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Answers to Questions and Exercises: Nested Classes

Questions

Question 1: The program Problem.java doesn't compile. What do you need to do to make it compile? Why?
Answer 1: Add final in front of the declaration of the timer variable. A nested class declared within a method has access to any final, local variables in scope. See ProblemSolved.java.

Question 2: Use the 5.0 API documentation for the Box class (in the javax.swing package) to help you answer the following questions.

a. What static nested class does Box define?
Answer 2a: Box.Filler

b. What inner class does Box define?
Answer 2b: Box.AccessibleBox

c. What is the superclass of Box’s inner class?
Answer 2c:[java.awt.]Container.AccessibleAWTContainer

d. Which of Box’s nested classes can you use from any class?
Answer 2d: Box.Filler

e. How do you create an instance of Box’s Filler class?
Answer 2e: new Box.Filler(minDimension, prefDimension, maxDimension)

Exercises

Exercise 1: Get the file Class1.java.

a. Compile and run Class1. What is the output?
Answer 2a:
InnerClass1: getString invoked.
InnerClass1: getAnotherString invoked.

b. Create a file called Class2.java that defines subclasses of both Class1 and its inner class, InnerClass1. (Call the subclasses Class2 and InnerClass2, respectively.) InnerClass2 should override the getAnotherString method to return "InnerClass2 version of getAnotherString invoked". Class2 should define one constructor and one method:

• A no-argument constructor that initializes the inherited ic instance variable to be an instance of InnerClass2
• A main method that creates an instance of Class2 and invokes displayStrings on that instance

What is the output when you run Class2?

Answer 2b:
InnerClass1: getString invoked.
InnerClass2 version of getAnother String invoked.

See Class2.java.

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