Java Interface Example Code: AP Computer Science A

📚 Quick Study Guide

🔍 A Java interface is a contract that defines a set of methods that a class must implement.
• 💡 Interfaces declare method signatures (name, return type, parameters) but do not provide implementations.
📝 Classes can implement multiple interfaces. ⚙️ Interfaces are declared using the `interface` keyword. 🔑 All methods in an interface are implicitly `public` and `abstract`. 📐 Interfaces can contain constant variables (implicitly `public`, `static`, and `final`). 🧪 Interfaces enable polymorphism, allowing objects of different classes to be treated as objects of the interface type. 📊 Common use cases include defining callbacks, creating abstract data types, and achieving multiple inheritance.

Practice Quiz

1. Which keyword is used to define an interface in Java?
   1. class
   2. struct
   3. interface
   4. object
2. Which of the following is true about methods declared in an interface?
   1. They must be `private`.
   2. They must be `public`.
   3. They must be `protected`.
   4. They can have any access modifier.
3. Can a class implement multiple interfaces?
   1. No, a class can only implement one interface.
   2. Yes, a class can implement multiple interfaces.
   3. Only if the interfaces are related.
   4. Only if the interfaces have no methods.
4. What happens if a class does not implement all the methods of an interface it implements?
   1. The class must be declared `abstract`.
   2. The program will not compile.
   3. The missing methods will be inherited from `Object`.
   4. The program will throw an exception at runtime.
5. Which of the following is NOT allowed inside an interface?
   1. Constant variables
   2. Abstract methods
   3. Method implementations
   4. Default methods (from Java 8 onwards)
6. What is the primary purpose of using interfaces in Java?
   1. To define the implementation of a class.
   2. To enforce a contract between classes.
   3. To create objects directly.
   4. To avoid inheritance.
7. What access modifiers are implicitly applied to variables declared in a Java Interface?
   1. private, static, final
   2. public, static
   3. public, static, final
   4. protected, static, final