.out.println("Only concrete methods");
    }
}

Why Abstract Class Cannot Be Fully Abstract

  • An abstract class cannot be fully abstract because:
  • Every abstract class inherits from the Object class
  • The Object class contains only concrete methods
  • Hence, every abstract class will always contain concrete methods indirectly
  • Therefore, an abstract class cannot achieve full abstraction.

Contracts Between Abstract Class and Subclass

Contract 1

If a subclass extends an abstract class, then it must provide implementations for all abstract methods of the abstract class.
👉 Implementation means method overriding

abstract class A {
    abstract void run();
}

class B extends A {
    void run() {
        System.out.println("Running");
    }
}

Contract 2

If a subclass does not implement the abstract methods of the abstract class, then the subclass must also be declared abstract.

abstract class C extends A {
    // No implementation provided
}

Important Notes

  • Abstract classes are used to provide a base structure for subclasses.
  • They are commonly used in frameworks and APIs.
  • Abstract class supports inheritance but not multiple inheritance.

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