CSE143X Notes for Monday, 10/29/12

It stores a list of integers by keeping track of an array of ints along with a field that stores the size. Any given ArrayIntList, therefore, has a capacity (how many ints it can store) versus a current size (how many ints it currently stores). This is a little like how a hotel has many rooms and often has vacancies.

The LinkedIntList class mentions various preconditions that must be satisfied before a method is called. As we move into the material from 143, it is important to understand the idea of documenting for a client of a class. We think of this as the contract for the class. If a method is making some kind of assumption, then that needs to be documented as a precondition.

The ArrayIntList class goes further than just documenting the preconditions. It throws an exception if the precondition is violated, as in this code at the beginning of the constructor that takes a capacity:

        // pre : capacity >= 0 (throws IllegalArgumentException if not)
        // post: constructs an empty list with the given capacity
        public ArrayIntList(int capacity) {
            if (capacity < 0) {
                throw new IllegalArgumentException("capacity: " + capacity);
            }
            elementData = new int[capacity];
            size = 0;
        }

Going forward, we will expect you to comment your classes in a much more detailed manner. In particular, we will expect you to document all of the following:

• all preconditions (assumptions)
• all exceptions that are thrown and a description of when it is thrown and exactly what type of exception is thrown (a client needs to know the type in order to "catch" an exception)
• the meaning of all parameters passed to the method
• a description of all meaningful behavior that a client will want to know about, including the postconditions (what action is performed and/or what value is returned)

• 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/143X/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/143X/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".

Then I discussed the built-in ArrayList class. Remember that we're studying the LinkedIntList and ArrayIntList class as a way to understand the built-in LinkedList and ArrayList classes. We know that for arrays, it is possible to construct arrays that store different types of data:

• an int[] to store an array of int values
• a double[] to store an array of double values
• a String[] to store an array of references to Strings

For example, suppose, we want an ArrayList of Strings. We describe the type as:

        ArrayList<String>

        ArrayIntList list = new ArrayIntList();

        ArrayList<String> list = new ArrayList<String>();

        List<String> list = new ArrayList<String>();

        List<String> list = new ArrayList<String>();
        list.add("four");
        list.add("score");
        list.add("seven");
        list.add("years");
        list.add("what was next?");
        list.add("ago");
        System.out.println("list = " + list);
        System.out.println(list.indexOf("seven"));

        list = [four, score, seven, years, what was next?, ago]
        2

        list.add(2, "and");
        list.remove(5);

        for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i));
        }

        for (String s : list) {
            System.out.println(s);
        }

There are some limitations of for-each loops. You can't use them to change the contents of the list. If you assign a value to the variable s, you are just changing a local variable inside the loop. It has no effect on the list itself.

I then reminded people that you can also traverse a list using an iterator. For our ArrayIntList class, we had a special class called ArrayIntListIterator for the iterator. With a structure like an ArrayList, we use an interface. Not surprisingly, the interface is called Iterator<E>. So we can also traverse our list of strings this way:

        Iterator<String> i = list.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }