ITEC 120 2009spring ibarland nokie jmdymacek

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lect03a
writing test methods
and state revisited

1. read docs;
2. call interactively (*after* figuring out the expected answer);
3. write a test case.

• add(int,int)
• grow(int,int)
• translate(int,int)
• union(Rectangle)

Note: in a test-method, you figure the answers *first*.
Tip: have *some* test cases involving zeroes and ones, whenever applicable.
Mindset: you'll get paid $100 if you find an incorrect result! ...er, rather, you won't lose 20pts (or, your job).

 1  class RectangleTester {
 2      
 3      public static void main( String[] args ) {
 4          java.awt.Rectangle r = new java.awt.Rectangle( 10, 15, 100, 200 );
 5          
 6          r.grow( 5, 7 );
 7          System.out.println( "r's width is now " + r.getWidth() + "; I expected 110." );
 8          System.out.println( "r's height is now " + r.getHeight() + "; I expected 214." );
 9          System.out.println( "r's NW corner is at "
10                            + "(" + r.getX() + "," + r.getY() + ")"
11                            + "; I expected (5,8)." );
12                            
13                            
14          // Also, verify that growing by 0,0 has no effect:
15          r.grow( 0, 0 );
16          System.out.println( "r's width is now " + r.getWidth() + "; I expected 110." );
17          System.out.println( "r's height is now " + r.getHeight() + "; I expected 214." );
18          System.out.println( "r's NW corner is at "
19                            + "(" + r.getX() + "," + r.getY() + ")"
20                            + "; I expected (5,8)." );
21          
22          }
23      
24      }

To test a method

1. Create an object
2. call the method to be tested
3. Compare the result to the expected value. For now, we'll print them.
   If it's a mutator method (which doesn't return a result), then you must call one or more getters after mutating, to make sure the object has changed as expected.
4. Repeat as necessary.
   (Usually, at least two-three tests are necessary to test a single method.)

Example: BankAccounts

If we had a class BankAccount,

• What state would you expect it to have, inside of each BankAccount?
• What methods would you expect a BankAccount to be able to do?

Somebody in the future will actually write class BankAccount. But for now, we'll go ahead and write class BankAccountTester right now!

• How will we declare a variable, to hold a BankAccount? solution
• How will we construct a new BankAccount? (This requires some discussion: does one need to be given any information, in order to construct a new BankAccount?) solution
• If we call the getBalance method right now, what do we expect the answer to be? Write code to print out the actual and expected answer. solution
• Make a call to deposit, and then print the actual and expected balance.
  Then, do the same thing with two more deposits. (Hmm, what's the smallest possible deposit?) solution
• Similarly, test making withdrawls. solution

Observation: We're writing the test-class before we write the actual class! That's the proper way for test-classes to be built.

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©2009, Ian Barland, Radford University Last modified 2009.Feb.09 (Mon)

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