ITEC 120 2008fall aaray, ejderrick, ibarland, jmdymacek

home—info—labs—hws—exams
textbook—java.lang docs—java.util docs—archive

lect03b
signatures; stub methods

Signatures are covered in §3.1, 3.2. The mantra for a signature is:

type_return name( parameter-type-and-name )

Stub functions

The problem with just writing, say,

public double getBalance() {
  ...
  }

  public double getBalance() {
    return -12.345;
    }

Of course, if a method's signature says it returns a String, then our stub function must comply:

  public String getOwnerName() {
    return "Otto von Schnitzelpusskrankengescheitmeyer";
    }

What about void methods, that don't return a value at all? You can leave those empty:

  public void deposit() {
    }

  // This compiles as-is.

(Constructors are like void methods, in this respect: they don't explicitly return a value, even though conceptually they are returning a whole new object.)

Another example

Problem 1: Our boss wants us to build GPSUnits for people to put in their car.

Solution 1: We can already write code for this:

public class GPSUnit {

   

  }

Problem 2: Our boss tells us what sort of things our GPSUnit can do: it can

• Give you directions to any given destination address.
  (That is, the user has to give a destination to the GPSUnit. Question: Does the user have to tell the GPSUnit where it currently is?)

  We start to wonder: How should we represent the information going on? In particular -- how should we represent the address: do we use int or double or String or Rectangle?
  A: An address, like "1001 E. Main St, Radford VA, 24142" might well be represented by Strings.

  We also wonder: How should we represent the directions? If it's a voice interface, it might be an mp3 file. For now, we'll have a more primitive system which just returns the directions as a String (for example: "Turn left now, go 1 mile, turn right, go 2 blocks."). We might later add other methods which can take this text and embed it in a web page, or can take this text and convert it into a mp3 using a speech-synthesizer, or can take this text and create a Braille printout :-).

  Once we've decided that this method takes in a String and returns a String, then we can write its signature:

  String getDirections( String destination )

  Finally, we can flesh out the method-body with a stub, and write valid code (which goes inside the class GPSUnit { ... }):

  public String getDirections( String destination ) { return "First star on the left; straight on 'til morning."; }

  (Of course, as a stub function, we're not claiming this is the correct answer — we're just returning some dumb String so that we have valid Java syntax (grammar).

• Tell you what latitude you are at right now.
  We again wonder about what information is going on: When the users asks to get the current latitude, they don't need to provide any further information. Our answer is best represented as a double (perhaps using negative numbers when south of the equator):

  double getLatitude()

  We flesh this out to a stub-method, so that it compiles:

  public double getLatitude() { return -1000.0; }

• Suppose the boss asks also wants the user to be able to set a the destination just once, and then (later) ask for directions to the most-recently-set-destination.

  This is two different behaviors (set the destination, and get directions to the destination), so we'll need two methods. What information is involved with each? What are your signatures?

  lect03b-soln.html

We can put this all together to see the entire class:

home—info—labs—hws—exams
textbook—java.lang docs—java.util docs—archive

©2008, Ian Barland, Radford University Last modified 2008.Sep.17 (Wed)

Please mail any suggestions (incl. typos, broken links) to iba�rlandrad�ford.edu