# 2.11: Exercises

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The code for this chapter is in the ch11 directory of ThinkJavaCode. See [Section 0.4] for instructions on how to download the repository. Before you start the exercises, we recommend that you compile and run the examples.

At this point you know enough to read Appendix B, which is about simple 2D graphics and animations. During the next few chapters, you should take a detour to read this appendix and work through the exercises.

Exercise $$\PageIndex{1}$$

Review the documentation of java.awt.Rectangle. Which methods are pure? Which are modifiers?

If you review the documentation of java.lang.String, you should see that there are no modifiers, because strings are immutable.

Exercise $$\PageIndex{2}$$

The implementation of increment in this chapter is not very efficient. Can you rewrite it so it doesn’t use any loops?

Hint

Remember the modulus operator.

Exercise $$\PageIndex{3}$$

In the board game Scrabble, each tile contains a letter, which is used to spell words in rows and columns, and a score, which is used to determine the value of words.

1. Write a definition for a class named Tile that represents Scrabble tiles. The instance variables should include a character named letter and an integer named value.
2. Write a constructor that takes parameters named letter and value and initializes the instance variables.
3. Write a method named printTile that takes a Tile object as a parameter and displays the instance variables in a reader-friendly format.
4. Write a method named testTile that creates a Tile object with the letter Z and the value 10, and then uses printTile to display the state of the object.
5. Implement the toString and equals methods for a Tile.
6. Create getters and setters for each of the attributes.

The point of this exercise is to practice the mechanical part of creating a new class definition and code that tests it.

Exercise $$\PageIndex{4}$$

Write a class definition for Date, an object type that contains three integers: year, month, and day. This class should provide two constructors. The first should take no parameters and initialize a default date. The second should take parameters named year, month and day, and use them to initialize the instance variables.

Write a main method that creates a new Date object named birthday. The new object should contain your birth date. You can use either constructor.

Exercise $$\PageIndex{5}$$

A rational number is a number that can be represented as the ratio of two integers. For example, 2/3 is a rational number, and you can think of 7 as a rational number with an implicit 1 in the denominator.

1. Define a class called Rational. A Rational object should have two integer instance variables that store the numerator and denominator.
2. Write a constructor that takes no arguments and that sets the numerator to 0 and denominator to 1.
3. Write an instance method called printRational that displays a Rational in some reasonable format.
4. Write a main method that creates a new object with type Rational, sets its instance variables to some values, and displays the object.
5. At this stage, you have a minimal testable program. Test it and, if necessary, debug it.
6. Write a toString method for Rational and test it using println.
7. Write a second constructor that takes two arguments and uses them to initialize the instance variables.
8. Write an instance method called negate that reverses the sign of a rational number. This method should be a modifier, so it should be void. Add lines to main to test the new method.
9. Write an instance method called invert that inverts the number by swapping the numerator and denominator. It should be a modifier. Add lines to main to test the new method.
10. Write an instance method called toDouble that converts the rational number to a double (floating-point number) and returns the result. This method is a pure method; it does not modify the object. As always, test the new method.
11. Hint: Finding the GCD only takes a few lines of code. Search the web for “Euclidean algorithm”.
12. There are several ways to add fractions. You can use any one you want, but you should make sure that the result of the operation is reduced so that the numerator and denominator have no common divisor (other than 1).

The purpose of this exercise is to write a class definition that includes a variety of methods, including constructors, static methods, instance methods, modifiers, and pure methods.

This page titled 2.11: Exercises is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .