Data Structures and Algorithms in Java[1].pdf - Fulvio Frisone

PositionList interfaces (defined for any generic
type E), and adds two more methods.
Implementing a Sequence with an Array
If we implement the sequence S ADT with a doubly linked list, we would get
similar performance to that of the java.util.LinkedList class. So suppose
instead we want to implement a sequence S by storing each element e of S in a
cell A[i] of an array A. We can define a position object p to hold an index i and a
reference to array A, as instance variables, in this case. We can then implement
method element(p) simply by returning A[i]. A major drawback with this
approach, however, is that the cells in A have no way to reference their
corresponding positions. Thus, after performing an add First operation, we
have no way of informing the existing positions in S that their indices each went
up by 1 (remember that positions in a sequence are always defined relative to their
neighboring positions, not their indices). Hence, if we are going to implement a
general sequence with an array, we need a different approach.
Consider an alternate solution, then, in which, instead of storing the elements of S
in array A, we store a new kind of position object in each cell of A, and we store
elements in positions. The new position object p holds the index i and the element
e associated with p.
With this data structure, illustrated in Figure 6.8, we can easily scan through the
array to update the index variable i for each position whose index changes
because of an insertion or deletion.
Figure 6.8: An array-based implementation of the
sequence ADT.
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