1. Introduction to Computing Using Java
Array and Table a
Michael Fung, CS&E, The Chinese University of HK

2. Array If we need 3 integer variables, we type:
int i, j, k;
If we need 1000 integer variables…?!
we want to use an index to access variables of the same name (identifier) and type:
i[0] = 5;
i[654] = -378;
How to do that?
int[] i = new int[1000];
OR ( C++ style )
int i[] = new int[1000];
integer array type
(a collection of integers) a
Michael Fung, CS&E, The Chinese University of HK

3. Array If we need 3 integer variables, we type:
int i, j, k;
If we need 1000 integer variables…?!
we want to use an index to access variables of the same name (identifier) and type:
i[0] = 5;
i[654] = -378;
How to do that?
int[] i = new int[1000]; OR
int i[] = new int[1000];
create an array of 1000 integer variables a
Michael Fung, CS&E, The Chinese University of HK

4. Declaration and Array Creation
int[] i = new int[1000];
is equivalent to
int[] i; // i is nothing yet
i = new int[1000]; // i keeps something a
Michael Fung, CS&E, The Chinese University of HK

5. Revision: Object Creation
Boy i = new Boy(3);
is equivalent to
Boy i; // i is nothing yet
i = new Boy(3); // i keeps something a
Michael Fung, CS&E, The Chinese University of HK

6. Another Form of Array Creation
By enumerating its initial values:
char[] vowels = {'a', 'e', 'i', 'o', 'u'};
Then vowels is an array of 5 char variables with
vowels[0]  'a'
vowels[1]  'e'
vowels[2]  'i'
vowels[3]  'o'
vowels[4]  'u'
Array index must be an integer: [0 to length – 1].
There are 5 char variables! a
Michael Fung, CS&E, The Chinese University of HK

7. Syntax of Creating Arrays
type[] array_name = new type[length];
type[] array_name = {value1, value2, ...};
e.g.
double[] GPA = new double[50];
String[] countryCode = new String[175];
String address[] = new String[30];
char[] vowels = {‘a’, ‘e’, ‘i’, ‘o’, ‘u’};
type may be primitive type, class type or even an other array type. a
Michael Fung, CS&E, The Chinese University of HK

8. Michael Fung, CS&E, The Chinese University of HK
Use of Array Elements
Each array element is a variable by itself: {
char[] vowels = {'a', 'e', 'i', 'o', 'u'};
int[] i = new int[1000]; // i[0] to i[999]
int j = 4; // serve as array index
vowels[0] = 'A'; // simple variable asg’t
if (vowels[1] == 'e')
System.out.println( vowels[j] );
i[4]++; // increment i[4] by 1
i[j] = i[3] – i[i[4]] * i[999]; }
vowels[4]
‘u’
vowels[3]
‘o’
vowels[2]
‘i’
vowels[1]
‘e’
vowels[0]
‘A’
vowels[0]
‘a’
i[4] 1
i[4] a
Michael Fung, CS&E, The Chinese University of HK

9. Michael Fung, CS&E, The Chinese University of HK
Properties of Array
A bounded (fixed length) and indexed collection of elements of the same type.
Array length is fixed at the time of creation.
Element access is done using an index [ ].
vowels[0] to vowels[vowels.length – 1]
vowels[-8]  ArrayIndexOutOfBoundsException
To get the length (size) of an array:
vowels.length
NOT vowel.length() [confused with String] a
Michael Fung, CS&E, The Chinese University of HK

10. Example: The Parameter in main()
class TestArgs {
public static void main(String[] args) {
System.out.println("There are " + args.length +
" arguments:");
int i;
for (i = 0; i < args.length; i++)
System.out.println( args[i] ); }
C:\LetSee\> java TestArgs
There are 0 arguments:
C:\LetSee\> java TestArgs Apple Orange
There are 2 arguments:
Apple
Orange a
Michael Fung, CS&E, The Chinese University of HK

11. Example: The Parameter in main()
A Java application program can receive some parameters at start-up.
These start-up parameters are called command-line arguments.
The main() method is the receiver of such arguments.
Such arguments are stored in a String array created by the JVM.
JVM sends a message to main() with such an array. a
Michael Fung, CS&E, The Chinese University of HK

12. Revision: Difference Between Primitive Types and Object References
/* 8 primitive types: byte, short, int, long,
float, double, char, boolean */
int i;
char c;
Octopus myCard; // object reference
Account ansonAccount; // object reference
i and c have their own storage spaces for storing the actual data values.
myCard and ansonAccount are just references to objects.
Without initialization, object references refer to nothing and are said to be null. a
Michael Fung, CS&E, The Chinese University of HK

13. Revision: Initializing Object References
int i; // an integer variable
i = 45 * 89; // it stores 0 by default
Octopus myCard; // a null object reference
myCard = new Octopus(); // create a new object and
myCard.addValue(100); // let myCard refer to it
Octopus yourCard; // a null object reference
yourCard = new Octopus(); // create another object
yourCard.addValue(50); // it’s your card
Octopus stolenCard; // a null object reference
stolenCard = myCard; // myCard and stolenCard
// now refer to the SAME card! a
Michael Fung, CS&E, The Chinese University of HK

14. Michael Fung, CS&E, The Chinese University of HK
Pictorially...
value
150.00
String
(object)
“Michael Fung”
String
(class)
age 21
Octopus
(object)
...
Octopus
(class)
name
(reference)
myCard
(reference)
double value = ;
int age = 21;
String name = “Michael Fung”;
Octopus myCard = new Octopus(); a
Michael Fung, CS&E, The Chinese University of HK

15. Array of Object References
int[] i; // a null integer array reference
i = new int[100]; // create a new integer array
i[5] = 87; // let i refer to the array
// initially, i[0] = … = i[99] = 0
Octopus[] deck; // a null Octopus array reference
deck = new Octopus[10];
// initially, deck[0] = … = null
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
Creating a new array  Creating members a
Michael Fung, CS&E, The Chinese University of HK

16. Array Itself is Also a Reference
deck[ ]
(reference) ?
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus(); a
Michael Fung, CS&E, The Chinese University of HK

17. Array Itself is Also a Reference
deck[ ]
(reference) ? ? ?
deck[2]
(reference)
deck[1]
(reference)
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
deck[0]
(reference) a
Michael Fung, CS&E, The Chinese University of HK

18. Array Itself is Also a Reference? ?
deck[ ]
(reference)
Octopus
(class)
Octopus
(object)
deck[2]
(reference)
deck[1]
(reference)
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
deck[0]
(reference) a
Michael Fung, CS&E, The Chinese University of HK

19. Array Itself is Also a Reference
deck[ ]
(reference) ?
Octopus
(class)
Octopus
(object)
deck[2]
(reference)
deck[1]
(reference)
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
deck[0]
(reference) a
Michael Fung, CS&E, The Chinese University of HK

20. Array Itself is Also a Reference
deck[ ]
(reference)
Octopus
(object)
Octopus
(class)
Octopus
(object)
deck[2]
(reference)
deck[1]
(reference)
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
deck[0]
(reference)
Class type array a
Michael Fung, CS&E, The Chinese University of HK

21. Array Itself is Also a Reference
(int) 9
i[1]
(int) 7
i[0]
(int) 7
Primitive type array 
Class type array
int[] i;
i = new int[3];
i[0] = 7;
i[1] = i[0];
i[2] = 9; a
Michael Fung, CS&E, The Chinese University of HK

22. Michael Fung, CS&E, The Chinese University of HK
7-Minute Break a
Michael Fung, CS&E, The Chinese University of HK

23. Assignment of the Whole Array
(reference)
They refer to the same array!
i[2]
(int) 9
i[1]
(int) 7
i[0]
(int) 7
j[ ]
(reference)
int[] i = new int[3];
int[] j;
j = i; // object reference copying
j[2] = 9; // i[2] = 9
i[1] = 7; // j[2] = 7
j[0] = 7; // i[0] = 7 a
Michael Fung, CS&E, The Chinese University of HK

24. Assignment of the Whole Array
(reference)
OR, Create another one!
i[2]
(int) 9
i[1]
(int) 7
j[ ]
(reference)
i[0]
(int) 7
int[] i = new int[3];
int[] j;
j = new int[5];
j[4]
(int)
j[3]
(int)
j[2]
(int)
j[1]
(int)
j[0]
(int) a
Michael Fung, CS&E, The Chinese University of HK

25. Assignment of the Whole Array
(reference)
Remember to Keep It Well!
i[2]
(int) 9
i[1]
(int) 7
j[ ]
(reference)
i[0]
(int) 7
int[] i = new int[3];
int[] j;
j = new int[5];
j = i;
j[4]
(int)
j[3]
(int)
Killed
j[2]
(int)
j[1]
(int)
j[0]
(int) a
Michael Fung, CS&E, The Chinese University of HK

26. Revision: Primitive Type Parameter Passing
class Student {
public static void studyHard(double newGPA) {
newGPA = 4.0; }
public static void main(String[] args)
double GPA = 1.0;
Student.studyHard(GPA);
System.out.println(GPA);
newGPA
4.0
newGPA
1.0
Start here 
GPA
1.0
Copy actual parameter to formal parameter when sending message.
Change to the formal parameter DOES NOT affect actual parameter! a
Michael Fung, CS&E, The Chinese University of HK

27. New Concept: Primitive Type Array Argument Passing
class Student {
public static void studyHard(double[] newGPAs) {
newGPAs[0] = 4.0;
newGPAs[1] = 4.0; }
public static void main(String[] args)
double[] GPAs = new double[2];
GPAs[0] = 1.0;
GPAs[1] = 1.5;
Student.studyHard(GPAs);
System.out.println(GPAs[0]);
System.out.println(GPAs[1]);
newGPAs
(Reference)
Start here 
GPAs[1]
4.0
GPAs[0]
GPAs[1]
1.5
GPAs[0]
1.0
GPAs
(Reference)
Copy array reference to formal parameter when sending message.
Change to the formal parameter DOES affect the actual parameter! a
Michael Fung, CS&E, The Chinese University of HK

28. Revision: Object Type Argument Passing
Employee
(class)
class CUHK {
public static void fire(Employee victim) {
victim.salary = 0; }
public static void main(String[] args)
Employee michael = new Employee(500);
CUHK.fire(michael);
if (michael.salary == 0)
System.out.println(“Fired!”);
class Employee {
public int salary;
public Employee(int initialSalary)
salary = initialSalary;
victim
(reference)
Start here 
Employee
(object)
salary
(int)
michael
(reference) a
Michael Fung, CS&E, The Chinese University of HK

29. New Concept: Object Type Array Argument Passing
class CUHK {
public static void fire(Employee[] victims) {
for (int i = 0; i < victims.length; i++)
victims[i].salary = 0; }
public static void main(String[] args)
Employee[] TAs = new Employee[3];
TAs[0] = new Employee(1000);
TAs[1] = new Employee(2000);
TAs[2] = new Employee(5000);
CUHK.fire(TAs);
class Employee {
public int salary;
public Employee(int initialSalary)
salary = initialSalary;
victims
(reference)
Employee
(object)
salary
1000
Start here 
TAs[0]
(reference)
Employee
(object)
salary
2000
TAs
(reference)
TAs[1]
(reference)
Employee
(object)
salary
5000
TAs[2]
(reference) a
Michael Fung, CS&E, The Chinese University of HK

30. Michael Fung, CS&E, The Chinese University of HK
What’s the type?
double
Table (2-Level Array)
// There are 176 students, 8 assignments
// record their marks in double
double[][] mark = new double[176][8];
mark[6][0] = 99.34; // mark: 7th student, Asg1
mark[175][6] = 89.12; // mark: last student, Asg7
double[] singleStudent;
singleStudent = mark[175]; // refer to the
singleStudent[6] = 45.67; // marks of the last one
System.out.println(mark[175][6]); // would print 45.67
Elements of an array could be arrays.
Array reference of array references.
double[] a
Michael Fung, CS&E, The Chinese University of HK

31. Michael Fung, CS&E, The Chinese University of HK
Table Illustrated
mark[ ][ ]
(reference)
mark[2]
(reference)
mark[1]
(reference)
mark[0]
(reference)
mark[2][3]
(double) 9.45
mark[1][3]
(double) 8.48
mark[0][3]
(double) 9.11
mark[2][2]
(double) 2.49
mark[1][2]
(double) 3.40
mark[0][2]
(double) 1.42
Array of Array
of double
mark[2][1]
(double) 3.43
mark[1][1]
(double) 6.13
mark[0][1]
(double) 5.43
mark[2][0]
(double) 1.75
mark[1][0]
(double) 1.15
mark[0][0]
(double) 0.35 a
Michael Fung, CS&E, The Chinese University of HK

32. Duplicating an int Array
(reference)
Copy the elements one-by-one
i[2]
(int) 9
i[1]
(int) 7
i[0]
(int) 7
int[] i = {7, 7, 9};
int[] j;
j = new int[i.length];
for (int count = 0;
count < i.length;
count++)
j[count] = i[count]; a
Michael Fung, CS&E, The Chinese University of HK

33. Duplicating an int Array
(reference)
Copy the elements one-by-one
i[2]
(int) 9
i[1]
(int) 7
j[ ]
(reference)
i[0]
(int) 7
int[] i = {7, 7, 9};
int[] j;
j = new int[i.length];
for (int count = 0;
count < i.length;
count++)
j[count] = i[count]; ? a
Michael Fung, CS&E, The Chinese University of HK

34. Duplicating an int Array
(reference)
Copy the elements one-by-one
i[2]
(int) 9
i[1]
(int) 7
j[ ]
(reference)
i[0]
(int) 7
int[] i = {7, 7, 9};
int[] j;
j = new int[i.length];
for (int count = 0;
count < i.length;
count++)
j[count] = i[count];
j[2]
(int)
j[1]
(int)
j[0]
(int) a
Michael Fung, CS&E, The Chinese University of HK

35. Duplicating an int Array
(reference)
Copy the elements one-by-one
i[2]
(int) 9
i[1]
(int) 7
j[ ]
(reference)
i[0]
(int) 7
int[] i = {7, 7, 9};
int[] j;
j = new int[i.length];
for (int count = 0;
count < i.length;
count++)
j[count] = i[count];
j[2]
(int) 9
j[1]
(int) 7
j[0]
(int) 7 a
Michael Fung, CS&E, The Chinese University of HK

36. Duplicating an Object Array
deck[ ]
(reference)
Octopus
(object)
Octopus
(class)
Octopus
(object)
deck[2]
(reference)
deck[1]
(reference)
Octopus[] deck;
...
Octopus[] newDeck;
newDeck = new Octopus[deck.length];
for (int count = 0;
count < deck.length;
count++)
newDeck[count] = deck[count];
deck[0]
(reference) a
Michael Fung, CS&E, The Chinese University of HK

37. Duplicating an Object Array
newDeck[ ]
(reference)
deck[ ]
(reference)
Octopus
(object)
Octopus
(class)
Octopus
(object) ?
deck[2]
(reference)
deck[1]
(reference)
Octopus[] deck;
...
Octopus[] newDeck;
newDeck = new Octopus[deck.length];
for (int count = 0;
count < deck.length;
count++)
newDeck[count] = deck[count];
deck[0]
(reference) a
Michael Fung, CS&E, The Chinese University of HK

38. Duplicating an Object Array
newDeck[ ]
(reference)
deck[ ]
(reference)
Octopus
(object)
Octopus
(class)
Octopus
(object)
newDeck[2]
(reference)
deck[2]
(reference)
newDeck[1]
(reference)
deck[1]
(reference)
Octopus[] deck;
...
Octopus[] newDeck;
newDeck = new Octopus[deck.length];
for (int count = 0;
count < deck.length;
count++)
newDeck[count] = deck[count];
newDeck[0]
(reference)
deck[0]
(reference) a
Michael Fung, CS&E, The Chinese University of HK

39. Duplicating an Object Array
newDeck[ ]
(reference)
deck[ ]
(reference)
Octopus
(object)
Octopus
(class)
Octopus
(object)
Only the object references are copied!
newDeck[2]
(reference)
deck[2]
(reference)
newDeck[1]
(reference)
deck[1]
(reference)
Octopus[] deck;
...
Octopus[] newDeck;
newDeck = new Octopus[deck.length];
for (int count = 0;
count < deck.length;
count++)
newDeck[count] = deck[count];
newDeck[0]
(reference)
deck[0]
(reference) a
Michael Fung, CS&E, The Chinese University of HK

40. Michael Fung, CS&E, The Chinese University of HK
End Note
Readings and References
Sections 7.1, 7.2, 7.3, 7.4, 7.6
Exercises
7.1, 7.2, 7.3, 7.4, 7.5
Programming Projects
7.1, 7.2, 7.3, 7.5, 7.6 a
Michael Fung, CS&E, The Chinese University of HK