Interface Summary

Interface Concept

• Contract specifying behavior
• Collection of abstract methods
• Can have constants (public static final)
• Declared using interface keyword
• Example:

  interface GetAreable {
    double getArea();  // implicitly public abstract
  }
  

Implementing Interfaces

• Classes use implements keyword
• Can implement multiple interfaces
• Must implement all methods
• Example:

  class Circle implements GetAreable {
    @Override
    public double getArea() {
      return Math.PI * radius * radius;
    }
  }
  

Interface Characteristics

• All methods implicitly public abstract
• All fields implicitly public static final
• Cannot have constructor
• Cannot be instantiated
• Can extend multiple interfaces
• Example:

  interface Drawable extends GetAreable {
    void draw();  // adds another abstract method
  }
  

Multiple Interface Implementation

• Class can implement many interfaces
• Must implement all methods from all interfaces
• Example:

  class Square implements GetAreable, Comparable<Square> {
    @Override
    public double getArea() { return side * side; }
  
    @Override
    public int compareTo(Square other) {
      return Double.compare(this.getArea(), other.getArea());
    }
  }
  

Interface vs Abstract Class

• Interface:

  • Pure abstraction
  • Multiple implementation
  • No state
  • All methods public

• Abstract Class:

  • Partial implementation
  • Single inheritance
  • Can have state
  • Methods any visibility

Best Practices

• Use interfaces to define behavior
• Keep interfaces focused (single responsibility)
• Prefer interfaces over abstract classes
• Use meaningful method names
• Document expected behavior
• Consider providing default methods in Java 8+