Answers to Questions and Exercises: Nested Classes

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Trail: Learning the Java Language
Lesson: Classes and Inheritance

Answers to Questions and Exercises: Nested Classes

Questions

Question 1: Match each situation in the first column with the most appropriate type of nested class in the second column.

a. The only users of this nested class will be instances of the enclosing class or instances of the enclosing class’s subclasses.	1. anonymous inner class
b. Anyone can use this nested class.	2. protected inner class
c. Only instances of the declaring class need to use this nested class, and a particular instance might use it several times.	3. public static nested class
d. This tiny nested class is used just once, to create an object that implements an interface.	4. protected static nested class
e. This nested class has information about its enclosing class (not about instances of the enclosing class) and is used only by its enclosing class and perhaps their subclasses.	5. private static nested class
f. Similar situation as the preceding (choice e), but not intended to be used by subclasses.	6. private inner class

Answer 1:

a. 2. (protected inner class)
b. 3. (public static nested class)
c. 6. (private inner class)
d. 1. (anonymous inner class)
e. 4. (protected static nested class)
f. 5. (private static nested class)

Question 2: The program Problem.java doesn't compile. What do you need to do to make it compile? Why?
Answer 2: 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 3: Use the 1.3 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 3a: Box.Filler b. What inner class does Box define?
Answer 3b: Box.AccessibleBox c. What is the superclass of Box’s inner class?
Answer 3c:[java.awt.]Container.AccessibleAWTContainer d. Which of Box’s nested classes can you use from any class?
Answer d: Box.Filler e. How do you create an instance of Box’s Filler class?
Answer 3e:new Box.Filler(minDimension, prefDimension, maxDimension)

Exercises

Exercise 1: First, get the source file InnerClassDemo.java.

a. Compile and run InnerClassDemo.java.
Answer 1a: N/A. [This step is here just to make sure you can compile and run Swing programs, before you try to change the programs.]
b. Make a copy of InnerClassDemo. Add to it an inner class named MyActionListener that implements the ActionListener interface. The ActionListener interface defines a single method. Put the following code into your implementation of the method: quit();
Delete the double forward slashes (//) in front of the following line of code:
//button.addActionListener(new MyActionListener());
Now compile and run the program. What is the difference in behavior between this version and the previous version of InnerClassDemo?
Answer 1b: See InnerClassDemo2.java. In this version, the button actually does something; it makes the program exit.
c. Make a copy of the program you created in exercise 1b. Change your ActionListener implementation to be an anonymous inner class. (Hint: The program has another anonymous inner class, a WindowAdapter, which you can refer to for syntax help.)
Answer 1c: See InnerClassDemo3.java.

Exercise 2: 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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