CSE143 Notes for Monday, 4/18/11

• a size method
• a get method
• a toString method
• an indexOf method
• a one-argument add method (the appending add)
• a two-argument add method (add at an index)
• and a remove method (remove at an index)

To underscore the similarity, I wrote the following client code that does parallel operations on two different lists, adding three values, removing one, and printing it before and after the remove:

        public class ListClient {
            public static void main(String[] args) {
                ArrayIntList list1 = new ArrayIntList();
                list1.add(18);
                list1.add(27);
		list1.add(93);
		System.out.println(list1);
		list1.remove(1);
                System.out.println(list1);

                LinkedIntList list2 = new LinkedIntList();
                list2.add(18);
                list2.add(27);
		list2.add(93);
		System.out.println(list2);
		list2.remove(1);
                System.out.println(list2);
            }
        }

        [18, 27, 93]
        [18, 93]
        [18, 27, 93]
        [18, 93]

        public class ListClient {
            public static void main(String[] args) {
                ArrayIntList list1 = new ArrayIntList();
                processList(list1);

                LinkedIntList list2 = new LinkedIntList();
                processList(list2);
            }

            public static void processList(ArrayIntList list) {
                list.add(18);
                list.add(27);
		list.add(93);
		System.out.println(list);
		list.remove(1);
                System.out.println(list);
            }
        }

        File: /Users/reges/143/ListClient.java  [line: 4]
        Error: processList(ArrayIntList) in ListClient cannot be applied to (LinkedIntList)

The point is that we want to be able to think of these lists as being the same thing. In computer science we try to use abstraction to find what is common between these two classes even though we recognize that there are things that are quite different about the two. We would imagine an "integer list" abstraction of which these are two possible implementations. They're both the same in the sense that they provide basic "integer list" functionality like an appending add. But they are different in the sense that they are implemented quite differently (one using an array and the other using a linked list).

With Java, we have actual language support for this concept. Not only can we talk abstractly about an "integer list" abstraction, we can actually define it using what is known in Java as an "interface".

In an interface, we want to specify that certain behaviors exist without saying how they are implemented. We want to specify the "what" part without specifying the "how" part. So I went to the ArrayIntList class and deleted all of its comments and all of the method bodies. That left me with this:

        public class ArrayIntList {
            public int size()
            public int get(int index)
            public String toString()
            public int indexOf(int value)
            public void add(int value)
            public void add(int index, int value)
            public void remove(int index)
        }

        public interface IntList {
            public int size();
            public int get(int index);
            public String toString();
            public int indexOf(int value);
            public void add(int value);
            public void add(int index, int value);
            public void remove(int index);
        }

So I went back to our code and changed the header for the processList method to use the interface instead:

        public static void processList(IntList list) {

        File: /Users/reges/143/ListClient.java  [line: 4]
        Error: processList(IntList) in ListClient cannot be applied to (ArrayIntList)

        public class ArrayIntList implements IntList {
             ...
        }
        
        public class LinkedIntList implements IntList {
             ...
        }

I then tried creating an instance of the IntList class:

        IntList list = new IntList();

I then asked people to think about the types of these objects. Consider our variable list1 from main:

        ArrayIntList list1 = new ArrayIntList();

It is a good idea to use interfaces when defining the types of variables. I changed our main method to use type intList for the variables instead of listing the individual types:

        public static void main(String[] args) {
            IntList list1 = new ArrayIntList();
            processList(list1);

            IntList list2 = new LinkedIntList();
            processList(list2);
        }

I then mentioned that this is an idea that has been used throughout the collections classes in Java (the java.util package). This idea is stressed by Joshua Bloch, the author of a book called Effective Java, which I said was one of the most useful books I've read about Java. Joshua Bloch was the primary architect of the collections framework and has influenced much of Sun's work. He now works at Google.

In the collections framework, Bloch was careful to define data structure abstractions with interfaces. For example, there are interfaces for List, Set and Map which are abstractions that we'll be discussing this quarter (we've already been examining the List concept). In addition to the interfaces, there are various implementations of each. For example, ArrayList and LinkedList both implement the List interface. TreeMap and HashMap both implement the Map interface. And TreeSet and HashSet both implement the Set interface.

Bloch's book is written as a series of 78 suggested practices for Java programmers. I have a link to the book and the suggested practices from our class web page (under "useful links"). His item 52 is to "Refer to objects by their interfaces." He says, "you should favor the use of interfaces rather than classes to refer to objects. If appropriate interface types exist, parameters, return values, variables and fields should all be declared using interface types." This last sentence was in bold face in the book, indicating how important Bloch thinks this is, and I've reproduced that here. He goes on to say that, "The only time you really need to refer to an object's class is when you're creating it."

We have been using this concept for many years in programming. It is traditionally referred to as an Abstract Data Type or ADT:

        A bstract
        D ata
        T ype

I also mentioned that the best analogy I have for interfaces is that they are similar to how we use the concept of certification. You can't claim to be a certified doctor unless you have been trained to do certain specific tasks. Similarly, to be a certified teacher you have to know how to behave like a teacher, to be a certified nurse you have to know how to behave like a nurse, and so on. In Java, if you want to claim to be a certified IntList object, then you have to have several different methods, including an appending add method . Java classes are allowed to implement as many interfaces as they want to, just as a single person might be certified to do several jobs (e.g., both a certified doctor and a certified lawyer). When there are more than one, you list them separated by commas after the word "implements".

In computer science, two of the most fundamental ADTs are called stacks and queues. They are so simple that they almost seem not worth studying. They are like the programming equivalent of drawers and shelves. Drawers and shelves are very simple and, therefore, sort of boring, and yet we find uses for them everywhere we turn.

It is useful to study stacks and queues as a way to understand a minimal kind of data structure. We'll find, for example, that they are less powerful than the list structures we have been looking at. But we often find ourselves wanting to think in terms of the simplest possible solution to a problem, as in, "You could solve that with a stack."

Like lists, stacks and queues store an ordered sequence of values. A minimal set of operations for such a structure would require at least:

• We need some way to put values into the structure (an adding operation)
• We need a way to take values out (a removing operation)
• We need a way to test whether there is anything left in the structure

Stacks and queues are similar in that they each store a sequence of values in a particular order. But stacks are what we call LIFO structures while queues are FIFO structures:

        stacks        queues

        L-ast         F-irst
        I-n           I-n
        F-irst        F-irst
        O-ut          O-ut

The analogy for queues is to think about standing in line at the grocery store. As new people arrive, they are told to go to the back of the line. When the store is ready to help another customer, the person at the front of the line is helped. In fact, the British use the word "queue" the way we use the word "line" telling people to "queue up" or to "go to the back of the queue".

In the case of a stack, the adding operation is called "push" and the removing operation is called "pop". All operations occur at one end of the stack, at the top. We push values onto the top and we pop them off the top. There is also a method for testing whether the stack is empty and an operation for requesting the current size of the stack. I showed people an interface with these four operations:

        public interface Stack<E> {
            public void push(E value);
            public E pop();
            public boolean isEmpty();
            public int size();
        }

For queues, we have a corresponding set of operations but they have different names. When values go into a queue we refer to it as "enqueueing" a value. When values are removed from a queue we refer to it as "dequeueing" a value. So the Queue interface looks like this:

        public interface Queue<E> {
            public void enqueue(E value);
            public E dequeue();
            public boolean isEmpty();
            public int size();
         }

        public class SimpleStackQueue {
            public static void main(String[] args) {
        	String[] data = {"four", "score", "and", "seven", "years", "ago"};
        	Queue<String> q = new LinkedQueue<String>();
        	Stack<String> s = new ArrayStack<String>();
        	
        	for (String str : data) {
        	    q.enqueue(str);
        	    s.push(str);
        	}
        	
                System.out.println("initial queue = " + q);
                while (!q.isEmpty()) {
                    System.out.println(q.dequeue());
                    System.out.println("now queue = " + q);
                }
                System.out.println();
        
                System.out.println("initial stack = " + s);
                System.out.println(s);
                while (!s.isEmpty()) {
                    System.out.println(s.pop());
                    System.out.println("now stack = " + s);
                }
            }
        }

        initial queue = front [four, score, and, seven, years, ago] back
        four
        now queue = front [score, and, seven, years, ago] back
        score
        now queue = front [and, seven, years, ago] back
        and
        now queue = front [seven, years, ago] back
        seven
        now queue = front [years, ago] back
        years
        now queue = front [ago] back
        ago
        now queue = front [] back
        
        initial stack = bottom [four, score, and, seven, years, ago] top
        bottom [four, score, and, seven, years, ago] top
        ago
        now stack = bottom [four, score, and, seven, years] top
        years
        now stack = bottom [four, score, and, seven] top
        seven
        now stack = bottom [four, score, and] top
        and
        now stack = bottom [four, score] top
        score
        now stack = bottom [four] top
        four
        now stack = bottom [] top