CS 2102 (B11): Object-Oriented Design Concepts
Linked List and For-Loops


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. 

   LinkedList L = 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.)

Iterating Over Java Lists

Java does not have an operator corresponding to rest in Racket. This changes how we write programs over lists: in particular, we cannot simply call a function recursively on the rest of a list. Instead, Java provides a for construct that iterates over lists. These are easiest explained by example.

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).

  1. Write a method numCheap that consumes a LinkedList and produces an integer indicating how many songs cost less than two dollars.

  2. 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.

  3. Repeat the previous problem, this time changing the price in the song classes in the original list.

  4. 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.