Espresso: A Concentrated Introduction to Java

Laboratory: Polymorphism in Java

Summary: In today's laboratory, you will explore issues of polymorphism in Java.

Equipment

• Mathematician.java
• TestMath.java

• TextBlock.java
• TBUtils.java
• Tester.java
• TextLine.java
• BoxedBlock.java
• HCompose.java
• VCompose.java

Contents

• Exercises
  • Exercise 0: Preparation
  • Exercise 1: Square Roots
  • Exercise 2: Simple Lines
  • Exercise 3: Boxed Text
  • Exercise 4: Composing Text
  • Exercise 5: Other Compositions
  • Exercise 6: Writing HCompose
  • Exercise 7: Writing VCompose
  • Exercise 8: Other Composition Classes

Exercises

Exercise 0: Preparation

In this laboratory, you will use two new packages entitled username.math and username.layout.

a. In a terminal window, type

/home/rebelsky/bin/espresso math
/home/rebelsky/bin/espresso layout

You should see messages about files being copied to a temporary directory.

b. Start Eclipse.

c. In Eclipse, build a project named Temp from /home/username/CSC152/Temp.

d. In the Temp project, you should see packages named username.math and usernamelayout. Drag those packages to your Code project.

e. Delete the Temp project.

You can now work with the new packages.

Exercise 1: Square Roots

a. Scan through Mathematician.java and verify that it has the form described in the reading.

b. Extend the main method of TestMath so that it computes the square root of an Integer, a Float, a Double, a BigInteger, and a BigDecimal.

c. Verify that the square roots are correct by printing out the squares of the computed roots.

Exercise 2: Simple Lines

As a first step in understanding the layout methods, extend the main method of username.layout.Tester so that it uses TBUtils.print to print a simple block. I would suggest something like

    TextBlock block = new TextLine("This is a test.");
    TBUtils.print(pen,block);

Exercise 3: Boxed Text

In the reading, I claimed that it was possible to combine text blocks together. Try creating a boxed phrase and a boxed boxed phrase.

Exercise 4: Composing Text

The classes HCompose and VCompose permit you to compose pairs of text blocks horizontally and vertically. For example, new HCompose(tb1,tb2) makes a text block by putting tb1 and tb2 side-by-side, and new VCompose(tb1,tb2) makes a text block by putting tb1 on top of tb2.

a. Using VCompose, TextLine, and BoxedBlock, build the following text block:

+-------+
|Hello  |
|Goodbye|
+-------+

b. Using VCompose, TextLine, and BoxedBlock, build the following text block:

+-----+
|Hello|
+-----+
+-------+
|Goodbye|
+-------+

c. Using HCompose, TextLine, and BoxedBlock, build the following block:

+-----+Goodbye
|Hello|
+-----+

d. Using HCompose, TextLine, and BoxedBlock, build the following block:

Goodbye+-----+
       |Hello|
       +-----+

Exercise 5: Other Compositions

Use the various implementations of TextBlock to make an interesting textual composition.

Exercise 6: Writing HCompose

Assume that HCompose has two fields,

  TextBlock left;
  TextBlock right;

a. Sketch how you might write

• String row(int rownum),
• int width(), and
• int height().

You can look at the source code of BoxedBlock for ideas, but please do not look at the source code of HCompose.

b. Compare your answer to the source code of HCompose.

Exercise 7: Writing VCompose

Assume that VCompose has two fields,

  TextBlock top;
  TextBlock bottom;

a. Sketch how you might write

• String row(int rownum),
• int width(), and

You can look at the source code of BoxedBlock and HCompose for ideas, but please do not look at the source code of VCompose.

b. Compare your answer to the source code of VCompose.

Exercise 8: Other Composition Classes

Pick one of the following composition mechanisms and implement it as a class.

a. Truncate, which, given a textblock and a maximum width, truncates the block to that width.

b. Center, which, given a textblock and a width, centers the block within that width.

c. RightJustify, which, given a textblock and a width, right-justifies the block within that width.

History

Spring 2005 [Samuel A. Rebelsky]

• Designed textblock example.

Tuesday, 27 September 2005 [Samuel A. Rebelsky]

• Created.

Wednesday, 28 September 2005 [Samuel A. Rebelsky]

• Added the "interesting composition" problem.
• Fixed a broken link. (Bad Sam!)

Tuesday, 28 February 2006 [Samuel A. Rebelsky]

• Updated for Eclipse.
• Reformatted.
• Minor edits.