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The Java™ Tutorials
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Interfaces and Inheritance
Interfaces
Defining an Interface
Implementing an Interface
Using an Interface as a Type
Evolving Interfaces
Default Methods
Summary of Interfaces
Questions and Exercises
Inheritance
Multiple Inheritance of State, Implementation, and Type
Overriding and Hiding Methods
Polymorphism
Hiding Fields
Using the Keyword super
Object as a Superclass
Writing Final Classes and Methods
Abstract Methods and Classes
Summary of Inheritance
Questions and Exercises
Trail: Learning the Java Language
Lesson: Interfaces and Inheritance
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# Questions and Exercises: Interfaces

## Questions

1. What methods would a class that implements the `java.lang.CharSequence` interface have to implement?
2. What is wrong with the following interface?

       public interface SomethingIsWrong {
           void aMethod(int aValue){
               System.out.println("Hi Mom");
           }
       }

3. Fix the interface in question 2.
4. Is the following interface valid?

       public interface Marker {
       }

## Exercises

1. Write a class that implements the `CharSequence` interface found in the `java.lang` package. Your implementation should return the string backwards. Select one of the sentences from this book to use as the data. Write a small `main` method to test your class; make sure to call all four methods.
2. Suppose you have written a time server that periodically notifies its clients of the current date and time. Write an interface the server could use to enforce a particular protocol on its clients.

Check your answers.

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