Interfaces

The Big Picture

Interfaces define contracts - method signatures that classes must implement. They enable polymorphism without inheritance.

• Classes can implement multiple interfaces
• Essential: List, Comparable, Runnable
• Allow unrelated classes to share behavior

Central to "program to an interface" design principle.

How It Works

Abstract interface methods have no body; default, static, and private methods can have bodies. JVM uses itables to dispatch.

• Abstract methods implicitly public abstract
• Fields are public static final constants
• Java 8+ allows default and static methods
• Java 9+ allows private helper methods

In the Java Landscape

Interfaces are central to Java's design philosophy.

• Collections, streams, concurrency APIs use them heavily
• Functional interfaces enable lambdas (Java 8+)
• Marker interfaces (Serializable) tag classes
• Sealed interfaces (Java 17+) restrict implementations
• default methods blur the line with abstract classes

Pitfalls and Best Practices

• Keep interfaces small and focused (ISP)
• Program to interfaces, not implementations
• Use default methods to evolve APIs without breaking clients
• Don't use interfaces as constant holders
• Prefer single-method interfaces for functional use