Lists are built into Java. There are several variations of them. We will use the LinkedList class in this course. These are a generic type of lists, so to create a variable containing a list, you need to supply the type of the data in the list. For example, the following statements defines an empty linked list of integers.
LinkedListL = new LinkedList ();
One key difference between Java lists and Racket lists is that adding elements to a list modifies the original list in Java. The analogous operator to cons in Java is called addFirst. The following expression adds 5 to the list L:
L.addFirst(5);However, in Java, L now contains the number 5. The original list L is no longer available. (Contrast this to Racket, in which (cons 5 L) would leave L unchanged.)
Here is the method to sum up a list of integers:
int sum (LinkedList<Integer> numList) { int result; result = 0; for ( Integer n : numList ) { result = result + n; } return result; }
To understand the structure of this code, it helps to step back and see the pieces of the construct. In the following code skeleton, substitute single expressions for terms in all-caps, and substitute computations for descriptions in square-brackets:
RETURN-TYPE result; // a local variable (field) in the class result = [the result when the list is empty] for ( TYPE VARNAME : SPECIFIC-LIST ) { result = [expression to build new result from current value of result and VARNAME]; } return result;For those who took CS1101, this is just the accumulator-style programs that you did in the last segment of that course.
Study this pattern and try it out on the following examples. The examples are for a list of Songs, where a song has an artist, title, and price (feel free to use an int for the price).
Write a method numCheap
that consumes a
LinkedList
and produces an integer indicating
how many songs cost less than two dollars.
Write a method discount
that consumes a
LinkedList
and produces
a LinkedList
in which each song is discounted
by 10%. Do this without modifying the individual songs.
Repeat the previous problem, this time changing the price in the song classes in the original list.
Write a method discography
that consumes
a LinkedList
and an artist (String) and produces
a LinkedList
with all songs by that artist.
Feel free to see us in office hours or ask on the board if you have questions on any of these.