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 index:
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 mentioned that the new version also has methods clear, which makes the list
empty, removeAll, and isEmpty.Then I said I wanted to discuss how to implement an iterator. Recall that an iterator as having three basic operations:
Iterator<Integer> i = list.iterator();
while (i.hasNext()) {
int n = i.next();
if (n % 3 == 0) {
i.remove();
}
}
System.out.println("product = " + product);
This code removes all multiples of 3 from the list.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 implements Iterator<Integer> {
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. The
calendar includes a complete version. You will notice some odd details that
will make more sense after we have learned more about the collections framework
(e.g., the class implements the Iterator<Integer> interface and the next method
returns a value of type Integer instead of int).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 manually checked the capacity and threw an exception if the array wasn'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.