ITEC120-ibarland (incl. office hrs)—info—lectures—labs—hws

lab11: for-each practice

Today's lab will have you modify a for-each loop, as well as a while-loop using an iterator.
You may work in pairs, or individually.
We will not check off today's lab, but its questions will be fair game for tomorrow's Chapter-6 quiz.

1. Download the code from class (.jar), saving it to the computer's disk (noticing which directory you're saving it in). Then, start BlueJ and select Project > Open non-BlueJ.... You should see a project with classes WhilePractice and ArrayIterator.
   If you have troubles with this, you can cut-and-paste the lecture code WhilePractice.java as well as ArrayIterator, an extra class needed for part 3 below.
2. Recall/review the example from lecture, where we used a for-each loop to look at each file inside a directory:

   import java.io.File; /** * Return The sum of the length of each file immediately inside directoryName. * Doesn't recur into any sub-directories (but does include any size a directory * itself takes up, aside from its sub-contents). * @param directoryName The pathname of a directory. * @return The sum of the length of each file immediately inside directoryName. */ public static String directoryListing( String directoryName ) { // Convert the directoryName into a File object, so we can get its size etc.: final File theDirectory = new File( directoryName ); // Note that we import'd java.io.File. if (theDirectory.isDirectory()) { long fileLengthsSoFar = 0; // Accumulate the sum of all immediate file sizes. String listingSoFar = ""; for ( File f : theDirectory.listFiles() ) { // For each `f' in the directory-listing... textualListing += f.toString() + "\t\t\t" + f.length() + "\n"; listingSoFar += f.length(); // Add f's length to our sum. } return listingSoFar + "\nTotal length: " + fileLengthsSoFar + "\n"; } else { // Hmm, this file wasn't a directory; print a message to System.err. String errMsg = "immediateDirectorySize: " + directoryName + " is not the name of a directory."; System.err.println( errMsg ); return errMsg; // Is this an appropriate sentinel value? I want to return 'false', but can't! } }

3. Now, write a similar function int countJavaFiles(String) which takes in a directory-name, and counts how many of the files in that directory end in¹ .java.
   Hint: use the String method endsWith.
   You can test your code by running it on various BlueJ project files from previous labs on your H: drive — the file name (on Windows) will be something like "H:\itec120\my-labs\", depending on exactly how you named your files.
4. Write a method boolean hasJavaFiles(String) which takes in a directory-name, and returns true if (and only if) that directory contains any .java files.
   (Hint: use the previous function, to make your program one line.)

5. While it is simple code to have #4 call #3, there is also a performance penalty: even if you find a .java right off the bat, your code still looks at every single file in the directory. This is a drawback of the for-each loop.

   Since a for-each loop seems not to be appropriate, instead write boolean hasJavaFiles_v2(String), which uses a while loop and an iterator.
   Hint: your while-loop will keep track a variable “foundSoFar”. You only want to continue your loop as long as there are more files to consider, and you haven't yet found any java file so far.

   Some notes:
   • For using iterators, recall the example from lecture notes. You can import java.util.Iterator to save yourself from typing the class's full name.

   • For comparison, also look at If you look at directoryListing_v2, which works just like directoryListing except it uses an iterator instead of a for-each loop. The one line of that function which is totally confusing to us (and which you can ignore) is a line which converts the result of theDirectory.listFile() from a collection into an Iterator:

     ArrayIterator<File> it = new ArrayIterator<File>( theDirectory.listFiles() );

     Once this is done, though, you can happily call it.hasNext() (returning a boolean) and it.next() (returning a File)

     Exactly what the <…> means must wait until ITEC220; why it is a ArrayIterator yet we use it as if it were an Iterator (with methods hasNext() and next()) will wait until we cover interfaces, section 6.5.

Wed.'s solution

ITEC120-ibarland (incl. office hrs)—info—lectures—labs—hws

2006.Nov.03 (Fri)

Experimenting with references

We can pass object references as parameters, just like anything else. What actually is being passed to the function is a piece of paper which has a reference written down on it. For example, we saw Kennel.setDog(), which is passed a piece of paper, upon which is a reference to some particular Dog.

1. Consider the method:

   class Dog { // ...fields and other methods omitted... /** Have this and otherDog exchange their ages. * A very useful method for sitcoms and Disney movies. * @param otherDog the dog who we'll swap ages with. */ void swapAge(Dog otherDog) { // ... WRITE THIS METHOD, in a moment ... } }

   1. Draw pictures, showing what happens when we evaluate:

      Dog yourDog = new Dog( "rex", 5); Dog myDog = new Dog( "fido", 7 ); Dog worldsBestDog = myDog; myDog.toString() (myDog == worldsBestDog) // Should be ...??? myDog.ageOneYear(); // myDog is still the same dog, just older. yourDog.swapAge( myDog ); // myDog is still the same dog, just younger. myDog.toString() (myDog == worldsBestDog) // Should be ...???

   2. Now, go ahead and write Dog.swapAge(Dog).
      (For starting code, save-to-disk lab11-Dog-Kennel.jar, and then from within BlueJ select Project > Open Non BlueJ….)
2. What does the following code do?

   Dog badClone() { // Not an evil clone, just broken: no new Dog is made. Dog d; d = this; // Note that "new Dog()" is *not* getting created. return d; // A three-line version of "return this". }

   which we then test by writing

   Dog d1 = new Dog( "lassie", 3); Dog d2 = d1.badClone(); (d1 == d2) // Should this be true, or should it be false?

   Draw pictures of what happens, when you run the tests.
3. If you want to (successfully) clone² a Dog, you must call new Dog:

   class Dog { /** * @return a new dog which looks just like this one, * but is not == to this one.. */ public Dog clone() { // ... WRITE THIS METHOD ... } }

   The method be called via:

   Dog d3 = new Dog( "lassie", 3); Dog d4 = d3.Clone(); (d3 == d4) // Should this be true, or should it be false?

4. Now that you're comfortable writing methods which return Dogs, try this:

   class Dog { /** * @param partner the dog who is breeding with this. * @return a new puppy dog, whose age is 0 and whose name is a blend * of this's and partner's names. * For example: * (new Dog("rex",4)).breed( new Dog("fifi", 6) ) = new Dog( "refi", 0 ); */ public Dog breed( Dog partner ) { // ... WRITE THIS METHOD ... } }

   (You can use a hyphenated compound name, instead of a blended name, if you prefer.)

Fri.'s solution (run testDog)

ITEC120-ibarland (incl. office hrs)—info—lectures—labs—hws

©2006, Ian Barland, Radford University Last modified 2006.Nov.10 (Fri)

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