1. Java 2 Collections ©1999 Trygve Reenskaug
Collections in Java 2 (1.2)
Compare Java Arrays with Smalltalk Collections
Declaration
Add and remove elements
Sorting
Enumeration
Compare Java Arrays with Java2 Collections
Summary
List interface
Mapinterface
Conclusion
Java 2 Collections ©1999 Trygve Reenskaug

2. Java 2 Collections ©1999 Trygve Reenskaug
The problem….
public class Project {
Activity[] activities = new Activity[0];
public void addActivity(Activity act) {
Activity[] newArr = new Activity[activities.length + 1];
System.arraycopy (activities, 0, newArr, 0, activities.length);
newArr[newArr.length-1] = act;
activities = newArr; }
public void paintBarChart (Graphics g, Dimension extent ) {
Activity[] sortedActivities = new Activity[activities.length];
System.arraycopy(activities, 0, sortedActivities, 0, activities.length);
quicksortEarlyStart(sortedActivities, 0, sortedActivities.length-1);
for (int i=0; i<sortedActivities.length; i++) {
int start = sortedActivities[i].getEarlyStart();
int end = sortedActivities[i].getEarlyFinish(); }}
Java 2 Collections ©1999 Trygve Reenskaug

3. The Smalltalk solution Class definition
public class Project {
Activity[] activities = new Activity[0]; … }
Object subclass: # Project
instanceVariableNames: ’activities'
classVariableNames: ''
category: ’planning2c’
initialize
activities := OrderedCollection new.
Java 2 Collections ©1999 Trygve Reenskaug

4. The Smalltalk solution Adding (and removing) elements
public void addActivity(Activity act) {
Activity[] newArr = new Activity[activities.length + 1];
System.arraycopy (activities, 0, newArr, 0, activities.length);
newArr[newArr.length-1] = act;
activities = newArr; }
addActivity: act
activities addLast: act
Java 2 Collections ©1999 Trygve Reenskaug

5. The Smalltalk solution Sorting
public void paintBarChart (Graphics g, Dimension extent ) {
Activity[] sortedActivities = new Activity[activities.length];
System.arraycopy(activities, 0, sortedActivities, 0, activities.length);
quicksortEarlyStart(sortedActivities, 0, sortedActivities.length-1); … }
private static void quicksortEarlyStart(Activity[] arr, int lo0, int hi0) {
int lo, hi; Activity mid;
if (hi0 > lo0) {
/* 15 lines of code */ }}
paintBarChart: g extention: extent
sortedActivities := activities asSortedCollection:
[:x :y | x earlyStart < y earlyStart].
Java 2 Collections ©1999 Trygve Reenskaug

6. The Smalltalk solution Enumeration
for (int i=0; i<sortedActivities.length; i++) {
int start = sortedActivities[i].getEarlyStart();
int end = sortedActivities[i].getEarlyFinish();
... }
sortedActivities do:
[:act || start end |
start := act getEarlyStart.
end := act getEarlyFinish.
…].
Java 2 Collections ©1999 Trygve Reenskaug

7. Collection Interfaces Java 2
Variables typed on interface to be
independent of concrete implementations
Dictionary
(key->value)
Group of
Objects
Unordered,
no duplicates
Sorted
Dictionary
OrderedCollection
Vector
SortedCollection
Java 2 Collections ©1999 Trygve Reenskaug

8. Collection Classes Java 2
Useful library Collection classes
Hashtable
(superclass:
Dictionary)
AbstractCollection
HashSet
TreeMap
ArrayList
TreeSet
Java 2 Collections ©1999 Trygve Reenskaug

9. The Java 2 solution Class definition
public class Project {
Activity[] activities; … }
public class Project {
List activities = new ArrayList(6); … }
Java 2 Collections ©1999 Trygve Reenskaug

10. The Java 2 solution Adding (and removing) elements
public void addActivity(Activity act) {
Activity[] newArr = new Activity[activities.length + 1];
System.arraycopy (activities, 0, newArr, 0, activities.length);
newArr[newArr.length-1] = act;
activities = newArr; }
addActivity: act
activities addLast: act
Java 2 Collections ©1999 Trygve Reenskaug

11. The Java 2 solution Sorting
private static void quicksortEarlyStart(Activity[] arr, int lo0, int hi0) {
int lo, hi; Activity mid;
if (hi0 > lo0) { // lines of code}
public List activitiesByEarlyStart() {
List sortedList = new ArrayList(activities.size());
sortedList.addAll(activities);
Collections.sort (sortedList
, (new Comparator() {
public int compare(Object o1, Object o2) {
if (o1.equals(o2)) return 0;
Activity a1 = (Activity) o1;
Activity a2 = (Activity) o2;
return a1.compareEarlyStart(a2); } }) );
return sortedList; }}
Java 2 Collections ©1999 Trygve Reenskaug

12. The Java 2 solution Enumeration with for () {}
for (int i=0; i<sortedActivities.length; i++) {
int start = sortedActivities[i].getEarlyStart();
int end = sortedActivities[i].getEarlyFinish();
... }
for (int i=0; i<sortedActivities.size(); i++) {
Activity act = (Activity)sortedActivities.get(i);
int start = act.getEarlyStart();
int end = act.getEarlyFinish();
Java 2 Collections ©1999 Trygve Reenskaug

13. The Java 2 solution Enumeration with Iterator
for (int i=0; i<sortedActivities.length; i++) {
int start = sortedActivities[i].getEarlyStart();
int end = sortedActivities[i].getEarlyFinish();
... }
ListIterator iterator = sortedActivities.listIterator();
while (iterator.hasNext()) {
Activity act = (Activity)iterator.next();
int start = act.getEarlyStart();
int end = act.getEarlyFinish();
Java 2 Collections ©1999 Trygve Reenskaug

14. List (ArrayList) Summary
void add(Object element) (optional operation).
void add(int index, Object element)
Object set(int index, Object element) (optional operation).
Object get(int index)
boolean addAll(Collection c) (optional operation).
boolean addAll(int index, Collection c) (optional operation).
Object remove(int index)
Object remove(Object o)
Int size()
boolean isEmpty()
boolean contains(Object o)
int indexOf(Object o)
Iterator iterator()
ListIterator listIterator()
List subList(int fromIndex, int toIndex)
Java 2 Collections ©1999 Trygve Reenskaug

15. Map (Hashtable) Summary
Object put(Object key, Object value) (optional operation).
Object get(Object key)
Object remove(Object key) (optional operation).
boolean isEmpty()
boolean containsKey(Object key)
boolean containsValue(Object value)
int size()
Set keySet()
Collection values()
Set entrySet()
Java 2 Collections ©1999 Trygve Reenskaug

16. Java 2 Collections ©1999 Trygve Reenskaug
Conclusion
Java 2 Collections:
are flexible and powerful
support changing size
support general enumeration
support generalized sorting
use (Interfaces) is independent of implementation (Classes)
think in terms of interacting objects -- not classes
based on “dirty typing” - need extensive type casting
I prefer them to Arrays and Vectors
Java 2 Collections ©1999 Trygve Reenskaug