abstract Keyword.md

abstract Keyword in Java

1. Overview

The abstract keyword in Java is used to define:

1. Abstract classes
2. Abstract methods

It allows you to create a base structure without specifying full implementation. Abstraction focuses on what needs to be done, not how it will be done.

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2. abstract Methods

An abstract method:

• Has a declaration (signature)
• Has no body
• Must be implemented by child classes

Syntax:

abstract void draw();

Example:

abstract class Shape {
    abstract void draw();   // no implementation
}

Child class must implement it:

class Circle extends Shape {
    void draw() {
        System.out.println("Drawing Circle");
    }
}

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3. Rules for abstract Methods

Rule	Explanation
Must be inside an abstract class	Cannot exist in a normal class
Cannot be private	Must be overridden by child
Cannot be final	Final methods cannot be overridden
Cannot be static	Abstract behavior belongs to instance, not class
Must be overridden in child class	Unless child is abstract

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4. abstract Classes

A class declared with abstract keyword cannot be instantiated.

Syntax:

abstract class Animal {
    abstract void sound();       // abstract method

    void sleep() {               // normal method
        System.out.println("Animal sleeps");
    }
}

Key Properties:

• Can contain abstract + concrete methods
• Can have constructors
• Can have variables, static methods, blocks
• Can extend another class (abstract or concrete)

Cannot Do:

Animal a = new Animal(); // error: cannot instantiate abstract class

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5. Constructor in Abstract Class

Even though abstract classes cannot be instantiated, they can have constructors. These are used for initial setup of fields.

Example:

abstract class Person {
    String name;

    Person(String name) {
        this.name = name;
    }

    abstract void work();
}

class Employee extends Person {
    Employee(String name) {
        super(name);
    }

    void work() {
        System.out.println(name + " is working");
    }
}

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6. Abstraction Level in abstract Classes

Abstract class provides partial abstraction.

• Some methods can be abstract
• Some methods can be fully implemented

This is different from interfaces (which originally gave 100% abstraction).

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7. Why Use abstract Classes?

When:

• You want to provide common functionality in base class
• You want child classes to implement specific behavior
• You want to avoid object creation of incomplete classes

Example Scenario:

A class like Shape is too generic to create objects:

Shape s = new Shape(); // meaningless

But: Circle, Rectangle, Triangle → meaningful.

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8. Real Example — Payment System

abstract class Payment {
    abstract void pay(double amount);

    void showReceipt() {
        System.out.println("Payment completed");
    }
}

class UPI extends Payment {
    void pay(double amount) {
        System.out.println("Paid via UPI: " + amount);
    }
}

Usage:

Payment p = new UPI();
p.pay(400);
p.showReceipt();

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9. abstract Class vs Interface

Feature	abstract class	interface
Methods	Abstract + concrete	Abstract, default, static
Variables	Any type	public static final only
Constructor	Allowed	Not allowed
Multiple inheritance	Only one abstract class	Multiple interfaces
Abstraction level	Partial	Complete (mostly)

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10. abstract + final + static — Valid or Not?

Combination	Valid?	Reason
abstract + final method	❌	final cannot be overridden
abstract + static method	❌	static cannot be abstract
abstract + private method	❌	private cannot be overridden
abstract class with final methods	✔	final concrete methods allowed
abstract class with static methods	✔	static concrete methods allowed

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11. Memory Behavior of abstract Classes

When creating a child object:

Payment p = new UPI();

Memory:

Reference type: Payment (abstract)
Object type: UPI (concrete)

• Abstract methods resolved via overriding
• Concrete methods inherited normally

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12. Common Interview Questions

Q1: Can abstract class have a constructor?

Yes. Used to initialize common fields.

Q2: Can abstract method be private?

No. It must be accessible to child classes for overriding.

Q3: Is it mandatory for child class to override all abstract methods?

Yes, unless the child class is also abstract.

Q4: What is the purpose of abstract class?

To provide a base template with some enforced behavior.

Q5: Can we declare an abstract class as final?

No. It contradicts the purpose of inheritance.

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Key Takeaways

• abstract keyword is used for classes and methods.

• Abstract methods have no body and must be implemented by child classes.

• Abstract classes cannot be instantiated, but can have constructors, fields, and methods.

• Provides partial abstraction and supports inheritance.

• Used to enforce a structure while allowing flexible implementations.

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