1. Outline ArrayLists standard for : shorthand version of while Admin:
Terms Test #2:
Monday 15th, 5-6pm or 6-7pm Rooms: TBA
Topics: Loops, Files, Defining Objects, Event Driven Input (GUIs),
Note: Chapter 6 of textbook is on GUI's. It uses a more complex design than this course (it uses inheritance and private inner classes). Read it only if you find it helpful.
Textbook: Ch 7 & 15
(but covers more than we do here)

2. Storing lots of values Assignment 3, Temperature Analyser:
got an ArrayList of numbers from the user
Used simple for loops to step through all the numbers to add them, draw them, etc.
ArrayLists allow you to work on lots of values
ArrayList of numbers or Strings
ArrayList of Gliders or Flowers.
What more can you do with an ArrayList?
Example programs
with lots of values

3. Using ArrayLists How do we create a new ArrayList?
add a new value to the ArrayList?
access the values in the ArrayList?
change the value at an index in the ArrayList?
remove a value from the ArrayList?
do something to every value in the the ArrayList
find out how big the ArrayList is? 1 2 3
names:

4. Using ArrayList Creating empty ArrayList:
ArrayList<String> names = new ArrayList<String>();
Must specify the type of value
Can’t be int or double or boolean!!! Have to use Integer or Double or Boolean (“wrapper” objects)
ArrayList<Integer> counts = new ArrayList<Integer>();
Must have the ( ) - standard constructor, with no arguments.
Type of value
in the ArrayList
in < …. >
Type of
variable
Constructing the
ArrayList

5. Using ArrayList For all actions, call methods on the ArrayList:
size, add, get, set, remove, clear, contains, indexOf, isEmpty, …
ArrayList<String> names = new ArrayList<String>();
names.add(“Jim”);
names.add(“Jan”);
if (names.get(0).equals(“Jim”) {
names.set(0, “Jane”); …
names.add(1, “Bob”);
names.remove(“Bob”);
names.remove(0);
Adds an item at the end of the list
Gets the item at a position
Replaces the item at a position with a new item
Adds an item at a position
names: 1 2
“Jim”
“Bob”
“Jan”
Removes item (1st occurrence)
Removes item at a position

6. Acting on each item in an ArrayList:
If you want to act on each element from 0 … end
⇒ use a simple for loop:
Print out all the names:
for (String name : names) {
UI.println( “Hello ”, name); }
for (Glider glider : gliders) {
glider.move(); 1 2 3
names: A B C D
Doesn't change the list 1 2 3
gliders:
Changes the gliders, but doesn't change the list – the gliders are still the same objects.

7. ToDo list Program
import ecs100.*; import java.util.*; public class ToDo{ private ArrayList<String> allItems = new ArrayList<String>(); public ToDo(){ UI.initialise(); UI.addButton("New", this::newList); UI.addTextField("Add", this::addItem); UI.addButton("List", this::displayList); UI.addTextField("Contains", this::doContains); UI.addTextField("Remove", this::doRemove); UI.addButton("Quit", UI::quit); } public void newList(){ this.allItems = new ArrayList<String>(); // or: this.allItems.clear(); this.displayList(); public void addItem(String item){ this.allItems.add(item);
public void displayList(){ UI.clearText(); UI.printf("List has %d items:\n", this.allItems.size()); for (String item : allItems){ UI.println(item); } public void doContains(String item){ if (this.allitems.contains(item)){ UI.println(item+" is in the list"); else { UI.println(item+" is not in the list"); public void doRemove(String item){ if (this.allItems.remove(item)) { UI.println(item+" was removed"); UI.println(item+" was not present");

8. ToDo list Program
public void saveList(){ try{ PrintStream ps = new PrintStream(new File("todolist.txt")); for (String item : allItems){ ps.println(item); } ps.close(); catch(IOException e){UI.println("todolist.txt is broken"+e);} public void loadList(){ this.allItems.clear(); Scanner sc = new Scanner(new File("todolist.txt")); while (sc.hasNext()){ this.allItems.add(sc.nextLine()); sc.close();

9. Garden Program
public class Garden { private ArrayList<Flower> flowers = new ArrayList<Flower>(); public Garden(){ UI.setMouseListener(this::doMouse); UI.addButton(“Clear”, this::doClear); UI.addButton(“Grow”, this::doGrow); UI.addButton(“Bloom”, this::doBloom); UI.addButton(“Pick”, this::doPick); } public void doMouse(String action, double x, double y){ if (action.equals(“released”)){ Flower fl = new Flower(x, y); this.flowers.add(fl); 1 2
10 30
bud
20 5
bud
73 49
bud

10. Garden program Operate on the ArrayList of Flowers:
private ArrayList<Flower> flowers = new ArrayList<Flower>();
public void doGrow(){
for (Flower flower : this.flowers) {
flower.grow(); }
public void doBloom(){
flower.bloom();
public void doClear(){
this.flowers.clear();
UI.clearGraphics(); 1 2
10 30
bud
20 5
bud
73 49
bud
grow
bloom
pick
clear

11. Garden program: Alternate design
Operate on a selected Flower:
private ArrayList<Flower> flowers = new ArrayList<Flower>();
private Flower selectedFlower;
public void doGrow(){
this.selectedFlower.grow(10); }
public void doBloom(){
this.selectedFlower.bloom();
public void doDig(){
this.flowers.remove(this.selectedFlower); 1 2
10 30
bud
20 5
bud
73 49
bud
grow
bloom
pick
clear

12. Garden program : Alternate design
Operate on a selected Flower:
private ArrayList<Flower> flowers = new ArrayList<Flower>();
private Flower selectedFlower; :
public void doMouse(String action, double x, double y){
if (action.equals(“released”)){
for (Flower flower : this.flowers) {
if (flower.touching(x, y) ) {
this.selectedFlower = flower ;
return ; }
} // if not touching a flower, plant one
this.flowers.add(new Flower(x, y)); 1 1 2
grow
bloom
pick
dig

13. Saving a Garden to a file
Get file name and open file
step through flowers, printing to file
public void doSave(){
File saveFile = new File(UIFileChooser.save("File for Garden"));
try{
PrintStream out = new PrintStream(saveFile);
for (Flower flower : this.flowers) {
out.println(flower.toString()); }
out.close();
catch(IOException e) { UI.println("File saving failed: "+e); } 1 2 3
flowers:
position(x,y) and
state(height, stage):
eg: bud

14. Loading Garden from a file
Get file name and open file
Step through file, reading
public void load(){
try {
this.flowers = new ArrayList<Flower>(); // or this.flowers.clear();
File file = new File(UIFileChooser.open("Choose Garden File"));
Scanner sc = new Scanner(file);
while (sc.hasNext()){
Flower fl= new Flower(sc.nextDouble(), sc.nextDouble(), sc.nextDouble(), sc.next () );
this.flowers.add(fl); }
sc.close();
catch(IOException e){UI.println("File loading failed: "+e);}
bud
bloom
picked
bud : 1 2 3
flowers:

15. Changing an ArrayList: loops
If you want to change an ArrayList, then can't use the simple for loop!
Eg: Change each name to uppercase:
for (String name : this.names) {
name = name.toUpperCase(); }
If you need to know the index of the items, can't use the simple for loop
Eg: print out the todo list is numbers:
for (String item : this.allItems) {
UI.println( index +": " + item ); 1 2 3
names:
anne
bill
cath
dave

16. Changing an ArrayList: loops
Need to use a while loop or a standard for loop
int i= 0;
while (i < names.size()) {
String upName = names.get(i).toUpperCase();
names.set(i, upName);
i++; }
for (int i= 0; i < names.size(); i++) {
for (int index = 0; index < this.allItems.size(); index++) {
UI.println( index +": " + item ); 1 2 3
names:
anne
bill
cath
dave
Standard "for loop"
Standard "for loop"

17. Standard for loops
Standard for loop is just a while loop with the elements rearranged:
int i = 0 ;
while (i < data.size() ){
… data.get(i) … ;
i = i+1 ; }
The for loop is a shorthand:
for (int i = 0 ; i < data.size(); i++ ) {
… data.get(i) … ;
Only got to this slide in 2016T1

18. Standard For loop For loop puts the together, at the front of the loop
intialisation  once, before the loop body is run at all
condition  tested each time, before loop body run
increment  run each time, after loop body run
together, at the front of the loop
But the meaning is (almost) exactly the same as the while loop
(scope of variables in initialisation is different)
Initialisation
Condition
Increment
for (
statement ;
expression ;
statement )
statement

19. Removing items from an ArrayList in a loop
Remove gliders past the right edge:
for (Glider glider : this.gliders) {
if (glider.getX() >= RIGHT_EDGE) {
gliders.remove(glider); }
Must use a standard for loop (or while loop)
for (int index = 0 ; index < this.gliders.size(); index++) {
Glider glider = this.gliders.get(index);
this.gliders.remove(glider);
BE CAREFUL WHEN MODIFYING THE LIST IN A LOOP!
Program will CRASH!
Not allowed to change the list while
iterating with a "for each" loop
up to here at 12pm
Won't crash,
But still won't work properly! 1 2 3
gliders:

20. Removing items from ArrayList in a loop
Step back after removing item
for (int num = 0 ; num < gliders.size(); num++) {
if (gliders.get(num).getX() >= RIGHT_EDGE) {
gliders.remove(num);
num = num - 1; }
Or iterate backwards from the end.
for (int num = gliders.size()-1 ; num >= 0; num--) {
gliders.remove(num);} 1 2 3
gliders:
up to here at 10am.

21. Menu More ArrayLists: Administration: ICT Careers fair today,
Tutorial on Tuesdays 5pm (Kirk 202!)
Test next Monday
I used bluej to demonstrate the Numbers program early in lecture.
Rest of lecture is going through the design process and explaining the code.
It is important to do the design process, not just present the resulting code.

22. More Examples of Methods on ArrayLists
mylist.size()
returns the number of items in mylist.
Note contrast to String: .size() vs .length()
UI.printf("The garden had %d flowers\n", this.flowers.size());
mylist.add(item)
adds the item at the end of the ArrayList
balloons.add( new Balloon(x, y, color) );
mylist.add(index, item)
inserts the item at position index ( 0 .. size )
int pos = UI.askInt(“Where do you want ” +name);
receptionOrder.add(pos, name);
item must be of
the right type
item must be of
the right type 1 2 3
receptionOrder: 4
“Jim”
“Ann”
“Cary”
“Bob”

23. More Examples of Methods on ArrayLists
mylist.get(index)
returns the item at position index (0 .. size-1)
mylist.set(index, item)
replaces the current value at position index with item
returns the old value at index
index must be size-1
UI.print(“Which two units do you want to swap?”);
int first = UI.askInteger("first");
int scnd = UI.askInteger("second");
String temp = attackList.get(first);
attackList.set(first, attackList.get(scnd));
attackList.set(scnd, temp);
attackList.set(first, attackList.set(scnd, attackList.get(first)));
Got to here on Friday.
if (first>=0 && first< attackList.size() && scnd>=0 && scnd<attackList.size() &&
first!=second){ Or

24. More Examples of Methods on ArrayLists
mylist.isEmpty()
returns true iff there are no items in mylist.
// Make each unit advance, if there are any units
if ( ! attackList.isEmpty() ) {
for (Unit unit : attackList) {
unit.checkPath();
unit.advance(3); }
mylist.clear()
removes all values from the list.
// Restart and clear the list of all elements.
public void doRestart() {
UI.clearGraphics();
this.flowers.clear();

25. More Examples of Methods on ArrayLists
mylist.contains(item)
returns true if the item is somewhere in mylist
mylist.remove(item)
removes the item, if it is present, and shuffles later items down
returns true iff item was removed
// Respond to a “Remove Person” button
String name = UI.askString("Person to remove");
if (receptionOrder.contains(name)){
receptionOrder.remove(name); }
else {
UI.println("That person is not in the reception order");
if ( ! receptionOrder.remove(name) ){
first occurrence of item
if item is at several
places in the list
“Bill” Or 1 2 3
receptionOrder: 4
“Jim”
“Cary”
“Bob”
“Ann”

26. More Examples of Methods on ArrayLists
mylist.indexOf(item)
returns the position of item in mylist
returns –1 if the item is not present
// Report position on waiting list
String name = UI.askString("Your name:");
int index = waitingList.indexOf(name);
if (index == –1) { UI.println("You are not on the waiting list"); }
else { UI.println("You are number " + index + " in order"); }
mylist.remove(index)
removes the item at position index (0 .. size-1)
returns the value that was removed
// Remove every third unit from attackList
for (int index=2; index<attackList.size(); index = index+3){
attackList.remove(index); } 1 2 3
attackList: 4 5 6 7 8 9 10 11

27. Using ArrayLists of numbers
public void analyseNumbers(String fileName){
ArrayList<Double> nums = new ArrayList<Double>();
try {
Scanner sc = new Scanner(new File(fileName))
while (sc.hasNextDouble()){
nums.add(sc.nextDouble()); }
sc.close();
} catch(IOException e){UI.println(“Reading numbers failed: "+e);}
double max = Double.NEGATIVE_INFINITY;
for (double num : nums) {
if ( num > max) {
max = num;
UI.printf("%.3f is the largest of %d numbers\n", max, nums.size());
Read numbers
into ArrayList
Step through
numbers to
find largest

28. Patterns
There are many common patterns for working with arrays or ArrayLists.
Do something with each value
Search for any item that satisfies some property
Check if all items have some property
Do something with every adjacent pair of items.
Do something with every possible pair of items.

29. "Any" pattern: Finding a value
Search array or ArrayList to see if it contains a given value
eg: array of names: finding if name is present in any element
public boolean containsName(String name){
boolean ans = false;
for (String n : this.names){
if ( n!=null && n.equals(name) ) {
ans = true; }
return ans;
return true;
return false; s0
s11 s5 s8 s2 s4
s10
(field)
names: s1 s3 s6 s7 s9
s12
default answer
if no such element
the answer if you find one
no else clause!
why not?
why keep searching once you know the answer?
default answer:
if no element was equal

30. "All" pattern: Check valid
Check ArrayList of names: all must have at least one space in them
public boolean checkValidNames(ArrayList<String> names){
boolean allOK= true;
for (int i = 0; i < names.size() ; i++){
if ( ! names.get(i).contains(" ") ) {
allOK = false;
break; }
return allOK;
for (String nm : names){
if ( ! nm.contains(" ") ) {
return false;
return true;
default: answer if all pass
to here at 10am
the answer if one fails.

31. Working on multiple items at once
Doing something to each adjacent pair
Given an array with a list of numbers, check if in order:
public boolean checkOrdered(ArrayList<Double> data){
for ( int i = 0; i<data.size()–1; i++){
if ( data.get( i ) > data.get(i+1)){
return false; }
return true;
for (int i = 1; i<data.size(); i++){
if ( data.get( i-1 ) > data.get( i ) ){
data: 51 67 68 68 79 78 82 90
Can’t use ‘for each’!
compare with next item:
stop before length-1
compare with previous item:
start at 1

32. Working on multiple items at once
smoothi = 0.25 datai datai datai+1
“Smooth” the data:
Given a list of numbers, generate a new list:
public ArrayList<Double> smooth(ArrayList<Double> data){
ArrayList<Double> answer = new ArrayList<Double>();
answer.add( 0.67*data.get(0) *data.get(1) ); // first element is special
for (int i = 1; i<data.size() –1; i++){
answer.add( 0.25 * data.get( i-1) * data.get( i) * data.get( i+1) ); }
answer.add( 0.33 * data.get(data.size() – 2) * data.get(data.size()-1); // last element
return answer;
data: 51 67 68 68 79 78 82 90

33. Changing the order: reverse
Reverse all the elements in an array/ArrayList
public void reverseList( ){
for (int i = 0; i < this.cartoons.size()/2; i++){
CartoonCharacter temp = this.cartoons.get( i );
this.cartoons.set( i, this.cartoons.get(this.cartoons.size() – 1 – i);
this.cartoons.set (this.cartoons.size() – 1 – i , temp); } Or
public void reverseList(){
for (int i = 0; i<this.cartoons.size()/2; i++){
cartoons.set( i, cartoons.set(cartoons.size() – 1 – i), cartoons.get( i) ) );
cartoons:
cc1
cc5 1
cc3 2
cc8 3
cc2 4
cc6 5
cc4 6
set returns the old value at the position!!

34. Acting on every pair of values
gliders:
Given ArrayList with Gliders check if any two Gliders are colliding with each other: (assume Glider class has a method called isTouching)
for ( int i=0; i<this.gliders.size(); i++ ){
for ( int j=0; j< this.gliders.size(); j++ ){
if (this.gliders.get( i ).isTouching(this.gliders.get( j ) ){
//do something }
for (int i=0; i<this.gliders.size(); i++){
for (int j=i+1; j<this.gliders.size(); j++){
if ( this.gliders.get( i ).isTouching(this.gliders.get( j )) ){ g1 g5 g3 g8 g2 g6 g4
What’s the problem?

35. Arrays vs ArrayLists Some lists have a fixed number of places:
The places may be empty
Arrays may be more convenient than using ArrayLists

36. SeedTray Program: just 12 flowers3 4 5 6 7 8 9 10 11
length: 12
null
from uploads/2010/04/tps_seedlings.jpg

37. Arrays An array is an object with a sequence of a fixed number places
Length determined when array is created)
All elements are of the same type
Special syntax, no methods
Each element specified by its index (an int expression)
seedtray[ 4 ] ← name of the element of shapes with index 4
seedtray[ n-3 ]
Counting from 0, just like ArrayLists!
Array knows its length: seedtray.length 1 2 3 4 5 6 7 8 9 10 11
length: 12
seedTray:
Confusion:
names.size() ← ArrayList
name.length() ← String
tray.length ← Array

38. Declaring and Creating Arrays
Declare a variable to hold an array object by putting [ ] after the type of the elements:
Flower[ ] seedtray;
String[ ] keywords;
private double[ ] marks;
Create an array object with new and the length of the array:
new Flower[12];
new String[50];
new double[200];
As usual, can combine declaration and initialisation:
String [ ] keywords = new String [50];
What does the new array contain?
Creates a place that can hold an array
Doesn’t create the array itself
NO ROUND BRACKETS !!!
Creates an array object
Doesn’t declare a variable to hold it

39. Initial values in a new array
Arrays of objects initialised with null (the “no object here” value)
Arrays of numbers initialised to 0.
seedtray:
null
null
null
null
null
null
null
null
null
null
null
null 1 2 3 4 5 6 7 8 9 10 11
length: 12
marks:
. . . 1 2 3 4 5 6 7 8
199
length: 200

40. No ‘Array’ in declaration!
SeedTray Program
public class SeedTray { private Flower[ ] seedtray = new Flower[12];
No ‘Array’ in declaration!
seedtray:
null
null
null
null
null
null
null
null
null
null
null
null 1 2 3 4 5 6 7 8 9 10 11
length: 12

41. Using an array Can act on the whole array to pass to a method
to assign to another variable :
this.processFlowers(seedtray);
int maxNum = this.findMax(numbers);
int [ ] windowSizes = numbers;
Note, passing as argument and assignment do not copy the array! (just the reference/ID of the object)
Just the same as with ArrayList.
numbers:
length:
public int findMax(int[] nums){ …

42. Index can be any int valued expression
Using an Array
Use [ .. ] to refer to an individual place in the array
to access the value in that place
to put a value in that place (using assignment: = )
double [ ] marks = new double [200];
int n=4; :
marks[5] = 45.6;
marks[6] = ( marks[5] + marks[7] ) / 2;
marks[n-1] = 80.0;
marks[n] = marks[n-1];
if (marks[ i ] == marks[ i+1 ]) {…
Not get() and set()
0.0
0.0
0.0
80.0
0.0
0.0
80.0
0.0
45.6
0.0
22.8
0.0
0.0
. . .
0.0 1 2 3 4 5 6 7 8
199
Note about index range added later – was explained during the lecture.
length
200
Index can be any int valued expression

43. SeedTray Program
public class SeedTray{ private Flower[ ] seedtray = new Flower[12]; : public void replant(){ for (int i = 0; i < this.seedtray.length; i++) { this.seedtray[ i ] = new Flower(70+i*50, 400); } public void growAll(){ public void growAll(){ for (int i = 0; i < this.seedtray.length; i++) { for (Flower flower : this.seedtray){ this.seedtray[ i ].grow(); flower.grow(); } } 1 2 3 4 5 6 7 8 9 10 11
length:12
seedtray:

44. Initialising the contents of an array
Can specify the initial values (and size) of an array by listing the values in {.. , .. , ..} :
String [ ] names = new String [ ] { “Ian”, “Pondy”, “Monique” , “Zarinah” };
int [ ] dimensions = new int [ ] { 20, 45, 8 };
Can’t do this
with ArrayLists!
names: 1 2 3
“Pondy”
“Monique”
“Zarinah”
“Sharon”
length: 4
dimensions: 1 2
length: 3 45 8 20

45. Arrays vs ArrayList Use an array if Use an ArrayList if
it will never change size, and
you know how big it will need to be, at the point you need to create it.
Use an ArrayList if
the size will change, or
you don’t know how big it will need to be.
Arrays have convenient syntax [ ]
ArrayLists have convenient methods.

46. Saving a SeedTray to a file
Get file name and open file
Write size of array
Step through seedtray, printing flowers or null to file
public void doSave(){
try{
PrintStream out = new PrintStream(new File(UIFileChooser.save("File for Seedtray")));
out.println(this.seedtray.length);
for (Flower flower : this.seedtray) {
out.println(flower); }
out.close();
catch (IOException e) { UI.println("File saving failed: "+e); } 4
bud
null
picked
bud : 1 2 3
seedtray:
println( object )
calls the toString() method
automatically.
if (flower == null) { out.println("null"); }
else { out.println(flower.toString(); }
Assumes that Flower has a
toString method that returns
a String in the form
bud

47. Loading Seedtray from a file4
bud
null
picked
bud :
Get file name and open file
Read the size & make array
Step through file, reading
public void load(){
try {
Scanner sc = new Scanner(new File(UIFileChooser.open("Choose Seedtray File")));
this.seedtray = new Flower [ sc.nextInt() ];
for (int i = 0; i < this.seedtray.length; i++ ){
if (sc.hasNextInt()) {
this.seedtray[i] = new Flower( sc.nextInt(), sc.nextInt(), sc.nextInt(), sc.next () ); }
else { sc.next(); }
sc.close();
catch (IOException e){UI.println("File loading failed: "+e);} 1 2 3
seedtray:
Assumes that Flower has a
constructor that has parameters
for all the fields to reconstruct
a Flower

48. Flower Class
import ecs100.*; import java.awt.Color; public class Flower{ // fields private double baseX; private double baseY; private double height; private String stage; /** constructor for planting; given position */ public Flower(double x, double y){ this.baseX = x; this.baseY = y; this.stage = "Bud"; this.height = 20; }
/** additional constructor; for reconstructing flower */
public Flower(double x, double y, double h, String s){
this.baseX = x;
this.baseY = y;
this.stage = s;
this.height = h;}
/** returns text description of the Flower*/
public String toString(){
return ( this.baseX + " " + this.baseY + " " +
this.height + " " + this.stage ); …

49. Arrays with Meaningful Indices
Sometimes, the index represents meaningful information
array of guest names for hotel rooms (numbered 1 to MaxRoom)
house info for each house on street
count of occurrences of each number
image n1 n5 n8
n10 n7 n3 n6 n4 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 -1
700
620
460
450
380
220
510
480 1 2 3 4 5 6 7 8 9 10 2 9 16 3 1 15 8 4 10 5 6 7 4 89 6 90 91 85 86 87 7 2 3 92 1

50. null: the “not-a-real-object” value.
Hotel Register
initialise empty
private String[ ] guests = new String[MaxRoom+1];
// gives indexes from 0 to MaxRoom, ignore index 0
assign to room
public void assignGuest(String name; int room){
this.guests[room] = name; }
look up to see if free
public boolean isFree(int room){
return (this.guests[room]==null); n1 n5 n8
n10 n7 n3 n6 n4
guests: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
null: the “not-a-real-object” value.
Can always be
assigned to a
place of an
object type
half way through slide at 12noon lecture
Don’t need to step
through array!

51. Hotel Register: remove
Checkout guest from room
/* returns true if name was checked out of room successfully,
false otherwise */
public boolean checkout(String name, int room){
if ( this.guests[room].equals(name) ){
this.guests[room] = null;
return true; }
return false;
if ( this.guests[room] != null && this.guests[room].equals(name) ) …
if ( name.equals(this.guests[room]) ) … n1 n5 n8
n10 n7 n3 n6 n4
guests: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Problem if null!
How do we fix it?

52. Alternative Checkout guest: search and remove:
/* returns true if name was checked out successfully,
false otherwise */
public boolean checkout(String name){
for (int rm=1; rm<this.guests.length; rm++){ // or <=MaxRoom
if (this.guests[rm] != null && this.guests[rm].equals(name) ) {
this.guests[rm] = null;
return true; }
return false; n1 n5 n8
n10 n7 n3 n6 n4
guests: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Example of "Any" pattern

53. Find an empty room
Find the index of an empty room (return -1 if no empty rooms)
public int findEmpty(){
for (int rm=1; rm<this.guests.length; rm++){ // or <=MaxRoom
if (this.guests[rm]==null) { return rm; } }
return -1;
Check a guest into an empty room (return room number)
public void checkIn(String name){
this.guests[ this.findEmpty() ] = name;
public boolean checkIn(String name){
int rm = this.findEmpty();
if (rm < 0) { return false; }
this.guests[rm] = name;
return true; n1 n5 n8
n10 n7 n3 n6 n4
guests: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
to here at 10am lecture
What’s the problem?
How do we fix it?

54. Arrays of Counts To keep track of numbers we have seen, efficiently
Counting the number of occurrences of each number a file of integers:
Scanner sc = new Scanner(new File(FileDialog.open("file to check")));
int[ ] numbersSeen = new int[100];
while ( sc.hasNext() ){
if ( sc.hasNextInt() ){
int num = sc.nextInt();
if ( num>=0 && num<100 ){
numbersSeen[num] = numbersSeen[num] + 1; // OR numbersSeen[num]++; }
else{
sc.next();
Initialised with 0 2 9 16 3 1 15 8 4 10 5 6 7 99 ⋯

55. Comparing arrays.
Be careful when comparing arrays (as with all objects)
int[ ] a = new int[ ]{ 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 43, 47};
int[ ] b = new int[ ]{ 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 43, 47};
int[ ] c = b;
if (a == b) ..  ?? no
if (b == c) ..  ?? yes
if (a.equals(b) ) ..  ?? no
if (Arrays.equals(a, b) ) ..  ?? yes
if (this.arrayEquals(a, b) ) ..  ?? yes
public boolean arrayEquals(int[ ] a, int[ ] b) {
if (a==null && b==null ) { return true; }
if (a==null || b==null ) { return false; }
if ((a.length != b.length ) { return false; }
for (int i = 0; i < a.length; i++) { if ( a[i] != b[i] ) { return false; } }
return true; }

56. Main method main takes an array of strings as a parameter:
Can be used to provide “command line inputs” to a program, eg. the name of a file to process. May be optional:
public static void main(String[ ] args) {
String fname;
if ( args.length > 0 ) { fname = args[0]; }
else { fname = UIFileChooser.open("Data files"); }
try {
Scanner scan = new Scanner (new File(fname));
while (scan.hasNext()) { … }
catch (IOException e){ …… }
> java Plotter mynumbers.txt

57. 2D arrays COMP 102 # T1

58. Menu
Some more array details
2D arrays
Administration

59. 2D Data data in rows and columns 2 4 8 X O ID A1 A2 A3 A4 30012 30031
30048
30056
30080
30118
30185
30302
30345
30382
30495
30515
30545 X O 2 4 8
Mon
Tue
Wed
Thu
Fri
Sat
Sun
9-10
10-11
11-12
12-1
1-2
2-3
3-4
4-5

60. 2D arrays: Creating 2D arrays require two indices, and two sizes
Declaring and creating:
int[ ] [ ] marks = new int [200][4];
ChessPiece[ ][ ] board = new ChessPiece [8][8];
Color[ ][ ] image = new Color [100][150];
int[ ][ ] matrix = new int [ ][ ]{{2, 4, 3},{5, 8, 4}, {8, 4, 9}};
First index, second index : which is the row and which is the column?
You choose! Choose your variable names carefully.
Typically use first index as the row.

61. 2D arrays: Accessing Assigning and accessing:
marks[10][3] = 72;
board[row][col] = board[row][col-1];
board[row][col-1] = null;
for (int row=0; row<height; row++){
for (int col=0; col<width; col++){
image[row][col] = image[row][col].darker(); }
In Java, can’t use commas
image[row, col]

62. 2D arrays 2D arrays are actually arrays of arrays:
int[ ] [ ] marks = new int[200] [4];
is the same as
int[ ] [ ] marks = new int[200] [ ];
for (int i = 0; i<200; i++){
marks[i] = new int[4]; }
marks 1 2 3 4 5 6

63. 2D arrays: length Number of rows and columns in a 2D array?
int[ ] [ ] marks = new int[200] [4];
marks.length  (number of rows)
marks[row].length  4 (number of columns)
If the first index is the row!
to here on Monday 10am
marks 1 2 3 4 5 6

64. We don’t use these in COMP102!
2D arrays
Can have non-square arrays:
int [ ] [ ] table = new int [7] [ ];
for (int row = 0; row<7; row++){
table[row] = new int [row+1]; }
We don’t use these in COMP102!
table 1 2 3 4 5 6

65. Processing 2D arrays
Typically use nested for loops to process each item
public void printTable( String[ ][ ] grades){
for (int row=0; row< grades.length; row++){
for (int col=0; col< grades[row].length; col++){
UI.printf(" %-2s ", grades[row][col]); }
UI.println();
for (String[ ] row : grades){
for (String grade : row){
UI.printf(" %-2s ", grade);
A+ B- A- B
B+ C A B-
A D A+ A
A- B+ B+ B
A A- C+ C+
To here on thursday
'-' flag means
left justified
If not modifying the array, can use foreach loops, but
must be careful!

66. Drawing a 2D array ♖ ♔ ♘ ♗ ♙ ♜ ♝ ♚
public void drawBoard(ChessPiece[ ][ ] board){
int rows = board.length;
int cols = board[0].length;
for (int row=0; row<rows; row++){
int y = TOP + SIZE*row;
for (int col=0; col<cols; col++) {
int x = LEFT + SIZE*col;
UI.setColor( (row%2==col%2) ? Color.gray : Color.white);
UI.fillRect(x, y, SIZE, SIZE);
if (board[row][col] !=null) {
board[row][col] .draw(x, y); }
}♔♕♗♖♘♙♚♛♜♝♞♟
UI.setColor(Color.black);
UI.drawRect(LEFT, TOP, SIZE * rows, SIZE * cols);
Shorthand for:
if (row%2==col%2) {
UI.setColor(Color.gray); }
else {
UI.setColor(Color.white); ♖ ♔ ♘ ♗ ♙ ♜ ♝ ♚
Make ChessPiece draw itself

67. Printing a table with headers
public void printTable(long[ ] IDs, String[ ][ ] grades){
int rows = grades.length;
int cols = grades[0].length;
UI.print(" ID |");
for (int col=0; col<cols; col++) {
UI.printf(" A%d |", col); }
UI.println();
for (int col=-1; col<cols; col++) {
UI.print("----+"); }
for (int row=0; row<rows; row++){
UI.printf("%4d|", IDs[row])
UI.printf(" %-2s |", grades[row][col]); } }
Assumes all rows same length
ID | A1 | A2 | A3 | A4 |
3012| A+ | B- | A- | B |
3052| B+ | C | A | B- |
3029| A | D | A+ | A |
3172| A- | B+ | B+ | B |
3094| A | A- | C+ | C+ |

68. Moving value in a 2D array
public void moveUp(ChessPiece[ ][ ] board, int row, int col){
if ( row > 0 && row < board.length
&& col >= 0 && col < board[row].length
&& board[row][col] != null
&& board[row-1][col] == null ) {
board[row-1][col] = board[row][col];
board[row][col] = null;♗♖♘♙♚♛♜♝♞♟ } 1 2 3 4 5 6 7 1 2 3 4 5 6 7 ♖ ♔ ♘ ♗ ♙ ♜ ♝ ♚

69. Moving all values in a 2D array
public void flipBoard(ChessPiece[ ][ ] board){
int rows = board.length;
int cols = board[0].length;
for (int row=0; row<rows/2; row++){
for (int col=0; col<cols; col++) {
ChessPiece temp = board[row][col];
board[row][col] = board[rows-row-1][col];
board[rows-row-1][col] = temp; }} }
to here on thurs 12noon 1 2 3 4 5 6 7 1 2 3 4 5 6 7 ♖ ♔ ♘ ♗ ♙ ♜ ♝ ♚ 1 2 3 4 5 6 7 ♜ ♚ ♝ ♙ ♗ ♔ ♘ ♖

70. Rotating (cw) all values in a 2D array♖ ♔ ♘ ♗ ♙ ♜ ♝ ♚
public void rotateBoard(ChessPiece[ ][ ] board){
int rows = board.length;
int cols = board[0].length;
ChessPiece[ ][ ] temp= new ChessPiece[rows][cols];
for (int row=0; row<rows; row++){
for (int col=0; col<cols; col++) {
temp[row][col] = board[row][col]; }}
int srow = cols-col-1;
int scol = row;
board[row][col] = temp[srow][scol]; } 1 2 3 4
Make a temporary copy of the source, to avoid "losing" values. 5
90º 6 7
10am Thursday to here 1 2 3 4 5 6 7 1 2 3 4 5 6 7 ♗ ♜ ♝ ♔ ♖ ♙ ♚ ♘
For each cell in the result, work out where its value comes from.

71. Processing multiple arrays
public double[ ][ ] matrixAdd(double[ ][ ] a, double[ ][ ] b){
int rows = a.length;
int cols = a[0].length;
if (b.length!=rows || b[0].length!=cols) { return null; }
double[ ][ ] ans = new double[rows][cols];
for (int row=0; row<rows; row++){
for (int col=0; col<cols; col++){
ans[row][col] = a[row][col] + b[row][col]; }
return ans; 2 4 5 6 3 2 5 1 4 7 8 7 4 9 6 2 + =

72. Processing multiple arrays
public double[ ][ ] matrixMultiply(double[ ][ ] a, double[ ][ ] b){
int rows = a.length; int cols = b[0].length;
if (cols != b.length) { return null; }
double[ ][ ] ans = new double[rows][cols];
for (int row=0; row<rows; row++){
for (int col=0; col<cols; col++){
for (int i=0; i<b.length; i++){
ans[row][col] = ans[row][col]+a[row][ i ] * b[ i ][col]; }
return ans; 2 4 5 6 3 2 5 1 4 7
6+4+6
4+16+0
8+20+0 * =

73. Saving a 2D array to a file
Write the grade table to a file: A B B+ A- B- C+
Writing the dimensions of the array first will help.
Three people with 4 assignments each or
Four people with 3 assignments each
Six people with 2 assignments each A B B+ A- B- C+ A B B+ A- B- C+ A B B+ A- B- C+

74. Saving a 2D array to a file
Writing the dimensions in the file helps:
public void saveTable( String[][] grades, String fileName){
try{
PrintStream file =new PrintStream(new File(fileName));
int rows = grades.length;
int cols = grades[0].length;
file.println(rows + " " + cols);
for (int row=0; row<rows; row++){
for (int col=0; col<cols; col++){
file.println(grades[row][col]); }
}catch(IOException e){UI.println("Table saving failed: "+e);}
Print number of rows and columns on first line of file.

75. Saving a 2D array to a file
Alternate design:
assume file has been opened elsewhere and passed as argument
use "foreach" because not modifying the array.
public void saveTable( String[ ][ ] grades, PrintStream file){
file.println(grades.length + " " + grades[0].length);
for (String[ ] row : grades){
for (String grd : row){
file.println(grd); }
Note, you could pass System.out to the method to make it print to the terminal window!
(useful for debugging)

76. Loading 2D array from a file
Assume first two tokens of file are the dimensions:
public String[ ][ ] loadTable( ){
try {
Scanner sc = new Scanner(new File(UIFileChooser.open()));
int rows =sc.nextInt();
int cols = sc.nextInt();
String[ ][ ] ans = new String[ rows ][ cols ];
for (int row=0; row<rows; row++){
for (int col=0; col<cols; col++){
ans[row][col] = sc.next(); }
return ans;
} catch(IOException e){UI.out.println("Table loading failed: "+e);}
return null;

77. Loading 2D array from a file
Alternate, assuming
scanner is passed as argument
array is stored in a field
public void loadTable(Scanner sc ){
this.dataArray = new String[ sc.nextInt() ] [ sc.nextInt() ];
for (int row=0; row<this.dataArray.length; row++){
for (int col=0; col<this.dataArray[row].length; col++){
this.dataArray[row][col] = sc.next(); }

78. Another file format for 2D arrays
Suppose the array has only a few entries and many nulls
ie, a "sparse" array
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec 1 T1 2
lects 3
Better to save just the non-null entries 4
5 lects
lects 6
Study 7 8 9 10
Exam 11
end T3 T2 12
end Ex
Grad 13 14 15 16 17
Grad
Study 18
Break 19

79. File format
size (months, days) [note: first and second index, not rows and columns!]
month day entry
To load:
read size and create calendar array
read month, day, entry, and assign entry to calendar[month][day]
To save:
print size
for each non-null entry, print month, day, entry
12 32
0 5 lects
end T3
2 1 T2
break :

80. Load sparse array public void loadSparse(){ try{
Scanner sc = new Scanner( new File(UIFileChooser.open()));
int months= sc.nextInt();
int days = sc.nextInt();
this.calendar = new String[months][days];
while (sc.hasNext()){
int month = sc.nextInt();
int day = sc.nextInt()
String entry = sc.next();
this.calendar[month][dayl] = entry; }
sc.close();
catch (IOException e){UI.println("Fail: " + e);}
Don't know how many entries there will be!

81. Save sparse array
public void saveSparse(){ try{ PrintStream ps = new PrintStream(new File(UIFileChooser.save())); ps.println(this.calendar.length+" "+ this.calendar[0].length); for (int month = 0 ; month < this.calendar.length; month++){ for (int day = 0 ; day< this.calendar[month].length; day++){ if (this.calendar[month][dayl] != null){ ps.println(month+" "+day+" "+this.calendar[month][dayl]); } sc.close(); catch (IOException e){UI.println("Fail: " + e);}

82. Doing without UI & ecs100 library #1
UI class lets you print and read from the text pane
What do you do without the UI?
Output:
System.out is the "terminal window".
You can print/println/printf to System.out, just like UI
System.out.println("Here is a message for you");
Input:
System.in is the keyboard via the "terminal window"
You have to wrap it with a Scanner to read:
Scanner userInput= new Scanner(System.in); :
System.out.print("Enter age: ");
int age = userInput.nextInt();
No "ask…()" methods

83. Doing without UI & comp102 library #2
How do you make a window with a text area, and buttons?
Using Swing:
Have to make a JFrame object and set its size
Make a JTextArea object and add to frame
Make a JPanel and add to frame
Make JButtons and add to the panel
make button
add listener
add to panel
Make the frame visible
Define methods to respond to events.
Writing to the text area is different.

84. DemoInterface
import javax.swing.*; import java.awt.event.*; import java.awt.BorderLayout; public class DemoInterface { private JFrame frame = new JFrame("Demo program"); private JTextArea textOut = new JTextArea(30, 30); public DemoInterface () { this.frame.setSize(600, 400); this.frame.add(this.textOut, BorderLayout.CENTER); JPanel panel = new JPanel(); this.frame.add(panel, BorderLayout.SOUTH); JButton button1 = new JButton("Doit"); button1.addActionListener(this::doIt); panel.add(button1); JButton button2 = new JButton("Clear"); button2.addActionListener((ActionEvent e)->{textOut.setText(“”);}); panel.add(button2); this.frame.setVisible(true); }

85. DemoInterface
public void doIt(ActionEvent e) { for (int i = 1; i <=10; i++){ this.textOut.append(i+ " squared is " + (i*i)); }

86. Makeup Assignment: GoGame
Very simple program
One field
Constructor
One button
One Mouse action
redisplay method
Key idea: program from scratch – no template code.

87. BalloonGame Simple game Key data structure: Collection of Balloons
Clicking on a Balloon expands it
If two Balloons touch, they pop.
Score = total area of live balloons minus area of popped Balloons.
Key data structure:
Like DominoGame: needs a list of Balloon objects:
Could use
ArrayList <Balloon>
Balloon[ ] (array of Balloon)

88. BalloonGame Simple game Key data structure: Collection of Balloons
Clicking on a Balloon expands it
If two Balloons touch, they pop.
Score = total area of live balloons minus area of popped Balloons.
Key data structure:
Like DominoGame: needs a list of Balloon objects:
Could use
ArrayList <Balloon>
Balloon[ ] (array of Balloon)
Actually want two lists:
live balloons
popped balloons

89. BalloonGame

90. Design of BalloonGame

91. Types and Interface
Variables, fields, array elements, ArrayList elements are all defined with a type
Only values of the specified type can be put into a place
What types are there
primitive types: int, double, boolean, long, float, char, ..
note: java will "coerce" some types: double number = 4;
Object types:
Arrays : int[ ], double[ ][ ], Balloon[ ],
ArrayLists: ArrayList<Integer>, ArrayList<ArrayList<Double>>, ArrayList<Balloon>,
Predefined: String, Color, File, Scanner, …
Programmer defined: CartoonFigure, Balloon, BalloonGame, …
Type of object determined at its creation:
new Balloon(100, 300) ⇒ a Balloon object
new File ("data.txt") ⇒ a File object
new int[100][100] ⇒ an array of arrays of int.
Every class
defines a type

92. Object types All object values have two types:
the type they were created as
the Object type
You can have a place of type Object:
Object value = new Balloon(100, 100);
value = "not any more";
ArrayList<Object> things = new ArrayList<Object>();
things.add("more“);
things.add( new Balloon(100, 100), new Double(4.5));
for (Object thing : things){
UI.println(thing.toString());
System.out.println(thing); }

93. Object types
There are limits on what you can do with a place of type Object:
value.pop();
value.toUpperCase();
for (Object thing : things){
if (thing.startsWith(“Ball”){
System.out.println(thing); }
You can only call methods on the value in a variable/field if the methods belong to the type of the variable/field.
value.toString()
Won't work if value contains a String
Won't work if value contains a Balloon
Won't work if value contains a Balloon

94. Collections of different types
Extending the Balloon Game:
several kinds of balloons:
ordinary balloons – expand uniformly, and pop on touching
bomb balloons – blow up, taking out lots of balloons at max size
long balloons – expand into sausage shapes, popping other balloons only, up to limit
bouncy balloons – don't expand, but bounce around T

95. New Balloon Game BalloonGame needs ArrayLists to hold all the balloons
Each class of balloons needs the same methods:
expand, on, pop, touches
but, they each behave differently.
BalloonGame
ArrayList of balloons
ArrayList of popped
balloons
RoundBalloon
expand()
on(int x, int y)
touches(…)
pop()
BombBalloon
expand()
on(int x, int y)
touches(…)
pop()
LongBalloon
expand()
on(int x, int y)
touches(…)
pop()
BouncyBalloon
expand()
on(int x, int y)
touches(…)
pop()

96. BalloonGame public class BalloonGame { :
Can use Object
Can use Object
public class BalloonGame { :
private ArrayList < > balloons = new ArrayList< > ();
public void doClick(double x, double y){
for ( b : this.balloons){
if ( b.on(x, y) ) {
b.expand();
Object
Object
RoundBalloon-2
BombBalloon-5
BouncyBalloon-3
LongBalloon-8
RoundBalloon-18
RoundBalloon-7
LongBalloon-9
Object
Java compiler will complain!
No such method for Object
Java will complain!
No such method for Object

97. Different Balloon classes
public class RoundBalloon {
private double x, y;
private double radius = 10;
private Color col;
public void expand(){ this.radius += 3;)
public boolean on(double u, double v){
return ( Math.hypot(this.x – u, this.y –v) < this.radius ); }
public class LongBalloon {
private double length = 0;
private double direction = Math.PI * 2 * Math.random();
private Color color;
public void expand(){ this.length += 10; }
return …. ;

98. Casting Can sometimes convert a value from one type to another
numbers:
int col = (int) (x /cellWidth);
double x = 100 * col;
Must be an explicit cast if information might be lost.
anything to a string: (by adding to a string)
"value is " + x
For an Object value, it automatically calls the toString() method
"value is " + x.toString()
any object value from one of its types to another
Object balloon = (Object) ( new RoundBalloon(34, 45) );
RoundBalloon rb = (RoundBalloon) balloon;
object must be of that type, otherwise a runtime error
got to here on friday, but not talked this slide

99. "Generic" values with Object
public class BalloonGame { :
private ArrayList<Object> balloons = new ArrayList<Object> ();
public void doClick(double x, double y){
for (Object b : this.balloons){
RoundBalloon rb = (RoundBalloon) b;
if ( rb.on(x, y) ) {
rb.expand();
RoundBalloon-2
BombBalloon-5
BouncyBalloon-3
LongBalloon-8
RoundBalloon-18
RoundBalloon-7
LongBalloon-9
Cast it to a RoundBalloon
Now compiler is happy
But if b is a BombBalloon or… the program will fall over!

100. "Generic" values with Object
public void doClick(double x, double y){
for (Object b : this.balloons){
if (b instanceof RoundBalloon) {
RoundBalloon rb = (RoundBalloon) b;
if (rb.on(x, y) ) { rb.expand();…..} }
else if (b instanceof LongBalloon) {
LongBalloon lb = (LongBalloon) b;
if (lb.on(x, y) ) { lb.expand();…..}
else if (b instanceof BombBalloon) {
BombBalloon lb = (BombBalloon) b; …
RoundBalloon-2
BombBalloon-5
BouncyBalloon-3
LongBalloon-8
RoundBalloon-18
RoundBalloon-7
LongBalloon-9
instanceof checks if the value is of the specified type
Correct, but not nice!

101. A Balloon type
RoundBalloon, LongBalloon, BombBalloon, BouncyBalloon are all kinds of Balloon.
⇒ should all be of a Balloon type.
RoundBalloon-2 should be a
a RoundBalloon,
a Balloon, and
an Object
public class BalloonGame {
private ArrayList<Balloon> balloons = new ArrayList<Balloon> ();
public void doClick(double x, double y){
for (Balloon b : this.balloons){
if ( b.on(x, y) ) {
b.expand(); …

102. Making a Balloon Type Problem: What is "a Balloon" ? Answer 1:
Something you can
expand, public void expand()
ask if a point is on it, public boolean on(int x, int y)
pop public void pop()
ask if touching a balloon public boolean touches(Balloon other)
ask for its size public double size()
Answer 2:
a RoundBalloon, or
a LongBalloon, or
a BombBalloon, or
a BouncyBalloon.
We need to make both answers true.
Balloon
RoundBalloon
LongBalloon
BouncyBalloon
BombBalloon

103. Interface classes
An interface class describes a supertype of other types.
It specifies
a name for the type
the headers for the methods that any object of the type can respond to.
constants (public static final double GRAVITY = 9.8)
No constructors!
public interface Balloon {
method headers
constants }
You cannot make a new instance of an interface.
Balloon b = new Balloon();
An interface just says "what"; not "how"

104. A Balloon Interface class.
public interface Balloon {
public void expand();
public boolean on(double x, double y);
public void pop();
public double size();
public boolean touches(Balloon other); }
Declares a type that you can use for fields,variables, arrays:
public class BalloonGame {
private int maxBalloons = 20;
private ArrayList<Balloon> balloons = new ArrayList<Balloon>();
public void doClick(double x, double y){
for (Balloon b : this.balloons){
if (b.on(x, y) ) {
b.expand();
Note the ; instead of { …. }
Java will NOT complain!
Balloons DO have these methods

105.  Making a Balloon Type Problem: What is "a Balloon" ?
Answer 1: Defined by interface Balloon {
Something you can
expand,
ask if a point is on it,
pop
ask for its size
ask if touching a balloon
Answer 2:
a RoundBalloon, or
a LongBalloon, or
a BombBalloon, or
a BouncyBalloon.
We need to make both answers true. 

106. Making classes have another type
If you define some class,
all instances are automatically of type Object also.
To make instances also be of some interface type:
declare that the class implements the interface type:
make sure that the class defines all the methods specified by the interface type:
public class RoundBalloon implements Balloon {
private double x, y; private double radius = 10;
private Color col;
public void expand(){ …… }
public boolean on(double x, double y){ …… }
public void pop() { …… }
public double size() { …… }
public boolean touches(Balloon other) { …… } }
implements is a “promise” that these objects will be of the interface type. ie, will have all the methods.
Must provide method bodies,
not just headers!

107. More classes implementing Balloon
public class LongBalloon implements Balloon {
private double x, y; private double length = 10;
private double dir = 1.26;
private Color baseCol, tubeCol;
public void expand(){ …… }
public boolean on(double x, double y){ …… }
public void pop() { …… }
public double size() { …… }
public boolean touches(Balloon other) { …… } }
public class BombBalloon implements Balloon {
private double x, y, maxRad; private double radius = 10;
private Color col;

108.   Making a Balloon Type Problem: What is "a Balloon" ?
Answer 1: Defined by interface Balloon {
Something you can
expand,
ask if a point is on it,
pop
ask for its size
ask if touching a balloon
Answer 2: Specified by … implements Balloon
a RoundBalloon, or
a LongBalloon, or
a BombBalloon, or
a BouncyBalloon.
We have now made both answers true.  

109. BalloonGame class structure
User Interface
ArrayList of Balloon
Balloon
RoundBalloon
expand
pop …
LongBalloon
expand
pop …
BombBalloon
expand
pop …
BouncyBalloon
expand
pop …

110. Interface class summary
If you define an interface class:
You have defined a type
You can declare variables (or arrays) to have that type.
You cannot make an object of the interface class
new Balloon(…) // NOT ALLOWED (there is no constructor)
Objects from classes implementing the interface are of that type.
You can call any of the specified methods on values in fields/variables of that type.
When defining an interface class:
You should include headers of methods
You may include static final constants
You may not include ordinary fields, constructors, or ordinary method bodies.