Thread: I'm stuck on this question in Java and we have to make a telephone directory?

What the question states is:


Having supplied an interface the question arises as to how it should be implemented. There are a
number of possibilities including the use of an array list or a linked list; however it is perfectly possible, and in fact instructive, to begin by implementing PhoneDirectory using a simple array of DirectoryEntry objects. This approach is illustrated below:

public class ArrayPhoneDirectory implements PhoneDirectory 
{ 
 private static final int INIT_CAPACITY = 100; 
 private int capacity = INIT_CAPACITY; 
 
 // holds telno of directory entries 
 
 private int size = 0; 
 
 // Array to contain directory entries 
 
 private DirectoryEntry[] theDirectory = 
 new DirectoryEntry[capacity]; 
 
 // Holds name of data file 
 
 private String sourceName = null; 
 
 // Flag to indicate whether directory was modified since 
 it was last loaded or saved 
 
 private boolean modified = false; 
 
 // add method stubs as specified in interface to compile 
 
 ............ 
 
 // add private methods 
 
 ........... 
}

Develop a tester class called ArrayPhoneDirectoryTester . As usual all methods in the class being tested should be executed at least once by the test driver. Supply standard test documentation – Test Plan, Test Data and Test Log.

In addition to the methods described in the interface the class ArrayPhoneDirectory contains three private methods listed below:

// Searches the array of directory entries for a specific name 
private int find(String name){} 
 
// Adds a new entry with the given name and telno to the array of 
// directory entries 
private void add(String name, String telno){} 
 
// Creates a new array of directory entries with twice the capacity 
// of the previous one 
private void reallocate(){}

These methods can be thought of as “helper” methods in that they assist the work of the public methods of the class. For example, the find() method is used by the lookUpEntry() method to locate the position of the a specific directory entry.

Pseudo-code algorithms for the methods of ArrayPhoneDirectory are listed below

Algorithm for load

create a Scanner to read file
while (not end of file)
use Scanner to read the name
use Scanner to read the telno
create a DirectoryEntry
add the new entry to theDirectory
close file

Hint. This method is a good place to start coding but note that load makes use of the add method
in case the number of entries stored in the text file has exceeded the initial capacity of the directory.
So to implement load fully you will need a working version of the ”helper” method add. However
you can do this in stages by first developing a basic version of add which does not increase the size
of the directory. In reading data from the file you may assume that the file only contains matched
pairs of names and numbers. There is no need to consider the case of a name not being associated
with a telephone number.

Algorithm for addChangeEntry
call find to see whether name is in directory
if name is in theDirectory
change the telno using setNumber
return the old telno
else
add a new entry to theDirectory using method add
return null

Algorithm for lookUpEntry
use find to locate position of entry
if entry is found
use getNumber to return the telno
else
return null

Algorithm for Method save
create a PrintWriter object
for each entry in theDirectory
call getName to get the name from the entry
write the name on a line
call getNumber to get the telno from the entry
write the telno on a line
close file

Pseudo code for the two private helper methods is listed below.

Algorithm for add
if size >= capacity
reallocate()
set theDirectory[size] to new DirectoryEntry(name, telno)
increment size

Algorithm for reallocate
set capacity to 2 * capacity
set newDirectory to new DirectoryEntry[capacity]
copy contents of theDirectory to newDirectory
set theDirectory to newDirectory

I don't know how to do any of this at all

Originally Posted by Norm

You should talk to your instructor to get some help understanding what is being asked.

I've asked other people and I have a vague idea on what to do and I've emailed my lecturer (but because it's just after Easter and we haven't started back yet) he hasn't got back to me

In cases like this, usually you just need to start coding. The details of the assignment will become clear progressively.

Important: Do not write a big bunch of code in one go. You will run into lots of problems. Always write small chunks, test, and once you have confirmed that it works properly, write the next small chunk. Testing in this case can be as simple as writing some println() statements so that you can visually check that the expected values are obtained when you run the program.

It is hinted that the load method "is a good place to start coding," and I agree because it's pretty straightfoward:

Algorithm for load

create a Scanner to read file
while (not end of file)
use Scanner to read the name
use Scanner to read the telno
...

You should have learned how to use Scanner by now, but if you're still unsure, go through Scanning (The Java™ Tutorials > Essential Classes > Basic I/O), and refer to Scanner (Java Platform SE 7 ). Just concentrate on reading the file, and extracting name and telno. As mentioned in my note above, at this point test your code by printing out name and telno to confirm at least this bit of code is working before tackling the next bit.

...
create a DirectoryEntry
add the new entry to theDirectory
close file

DirectoryEntry is a custom class. If you're unsure how to create one, see Classes (The Java™ Tutorials > Learning the Java Language > Classes and Objects). The pseudocode above states that you'll need to call the add method after creating a DirectoryEntry. As a start, you can write a simple add method that simply prints out name and telno that are passed to it to confirm that add is indeed called with the correct name and telno. Test it again.

Once you have confirmed that the above are working properly, you're done with the load method, and you're ready to proceed to the add method.