Implementing a Collection Class - Building Java Programs: A Back to Basics Approach - page 915

itself. You can use the this keyword to say, “Construct an iterator that is iterating
over me”:
public ArrayIntListIterator iterator() {
return new ArrayIntListIterator(this);
}
You can find a complete listing of this fourth version of the ArrayIntList class
along with the ArrayIntListIterator class on the web page for this textbook at
http://buildingjavaprograms.com.
15.4 ArrayList<E>
In this section, we will explore how to convert the version of ArrayIntList from
the previous section into a generic ArrayList<E> . To start, you can simply replace
all occurrences of ArrayIntList with ArrayList<E> and change references to int
that refer to values to E . Of course, there are other uses of int that specify capacity,
size, and indexes, and these don't change.
This approach almost works, but there are a few places where you have to be care-
ful. For example, when you define constructors, you don't use the generic E when
you're naming the constructor. So the zero-argument constructor becomes:
public ArrayList() {
this(DEFAULT_CAPACITY);
}
You also run into trouble in the second constructor. After you perform the simple
substitution, you end up with the following method:
public ArrayList(int capacity) {
if (capacity < 0) {
throw new IllegalArgumentException("capacity: " + capacity);
}
elementData = new E[capacity]; // illegal
size = 0;
}
When you try to compile this version, you get an error indicating that you are not
allowed to construct a generic array. This is a limitation of generic types. You can
construct an array of type Object[] , but not an array of type E[] . You can solve this
problem by introducing a cast. Replace the following line of code:
elementData = new E[capacity];