I asked people to consider the situation where a client wants to replace a value at a particular location. The only option we have given the client is to remove the value and then to add a new value back in its place. This requires shifting values twice, which can be very inefficient if the list is long and if the change occurs towards the front of the list. So I said that we would include a method called set that can be used to replace the value at a given inde:
public void set(int index, int value) {
elementData[index] = value;
}
Of course, we have to indicate the precondition on the index and we have to
check the index to make sure it is legal. We introduced a private method
called checkIndex that performs the check for us:
// pre : 0 <= index < size()
// post: replaces the integer at the given index with the given value
public void set(int index, int value) {
checkIndex(index);
elementData[index] = value;
}
I reminded people that in terms of grading, we require that all preconditions
be commented along with the fact that an exception is thrown. We require that
you mention exactly what type of exception is thrown, as in the comment
above.We also discussed a "bulk add" method called addAll. It may seem a little odd to have one ArrayIntList dealing with another ArrayIntList, but this actually happens fairly often and you will have to practice it in your next homework assignment. The idea is that "this" ArrayIntList is supposed to add all of the values from the "other" ArrayIntList. So its header looks like this:
public void addAll(ArrayIntList other) {
...
}
We can call the appending add method to add values to the list. We just have
to figure out how to write a loop that indicates what values to add. I first
wrote the method this way:
public void addAll(ArrayIntList other) {
for (int i = 0; i < other.size(); i++)
add(other.get(i));
}
This approach works. Here were are using the size and get methods to access
the values in the other list. But it also works to directly refer to the
private fields themselves, as in:
public void addAll(ArrayIntList other) {
for (int i = 0; i < other.size; i++)
add(other.elementData[i]);
}
It may seem odd that one ArrayIntList object can access the private fields of
another, but that's how it works in Java. Private means private to the class.
That doesn't match our own human intuitions about privacy. Often it can be
more efficient to directly access private fields and methods and sometimes a
problem can't be solved at all without such access.I mentioned that I also included bulk removeAll method along with a method called clear that empties the list.
Then I said I wanted to discuss a concept that we will be exploring a lot as we look at other collection classes. First I wrote some code to construct an ArrayIntList and to fill it up with values:
public class ArrayIntListExample {
public static void main(String[] args) {
ArrayIntList list = new ArrayIntList();
list.add(12);
list.add(3);
list.add(3);
list.add(72);
list.add(42);
list.add(3);
list.add(-19);
System.out.println("list = " + list);
}
}
When we ran it, it produced the following output:
list = [12, 3, 3, 72, 42, 3, -19]
I then asked how we could find the sum of the values in the list. Someone said
we should use a for loop and that we should use the size method of ArrayIntList
to figure out how many times to loop and that we should use the get method of
ArrayIntList to get the individual values:
int sum = 0;
for (int i = 0; i < list.size(); i++) {
sum += list.get(i);
}
System.out.println("sum = " + sum);
When we ran this new version of the program, it produced the following
output:
list = [12, 3, 3, 72, 42, 3, -19]
sum = 116
This is often a reasonable way to manipulate a list, but I said that I wanted
to explore a different approach using what is known as an iterator. The
code we've written to sum the list works, but it relies on the "get" method
being able to quickly access any element of the structure. This property is
known as random access. We say that arrays and the ArrayList and
ArrayIntList classes that are built on top of them are random access structures
because we can quickly access any element of the structure. If you knew that
for the rest of your life, you'd always be working with arrays, then you'd have
little use for iterators. You'd just call the get method because with arrays
you get fast random access.But many of the other data structures we will be looking at don't have this kind of quick access. Think of how a DVD works, quickly jumping to any scene, or how a CD works, quickly jumping to any track, versus how a VHS tape works, requiring you to fast forward through every scene until you get to the one you want. For those other structures we will study, iterators will make a lot more sense. So iterators will seem a bit silly when tied to an array-based structure, but we'll eventually see much more interesting examples of iterators.
In general, we think of an iterator as having three basic operations:
ArrayIntListIterator i = list.iterator();
int product = 1;
while (i.hasNext()) {
int next = i.next();
product *= next;
}
System.out.println("product = " + product);
This involves a new kind of object of type ArrayIntListIterator. Once we have
our iterator, we can use it to go through the values in the list one at a
time.To make this even more clear, we added a println inside the loop:
ArrayIntListIterator i = list.iterator();
int product = 1;
while (i.hasNext()) {
int next = i.next();
System.out.println("product = " + product + ", next = " + next);
product *= next;
}
System.out.println("product = " + product);
This loop produced the following output:
product = 1, next = 12
product = 12, next = 3
product = 36, next = 3
product = 108, next = 72
product = 7776, next = 42
product = 326592, next = 3
product = 979776, next = -19
product = -18615744
I also briefly mentioned that iterators often support a method called remove
that allows you to remove the value that you most recently get from a call on
next. For example, this variation of the code prints each value and removes
any occurrences of the value 3:
ArrayIntListIterator i = list.iterator();
int product = 1;
while (i.hasNext()) {
int next = i.next();
System.out.println("product = " + product + ", next = " + next);
product *= next;
if (next == 3)
i.remove();
}
System.out.println("product = " + product);
This code examines each value in the list and removes all the occurrences of
3.Then we spent some time discussing how the ArrayIntListIterator is implemented. The main function the iterator performs is to keep track of a particular position in a list, so the primary field will be an integer variable for storing this position:
public class ArrayIntListIterator {
private int position;
public ArrayIntListIterator(?) {
position = 0;
}
public int next() {
position++;
}
...
}
I asked people how we would implement hasNext and someone said we'd have to
compare the position against the size of the list. I then said, "What list?"
Obviously the iterator also needs to keep track of which list it is iterating
over. We can provide this information in the constructor for the iterator. So
the basic outline became:
public class ArrayIntListIterator {
private ArrayIntList list;
private int position;
public ArrayIntListIterator(ArrayIntList list) {
position = 0;
this.list = list;
}
public int next() {
use get method of list & position
position++;
}
public boolean hasNext() {
check position against size
}
...
}
We briefly discussed how to implement remove. We have to keep track of when
it's legal to remove a value. Recall that you can't remove before you have
called get and you can't call remove twice in a row. We decided that this
could be implemented with a boolean flag inside the iterator that would keep
track of whether or not it is legal to remove at any given point in time.
Using this flag, we can throw an exception in remove if it is not legal to
remove at that point in time:
public class ArrayIntListIterator {
private ArrayIntList list;
private int position;
private boolean removeOK;
public ArrayIntListIterator(ArrayIntList list) {
position = 0;
this.list = list;
removeOK = false;
}
public int next() {
use get method of list & position
position++
removeOK = true;
}
public boolean hasNext() {
check position against size
}
public void remove() {
if (!removeOK)
throw new IllegalStateException()
call remove method on list
removeOK = false;
}
}
This is a fairly complete sketch of the ArrayIntListIterator code.Then I discussed the fact that the new version of the list "grows" the list as needed if it runs out of capacity. It isn't, in general, easy to make an array bigger. We can't simply grab the memory next to it because that memory is probably being used by some other object. Instead, we have to allocate a brand new array and copy values from the old array to the new array. This is pretty close to how shops and other businesses work in the real world. If you need some extra space for your store, you can't generally break down the wall and grab some of the space from the store next door. More often a store has to relocate to a larger space.
The new version of ArrayIntList has this functionality built into it. In the previous version we had a checkCapacity method that throws an exception if the array isn't big enough. In the new version that has been replaced by an ensureCapacity method that constructs a new array if necessary.
Obviously you don't want to construct a new array too often. For example, suppose you had space for 1000 values and found you needed space for one more. You could allocate a new array of length 1001 and copy the 1000 values over. Then if you find you need space for one more, you could make an array that is 1002 in length and copy the 1001 old values over. This kind of growth policy would be very expensive.
Instead, we do something like doubling the size of the array when we run out of space. So if we have filled up an array of length 1000, we double its size to 2000 when the client adds something more. That makes that particular add expensive in that it has to copy 1000 values from the old array to the new array. But it means we won't need to copy again for a while. How long? We can add another 999 times before we'd need extra space. As a result, we think of the expense as being spread out or "amortized" over all 1000 adds. Spread out over 1000 adds, the cost is fairly low (a constant).
You will find that the built-in ArrayList class does something similar. The documentation is a little coy about this saying, "The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost." If you look at the actual code, you'll find that increase by 50% each time (a multiplier of 1.5).
The latest version of the ArrayIntList class along with the ArrayIntListIterator class are included in the calendar for this lecture.