handout #6
CSE143—Computer Programming II
Programming Assignment #2
due: Thursday,
In this programming assignment we will continue
to practice using arrays and classes.
You are to implement a class called LetterInventory that can be used to keep
track of an inventory of letters of the alphabet. The constructor for the class takes a String
and computes how many of each letter are in the
String. This is the information the
object keeps track of (how many a’s, how many b’s, etc). It ignores the case of the letters and
ignores anything that is not an alphabetic character (e.g., it ignores
punctuation characters, digits and anything else that is not a letter).
Your
class should have the following public methods.
Method |
Description |
LetterInventory(String
data) |
Constructs
an inventory (a count) of the alphabetic letters in the given string,
ignoring the case of letters and ignoring any non-alphabetic characters. |
int
get(char letter) |
Returns
a count of how many of this letter are in the inventory. Letter might be lowercase or uppercase
(your method shouldn’t care). If a
nonalphabetic character is passed, your method should throw an IllegalArgumentException. |
void
set(char letter, int value) |
Sets
the count for the given letter to the given value. Letter might be lowercase or uppercase. If a nonalphabetic character is passed or
if value is negative, your method should throw an IllegalArgumentException |
int
size() |
Returns
the sum of all of the counts in this inventory. This operation should be “fast” in that it
should store the size rather than having to compute it each time this method
is called. |
boolean
isEmpty() |
Returns
true if this inventory is empty (all counts are 0). This operation should be fast in that it
should not need to examine each of the 26 counts when it is called. |
String
toString() |
Returns
a String representation of the inventory with the letters all in lowercase
and in sorted order and surrounded by square brackets. The number of occurrences of each letter
should match its count in the inventory.
For example, an inventory of 4 a’s, 1 b, 1 l and 1 m would be
represented as “[aaaablm]”. |
LetterInventory
add(LetterInventory other) |
Constructs
and returns a new LetterInventory object that represents the sum of this
letter inventory and the other given LetterInventory. The counts for each letter should be added
together. The two LetterInventory
objects being added together (this and other) should not be changed by this
method |
LetterInventory
subtract(LetterInventory other) |
Constructs
and returns a new LetterInventory object that represents the result of
subtracting the other inventory from this inventory (i.e., subtracting the
counts in the other inventory from this object’s counts). If any resulting count would be negative,
your method should return null. The
two LetterInventory objects being subtracted (this and other) should not be
changed by this method |
Below is an example of how the add method would
be called.
LetterInventory inventory1 = new LetterInventory("George W. Bush");
LetterInventory inventory2 = new
LetterInventory("Hillary Clinton");
LetterInventory sum = inventory1.add(inventory2);
The first inventory would correspond to [beegghorsuw],
the second would correspond to [achiilllnnorty] and the third would correspond
to [abceegghhiilllnnoorrstuwy].
You should implement this class with an array of
26 counters (one for each letter) along with any other data fields
you find that you need. Remember,
though, that we want to minimize the number of data fields when possible. You might be tempted to implement the add
method by calling the toString method but you are not allowed to use that
approach because it would be inefficient for inventories with large character
counts.
You will need to know certain things about the
properties of letters and type char.
There is a section about type char in chapter 4 of the textbook. One of the most important ideas is that the
values of type char have corresponding integer values. There is a character with value 0, a
character with value 1, a character with value 2 and so on. You can compare different values of type char
using less-than and greater-than tests, as in:
if (ch
>= 'a') {
...
}
All of the lowercase letters appear grouped
together in type char ('a' is followed by 'b' followed by 'c', and so on) and
all of the uppercase letters appear grouped together in type char ('A' followed
by 'B' followed by 'C' and so on).
Because of this, you can compute a letter’s displacement (or distance)
from the letter 'a' with an expression like the following (this expression
assumes the variable letter is of type char and stores a lowercase letter):
letter -
'a'
Going in the other direction, if you know a
character’s integer equivalent, you can cast the result to char to get the
character. For example, suppose that you
want to get the letter that is 8 away from 'a'.
You could say:
char
result = (char) ('a' + 8);
This assigns the variable result the value 'i'.
As in these examples, you should write your code
in terms of displacement from a fixed letter like 'a' rather than including the
specific integer value of a character like 'a'.
You probably want to look at the String and Character
classes for useful methods (e.g., there is a toLowerCase method in each). You will have to pay attention to whether a
method is static or not. The String
methods are mostly instance methods because Strings are objects. The Character methods are all static because
char is a primitive type. For example,
assuming you have a variable called s that is a String, you can turn it to
lowercase by saying:
s = s.toLowerCase();
This is a call on an instance method where you
put the name of the object first. But
char values are not objects and the toLowerCase method in the Character class
is a static method. So assuming you have
a variable called ch that is of type char, you'd turn it to lowercase by
saying:
ch =
Character.toLowerCase(ch);
You can read about String operations on pages
137—143 of the textbook.
In terms of correctness, your class must provide
all of the functionality described above and must satisfy all of the
constraints mentioned in this writeup.
In terms of style, we will be grading on your use of comments, good
variable names, consistent indentation, minimal data fields and good coding
style to implement these operations.
You should name your file LetterInventory.java
and you should turn it in electronically from the “assignments” link on the
class web page.