CS 5 Spring 2000
Lecture 34
May 26

A linked list application

A simple application that uses linked lists is given in the program ScoresDriver.java and the class Scores.java. Here we have a class Scores, consisting of an array theScores, where each entry is a reference to a List object. The idea is that the array entry at index i is a linked list of the names of all students who got a grade of i. The array indices run from 0 to 100.

This example uses a slightly modified version of the circular, doubly linked list with a sentinel. You can find the class in List.java. There are only minor differences between these linked lists and those that we previously saw. The first two matter for this program, and the last two will be used in the next program we look at.

Element::print() is overloaded - one version takes a PrintWriter object and writes to a file.
List::print() is overloaded. The version that does not take an PrintWriter as a parameter prints to System.out. The other version takes a PrintWriter object as a parameter.
firstOf() makes the current element be the first one on the list (sentinel.next).
getCurrentData() returns a copy of the string in the current element.

Ideally the versions that don't take a PrintWriter object would just pass System.out to the versions that do. Unfortunately System.out is of type PrintStream, rather than PrintWriter, so this will not work unless we learn a new way of opening a file for writing.

Observe how the member functions for Scores in Scores.java make simple calls to the linked list member functions for the List objects in the appropriate array slots.

Try running this program. You can load scores in by using the command "l" and loading scores from the file Student Scores. To see that error handling works, load scores from the file Student Scores bad

Abstract Data Types

An abstract data type (ADT) is a data structure that is defined solely by its operations. Users of an ADT need not and should not know how the ADT is implemented. All that a user needs to know is what the operations do.

Stacks

A stack is defined by the following operations:

push: put an item onto the stack.
pop: remove from the stack the item most recently pushed onto it that has not yet been removed.
isEmpty: determine whether or not the stack has any items on it.
print: print the contents of the stack.

Not surprisingly, the runtime stacks that you drew several weeks back are examples of a stack. Calling a method pushes a new stack frame, and returning from a method call pops the top stack frame.

Push, pop, and isEmpty are the "traditional" stack operations. With the push and pop operations, stacks are "last-in, first-out", or "LIFO". The print method, though nontraditional, is helpful to have in a demonstration program.

We can easily use a linked list as a stack. The class Stack is in Stack.java. The only private data is a List object. The functions push() and pop() are both extraordinarily simple, as are the functions isEmpty() and print().

You can test the stack code with the driver program StackTest.java.

Queues

A queue is like a line at the bank or at the registrar. Service is "first-in, first-out", or "FIFO". Items enter the queue at one end (the tail) and leave at the other end (the head). Compare this to a stack, where items enter at one end and leave at the same end.

A queue is defined by the following operations:

enqueue: add an item to the tail of the queue.
remove: remove the item at the head of the queue.
isEmpty: determine whether or not the queue has any items on it.
print: print the contents of the queue.

We can easily use a linked list as a queue, as the class Queue does in Queue.java. You can test this code with QueueTest.java.

Dequeues

A dequeue (pronounced "deck") is a double ended queue. In other words, you can add or remove from either end of a dequeue. The operations are

enqueueAtHead: add an item to the head of the queue.
removeFromHead: remove the item at the head of the queue.
enqueueAtTail: add an item to the tail of the queue.
removeFromTail: remove the item at the tail of the queue.
isEmpty: determine whether or not the queue has any items on it.
print: print the contents of the queue.

A linked list makes a fine dequeue, as the class DEQueue does in Dequeue.java. You can test this code with DequeueTest.java.

Scot Drysdale <scot@cs.dartmouth.edu>