Difference Between Abstract Class and Interface in Java

In Java, both abstract classes and interfaces are used to achieve abstraction. Many beginners get confused between these two concepts because they look similar, but they serve different purposes in application design.

Understanding their differences is very important for Java interviews and real-world development.

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What is an Abstract Class?

An abstract class is a class declared using the abstract keyword. It can contain both:

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

An abstract class is used when classes share a common base with partial behavior already defined.

Example

abstract class Animal {

    abstract void sound();

    void eat() {
        System.out.println("Animal eats food");
    }
}

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What is an Interface?

An interface is a blueprint of a class that contains method declarations. The implementing class provides the method implementation.

Interfaces are mainly used to achieve full abstraction and multiple inheritance.

Example

interface Vehicle {
    void start();
}

class Car implements Vehicle {
    public void start() {
        System.out.println("Car starts");
    }
}

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Key Differences Between Abstract Class and Interface

Feature	Abstract Class	Interface
Keyword	abstract	interface
Methods	Abstract + Concrete methods allowed	Mostly abstract methods
Variables	Can have instance variables	Variables are public, static, final by default
Constructors	Allowed	Not allowed
Access Modifiers	Any access modifier	Methods are public by default
Multiple Inheritance	Not supported (with classes)	Supported
Implementation	Use extends	Use implements
Purpose	Partial abstraction	Full abstraction / contract

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When to Use Abstract Class?

Use an abstract class when:

• Classes share common code
• You want default behavior
• There is a strong parent-child relationship

Example: Animal → Dog, Cat

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When to Use Interface?

Use an interface when:

• Different classes share common capability
• Multiple inheritance is required
• You want to define a contract

Example: Flyable → Bird, Airplane

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Interview Tip

A simple answer:

An abstract class provides partial abstraction with both implemented and unimplemented methods, while an interface provides a contract that classes must implement and supports multiple inheritance.

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Conclusion

Both abstract classes and interfaces are powerful tools in Java design. Choosing between them depends on whether you need shared implementation (abstract class) or a common contract across unrelated classes (interface).

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