Difference Between Interface and Abstract Class in Java

1. Introduction

Both Interface and Abstract Class are used to achieve abstraction in Java, but they differ in:

• Design purpose
• Implementation
• Flexibility

Understanding this difference is very important for interviews and real-time design.

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2. Abstract Class

Explanation

• An abstract class can have:

  • Abstract methods (without body)
  • Concrete methods (with implementation)

• Can have constructors, variables, and method implementations

• Supports partial abstraction (0–100%)

Example

abstract class Vehicle {
    abstract void start();

    void fuel() {
        System.out.println("Fuel type: Petrol/Diesel");
    }
}

class Car extends Vehicle {
    void start() {
        System.out.println("Car starts with key");
    }
}

Explanation of Code

• Vehicle defines common behavior
• Car provides implementation for abstract method
• Allows code reuse

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3. Interface

Explanation

• An interface provides 100% abstraction (traditionally)

• Contains:

  • Abstract methods
  • (Java 8+) default and static methods

• Variables are public static final by default

• Supports multiple inheritance

Example

interface Payment {
    void pay();
}

class UpiPayment implements Payment {
    public void pay() {
        System.out.println("Paid using UPI");
    }
}

Explanation of Code

• Payment defines contract
• UpiPayment implements it
• Focus is on what to do, not how

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4. Key Differences Table

Feature	Abstract Class	Interface
Abstraction	Partial (0–100%)	100% (mostly)
Methods	Abstract + Concrete	Abstract (default/static allowed)
Variables	Instance variables allowed	Only public static final
Constructors	Yes	No
Inheritance	Single inheritance	Multiple inheritance
Keywords	extends	implements
Performance	Slightly faster	Slight overhead
Use Case	Common base class	Contract definition

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5. Real-Time Use Case

Abstract Class

• When classes share common functionality

• Example:

  • Vehicle → Car, Bike

Interface

• When different classes share common behavior

• Example:

  • Payment → UPI, CreditCard, NetBanking

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6. When to Use What?

• Use Abstract Class when:

  • You need code reuse
  • You have a strong is-a relationship

• Use Interface when:

  • You want loose coupling
  • You need multiple inheritance
  • You define a contract

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7. Summary

• Abstract Class → Partial abstraction + implementation
• Interface → Full abstraction + contract
• Abstract class = “IS-A” relationship
• Interface = “CAN-DO” capability

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Java Full Stack Developer Roadmap

To master OOP concepts like interfaces and abstract classes:

👉 https://www.ashokit.in/java-full-stack-developer-roadmap

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