How do you show an interface and its implementations in a class diagram?

Direct Answer

Draw the interface as a box with <<interface>> above the class name and only method signatures inside (no attributes). Each implementing class uses a dashed line with a hollow arrow pointing to the interface. If you have an abstract class, write the name in italics and use a solid line with hollow arrow for subclasses. You can mix both: an interface with multiple implementations, one of which is abstract with its own subclasses.

Deep Dive & Architecture

Realization means implements

An interface is normally shown as a rectangle marked «interface» with its operation signatures. A class that implements it connects with a dashed line and a hollow triangle pointing toward the interface. The arrow direction says the concrete class fulfils the more general contract.

Generalization means extends

Inheritance uses a solid line with a hollow triangle pointing toward the parent class. An abstract class may be shown with an italic name or an {abstract} marker. Visibility symbols such as + and - can be added when method and field access matters.

A payment hierarchy uses both notations

PaymentMethod can be an interface. CardPayment and UpiPayment point to it with dashed realization arrows. If CardPayment is an abstract base for CreditCardPayment and DebitCardPayment, those subclasses use solid generalization arrows. Line style distinguishes implementing a contract from inheriting a base implementation.

Code Example

interface PaymentMethod {        // <<interface>> in UML
    void pay(BigDecimal amount);
}
class CreditCard implements PaymentMethod {   // dashed + hollow triangle
    public void pay(BigDecimal amount) { /* charge card */ }
}
class UpiPayment implements PaymentMethod {
    public void pay(BigDecimal amount) { /* UPI */ }
}
abstract class Wallet implements PaymentMethod { /* italic name; solid arrow from subclasses */ }

Two implementers realize the interface (dashed); an abstract Wallet can implement it and be extended (solid).

Common Mistakes & Pitfalls

• Solid line for implementation — implements is a dashed line; solid is for extends.
• Attributes inside an interface box — interfaces show methods (and constants), not instance fields.
• Forgetting the «interface» / italic markers — without them the reader can't tell the type.
• Arrow pointing the wrong way — the triangle points to the interface/parent.