IntermediateIn-depth2 min readUpdated 30 Sept 2026

What is the difference between an abstract class and an interface?

SHORT ANSWER

An abstract class is a partially built parent that can hold state, constructors and shared code, and a class can extend only one. An interface is a contract of methods that a class promises to provide, and a class can implement many.

Explain it simply

An abstract class is like a half-built house from a builder: walls and wiring are done (shared code and fields), and you finish the rooms your way. You can buy only one such house. An interface is like a certificate, such as "fire safe" or "wheelchair accessible". It lists rules you must meet, and one building can hold many certificates.

Example

java

abstract class Employee {                    // shared state + shared code
    protected final String name;
    Employee(String name) { this.name = name; }
    String greet() { return "Hi, I am " + name; }
    abstract double monthlyPay();            // children must decide
}

interface Payable { double amountDue(); }    // contracts
interface Reportable { String report(); }

class Intern extends Employee implements Payable, Reportable {
    Intern(String name) { super(name); }
    double monthlyPay() { return 15000; }
    public double amountDue() { return monthlyPay(); }
    public String report() { return name + " earns " + monthlyPay(); }
}

How it works

	Abstract class	Interface
How many per class	One (single inheritance)	Many
Fields	Any fields, with state	Only constants (`static final`)
Constructors	Yes	No
Method bodies	Yes	Default and static methods (Java 8+)
Relationship	"is a"	"can do"

Real-world use

Abstract class: a base Shape, Employee or Repository with shared logic and fields.
Interface: capabilities such as Comparable, Runnable, Payable, or anything you want to mock in tests.

Common mistakes

Using an abstract class when there is no shared state or code; an interface is lighter.
Putting too many methods in one interface. Small, focused interfaces are easier to implement.
Forgetting that interface methods are public, so implementing methods must be public too.

What to say in the interview

"An abstract class can have fields, constructors and implemented methods, and a class can extend only one, so I use it when related classes share state and behaviour, an 'is-a' relationship. An interface defines a contract a class promises to fulfil, and a class can implement many, so I use it for capabilities like Comparable or Payable. Since Java 8 interfaces can have default methods, but they still cannot hold instance state."

Follow-up questions

Why does Java not allow multiple inheritance of classes?
What problem do default methods solve?
How do interfaces help with testing?

#oop #interfaces #java