For example, suppose, we want an ArrayList of Strings. We describe the type as:
ArrayList<String>
When we construct an object like a Point, we say:
Point p = new Point();
Imagine replacing both occurrences of "Point" with "ArrayList<String>" and
you'll see how to construct an ArrayList<String>:
ArrayList<String> list = new ArrayList<String>();
Java has a special notation known as the diamond operator that allows you to
simplify the line above to be:
ArrayList<String> list = new ArrayList<>();
You need to include "String" on the left side of the assignment statement, but
not on the right side. Java is able to infer the proper type to use based on
the type of the variable, which allows us to leave off "String" and just say
"<>" (which is known as the diamond operator).Once you have declared an ArrayList<String>, you can call the add method to append values to the end of the list:
ArrayList<String> list = new ArrayList<>();
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);
which produces this output:
list = [four, score, seven, years, what was next?, ago]
Obviously this isn't the correct quote. We could fix the individual calls on
the appending add method, but it was a more interesting exercise to explore how
to fix the list after it is constructed.The first problem is that there is a missing word. The word "and" should appear between "score" and "seven". We want to include it at index 2 in the structure, so we used this line of code to do so:
list.add(2, "and");
The other problem is that it includes the string "what was next?". This was
originally at index 4 of the list, but now that we have inserted a new value,
it has been shifted over so that it appears at index 5. So we added this line
of code:
list.remove(5);
With those two lines of code added after constructing the list, the output is
now correct:
list = [four, score, and, seven, years, ago]
I showed a list of useful methods for the ArrayList class that will be included
in the section cheat sheet when we discuss this and that will be included with
the final exam as part of its cheat sheet.
List<E> Methods (10.1)
----------------------
add(value) appends value at end of list
add(index, value) inserts given value at given index,
shifting subsequent values right
clear() removes all elements of the list
indexOf(value) returns first index where given value is
found in list (-1 if not found)
get(index) returns the value at given index
remove(index) removes/returns value at given index if
it is present, shifting subsequent
values left
set(index, value) replaces value at given index with given
value
size() returns the number of elements in list
addAll(list) adds all elements from the given
collection to the end of the list
contains(value) returns true if the given value is found
somewhere in this list
remove(value) finds and removes the given value from
this list
removeAll(list) removes any elements found in the given
collection from this list
iterator() returns an object used to examine the
contents of the list
Then I said that I wanted to explore all of the ways to traverse the list,
printing the values one per line. The most basic is to use the standard
traversal loop for a list that uses calls on the size and get methods:
for (int i = 0; i < list.size(); i++) {
System.out.println(list.get(i));
}
I mentioned that another way to do this is with a foreach loop. Chapter 7
describes how to use the foreach loop for arrays and chapter 10 describes how
to use it for an ArrayList. It is the best structure to use when you are
performing a "read only" operation on a list that doesn't require keeping track
of the index.The foreach loop syntax is fairly simple:
for (String s : list) {
System.out.println(s);
}
We read this loop as, "for each string s that is in list..." The choice of
"s" is arbitrary. It defines a local variable for the loop. I could just as
easily have called it "x" or "foo" or "value". Each time through the loop Java
sets the variable s to the next value from the list. You don't have to test
for the size of the list or to use a call on the get method. Java does that
for you when you use a for-each loop.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 the variable s, you are just changing a local variable inside the loop. It has no effect on the list itself.
Then we explored a different approach to traversing a list using what is known as an iterator. In Java there are three fundamental operations that an iterator performs:
Iterator<String> itr = list.iterator();
while (itr.hasNext()) {
String s = itr.next();
System.out.println(s);
}
Then I then spent a little time discussing the issue of primitive data versus
objects. Even though we can construct an ArrayList<E> for any class E, we
can't construct an ArrayList<int> because int is a primitive type, not a class.
To get around this problem, Java has a set of classes that are known as
"wrapper" classes that "wrap up" primitive values like ints to make them an
object. It's very much like taking a candy and putting a wrapper around it.
For the case of ints, there is a class known as Integer that can be used to
store an individual int. Each Integer object has a single data field: the int
that it wrapped up inside.Java also has quite a bit of support that makes a lot of this invisible to programmers. If you want to put int values into an ArrayList, you have to remember to use the type ArrayList<Integer> rather than ArrayList<int>, but otherwise Java does a lot of things for you. For example, you can construct such a list and add simple int values to it:
ArrayList<Integer> numbers = new ArrayList<>();
numbers.add(18);
numbers.add(34);
In the two calls on add, we are passing simple ints as arguments to something
that really requires an Integer. This is okay because Java will automatically
"box" the ints for us (i.e., wrap them up in Integer objects). We can also
refer to elements of this list and treat them as simple ints, as in:
int product = numbers.get(0) * numbers.get(1);
The calls on list.get return references to Integer objects and normally you
wouldn't be allowed to multiply two objects together. In this case Java
automatically "unboxes" the values for you, unwrapping the Integer objects and
giving you the ints that are contained inside.Every primitive type has a corresponding wrapper class: Integer for int, Double for double, Character for char, Boolean for boolean, and so on.
Then I used an array of data to initialize a list and to print it out:
int[] data = {18, 4, 97, 3, 4, 18, 72, 4, 42, 42, -3};
ArrayList<Integer> numbers = new ArrayList<>();
for (int n : data) {
numbers.add(n);
}
System.out.println("numbers = " + numbers);
This produced the following output:
numbers = [18, 4, 97, 3, 4, 18, 72, 4, 42, 42, -3]
Then I switched my attention to the ArrayIntList class that I use as a way to
talk about how to implement ArrayList<E>. In the 143 class, we spend a lot
more time developing the ArrayIntList class and it is the subject of chapter 15
of the textbook. We don't have time to do the full version, but I pointed out
some important things to notice about the class.Each ArrayIntList 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 ArrayIntList 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;
}
The string passed to the IllegalArgumentException constructor is optional. If
you have useful information to provide, however, it is helpful to the client
to pass that along. In this case, we show the client the illegal capacity that
was passed to the constructor.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:
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);
}
}
The program produced the following output:
[18, 27, 93]
[18, 93]
[18, 27, 93]
[18, 93]
As expected, the two kinds of list behave the same way. I pointed out that in
CSE142 we tried to emphasize the idea that you shouldn't have redundant code
like this. So ideally we'd like to move this code into a method. As a first
attempt, I said:
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);
}
}
This is obviously a better way to write this program, but it didn't compile.
jGRASP highlighted the second call on processList and said:
File: /Users/reges/143X/ListClient.java [line: 4]
Error: processList(ArrayIntList) in ListClient cannot be applied to (LinkedIntList)
The error indicates that the method processList takes an ArrayIntList as an
argument and that it cannot be applied to a call that passes a LinkedIntList as
a parameter. So I tried changing the parameter type to LinkedIntList and then
it produced an error for the other call.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)
}
To turn this into an interface, I had to change the name to something new. I
decided to call it IntList. I also had to change the word "class" to
"interface". The method have no curly braces because I deleted those lines of
code. I replaced each of those with a semicolon. That left me with:
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);
}
This is how you define an interface. In place of the method bodies, we have a
semicolon to indicate, "The implementation isn't given." You can think of an
interface as being like a hollow radio. It has all of the knobs and buttons
that are used to control the radio, but it has none of the "innards" that make
it work.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) {
Unfortunately, this led to two errors. Both of the calls now failed with
messages like this:
File: /Users/reges/143X/ListClient.java [line: 4]
Error: processList(IntList) in ListClient cannot be applied to (ArrayIntList)
That seems a bit odd, because both ArrayIntList and LinkedIntList have the
methods mentioned in the IntList interface. The explanation is that Java
requires classes to explicitly state what interfaces they implement. So we had
to modify the two classes to include this notation:
public class ArrayIntList implements IntList {
...
}
public class LinkedIntList implements IntList {
...
}
With this change, the code compiled and executed properly.I then tried creating an instance of the IntList class:
IntList list = new IntList();
This produced an error. Interfaces cannot be instantiated because they are
incomplete.I then asked people to think about the types of these objects. Consider our variable list1 from main:
ArrayIntList list1 = new ArrayIntList();
This describes an object of type ArrayIntList, but it also describes an object
of type IntList. That's why the method call works. The object is of more than
one type. This idea is important in object oriented programming because Java
objects can typically fill many roles. This is related to the notion of type
because each role is defined by a new type. Interfaces are a way to define a
new role, a new type. By saying that ArrayIntList and LinkedIntList both
implement the IntList interface, we say that they both fill that role. So
given an ArrayIntList, we can say that it is of type ArrayIntList, but we can
also say that it is of type IntList. Similarly, a LinkedIntList is of type
LinkedIntList and of type IntList.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);
}
These variables are more flexible than the old variables. The variable list1,
for example, can now refer to any IntList object, not just one of type
ArrayIntList. In fact, list1 can even store a reference to other kinds of
objects, as long as they implement the IntList interface.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 in the coming weeks. His item 64 is, "Refer to objects by their interfaces," which begins by saying:
You should favor the use of interfaces over classes to refer to objects. If appropriate interface types exist, then parameters, return values, variables and fields should all be declared using interface types. The only time you really need to refer to an object's class is when you're creating it with a constructor.The middle sentence was in bold face in the book, indicating how important Bloch thinks this is, and I've reproduced that here.
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
The List, Set and Map abstractions from the collections framework are all
ADTs.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 nncertified doctor and a certified lawyer). When there are more than one, you list them separated by commas after the word "implements".