CS61B Lecture #7
Reminder:
•Discussion section 114 (3-4 Th) moves to 3102 Etch., starting to-
morrow.
Today:
•Java Library Classes for lists.
•Iterators, ListIterators
Last modified: Thu Feb 2 22:55:39 2006 CS61B: Lecture #7 1
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CS61B Lecture #7: Java Library Classes for Lists and Iterators, Slides of Data Structures and Algorithms
A part of the cs61b lecture notes discussing java library classes for lists and iterators. It covers the use of arraylist and linkedlist, the advantages and disadvantages of both, and the introduction of iterators for efficient item access. The document also explains how to read and interleave two lists using iterators.
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CS61B Lecture
Reminder:
- Discussion section 114 (3-4 Th) moves to 3102 Etch., starting to- morrow.
Today:
- Java Library Classes for lists.
- Iterators, ListIterators
Abstracting “Listness”
- So far, we’ve seen fairly primitive types for representing lists of things.
- Arrays:
- Good: random access to items.
- Bad: hard to expand, insert items, or delete items.
- Linked lists (e.g., IntList):
- Good: easy to expand, insert, delete.
- Bad: must access in sequence, pointer manipulation can be tricky.
- Both used to represent same thing (sequence of things), but syntax for using very different,
- So hard to switch from one to the other if you change your mind.
Lists
- The list classes ArrayList and LinkedList both share many public methods, including: - size(), isEmpty(): Number of items, test for 0 items. - get(k): Get item #k, where 0 ≤ k < size(). - remove(k): Remove item #k. - clear(): Make the list empty. - set(k, x): Set item #k to x. - add(x), add(k,x): Add item to end, or a position k. - contains (x): True iff there is an item that equals x (according to
.equals method).
- indexOf (x): Gives the position ( 0 ≤ · <size()) of the first item
that .equals x, or -1 if there is none.
- Both expand as needed (automatically).
- A few methods specialized to one or the other (e.g. LinkedList.removeFirst().
Example: Read and interleave two lists
/** Read the sequence of words on INPUT, and print on
- OUTPUT in reverse order. */ static void readAndReverse (Scanner input, PrintStream output) { ArrayList
L = new ArrayList (); while (input.hasNext ()) L.add (input.next ()); for (k = L.size ()-1; k >= 0; k -= 1) output.printf ("%s ", L.get (k)) }
- Not shown: import java.util.ArrayList; before class.
- Could also use a LinkedList
. What problem might there be with that?
ListIterator
- Library also has type ListIterator
- These have both .previous() and .next() methods.
- Also allow insertion.
- Look at reversal again:
/** Read the sequence of words on INPUT, and print on
- OUTPUT in reverse order. */ static void readAndReverse (Scanner input, PrintStream output) { ArrayList
L = new ArrayList (); ListIterator place = L.listIterator ();
while (input.hasNext ()) place.add (input.next ()); while (place.hasPrevious ()) System.out.printf ("%s ", place.previous ()); }
Primitive Types and Wrappers
- ArrayLists and the like can only take elements that are pointers, no
ints, doubles, booleans, etc.
- So, Java library contains corresponding wrapper classes: Integer,
Double, Boolean, etc.—all pointed-to objects
- So, new Integer(3) is 3. The intValue() method retrieves the 3.
- All very tedious, so Java 1.5 converts int ⇔ Integer automatically—
boxes 3 to make an Integer, unboxes to get 3 back.
- So we can do things like this:
ArrayList
- Almost painless, but, alas, expensive.