ITEC120

lab02:
Strings

Aug.30 (Wed)

We'll experiment with Strings, which were barely introduced in lecture yesterday.

Today's project can be done individually, or in a pair.

Preliminary:

• Start up BlueJ.
• Create a new project; a plausible name would be “lab-week2a” or so.
• Make a new class; as before, take the file which BlueJ provides (with a few boilerplate definitions) and gut it out, replacing it with a single class definition, perhaps

  class StringsInLab { }

Experimenting on Strings

Go to BlueJ's code pad in the lower-right corner of the main screen¹. Type in some sample strings, such as:

• "hello"
• "!&$^%, and &*(# too!"
• ""

As just discussed in the first few minutes of the Lab section, +, if applied to strings, will concatenate them. Try these in the Code Pane:

• "hi" + "there"
• "Ja" + "va a la" + "ng" + "u" + "age."
• Here's the function you were just shown:

  /* Return the results heard when shouting `shoutOut' near a cliff. * echo( "Ahoy" ) = "AhoyAhoy" * echo( "" ) = "" */ String echo( String shoutOut ) { return shoutOut + shoutOut; }

  Take this function and modify it to sound a bit more realistic -- make it match the test case

  /* echo2( "Ahoy" ) = "Ahoy! ...Ahoy, Ahoy"

• Agent 007 is known for his suave introduction²: "Bond... James Bond." Write the function introduceSpy, which creates such a greeting given a first and last name:

  /* Test cases: * spyIntroduction( "Dweezil", "Zappa" ) = "Zappa... *Dweezil* Zappa." */

  Before you start writing code, write a few more test cases. What are some degenerate inputs to try? (When you have two inputs, you have more choices -- you can have one or the other or both inputs be degenerate.)

  Hmmm, this is new -- a function which accepts two inputs, we haven't seen that before!
  Well, give it a whirl anyway. If you like, here's a syntax rule for defining simple two-argument functions:

  [return-type] [name]( [type] [arg1name], [type] [arg2name] ) { return [expression-involving-arg1name-and-arg2name]; }

  (If you get your function to work, can you match up piece-for-piece, your code with this syntax rule?)

If you finish these tasks, you are free to start working on hw02i (in a new BlueJ project window), and/or line up a partner for hw02g.

Sep.01 (Fri)

A preview to objects

Yesterday in class we say that when you write 3+"musketeers", Java realizes that neither meaning of + (adding numbers, and concatenating strings) makes any sense, and as a (perhaps confusing) courtesy it inserts a function call which converts the 3 into a String for you. After Java does that, string concatenation makes sense, so:
  3 + "musketeers"
= Integer.toString(3) + "musketeers"
= "3" + "musketeers"
= "3musketeers"

Not only does Java provide a function + to concatentate strings, but it also provides a function to extract pieces of a string: substring. Try the following, in BlueJ's codepane:

String menuEntry = "hamburger";
menuEntry.substring(1,3)

• I thought it was giving me back characters starting at #1, so why does the returned string start with ‘a’? Oh -- the 1 and 3 are indices, and indices start counting from 0.
  Let's confirm that we can ask for a substring starting at index 0: menuEntry.substring(0,3).

• Okay, but if we start counting at 0, then the character at index three is 'b', so why didn't the returned string include 'b'?

  Because it turns out, the function substring( int start, int stop ) returns the letters starting at index start, up to (not including) index stop.

  The only way you could have known this was by reading the documentation (comments) for substring. This is a live example of why comments are so important!

• Much more unsettling -- if this is a function which takes a string, a start-point, and an end-point, then why didn't we write

  substring(menuEntry, 1, 3) /* Does not work?! */

  The answer is a bit odd: Strings are actually objects. If you want to ask an object (like menuEntry) a question, you walk up to it and ask “Hey menuEntry, what is your substring from indices 1 to 3?” In Java we do that by writing menuEntry.substring(1,3).

  We've actually seen this before, when using BlueJ: in the first week, we created a PizzaServer object, we named it jo, and then (by clicking in BlueJ) we asked jo.pieArea(20). In fact, in the previous part of this very lab, when we tested spyIntroduction, we made objects: From the template class StringsInLab, we made a new object (an “instance”) and associated it with the identifier (say) stringsInLa1. Then we walked up to stringsInLa1 and asked it “hey, give me spyIntroduction( "James", "Bond" ).” Look closely at the first words BlueJ's results-window:

  stringsInLa1.spyIntroduction("James", "Bond") returned: …

  That same dot notation! The exact same thing is happening with our calls to substring: we walk up to an object, and ask it a substring question.

Note that you don't need to name every String object, you can walk up to any String and ask it for substrings

"hamburger".substring(4,8)
"smiles".substring(1,5)

hands-on exercise

Today's project can be done individually, or in a pair. Before leaving, you'll need to check off your work (see below).

Evaluate each of these in BlueJ's code pane. Before hitting return, make a guess to yourself (or your partner), what you think your answer will be:

• "hi" + 3 + 4
• "hi" + (3 + 4)
• ("hi" + 3) + 4
• 3 + 4 + "hi"
• (3 + 4) + "hi"
• 3 + (4 + "hi")

Although we won't use substring in a program yet, go ahead and try these out:

• "hamburger".substring(4,8)
• "smiles".substring(1,5)
• "smiles".substring(1,2)³
• "smiles".substring(1,1) (Think, before you press return: what will the answer be?)
• "smiles".substring(0,5) (Think, before you press return: what will the answer be?)
• "smiles".substring(0,6) (Think, before you press return: what will the answer be?)
• "smiles".substring(0,7) (Think, before you press return: what will the answer be?)

Now, we are ready for another short program:

You are working for a pet store, and you need to write a function which takes two pieces of information: an animal type, and the number of those animals currently in stock; it returns a string conveying that information.

• What is an appropriate Java type, to represent the number of animals in stock?
  ... Clearly, int. What is an appropriate Java type, to represent an animal type? Strings seem like a good choice (after all, that's what we're talking about in class :-)
• One test case might be

  inventoryMessage( "dog", 3 ) = "There are 3 dogs in stock."

  What are some other test cases? (Include degenerate, oddball cases.)
  (Note that the problem does not ask you to print out the string, only to return it.)
• Write the function.
• Test it.
  What is a numeric input which yields poor English in the result? (But we don't know how to fix it, at this point in the semester.)
  What is a string input which yields poor English in the result? (But fixing it would require... gosh, lots of work or info. Think about/discuss, what would be needed, to get the program to work correctly?)
• Does your program violate any of the Laws of Programming? You can compare your solution to this one.

If you finish this, you can leave early, or begin working on hw02g.

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Please mail any suggestions (incl. typos, broken links) to ibarlandradford.edu

Last modified 2006.Sep.01 (Fri)