1. Chapter 9
Array Basics
Suppose you need to process daily temperatures for a 12-month period in a science project, would you use 365 variables?
You can, but would you?
An array is a collection of data values of the same data type.
If your program needs to deal with 100 integers, 365 real numbers, etc., you will use an array.
Intro to OOP w/Java--Wu

2. Chapter 9
Primitive Arrays
A primitive array stores multiple values of the same primitive data type.
rainfall 1 2 3 4 5 6 7 8 9 10 11
The index of the first
position in an array is 0.
Intro to OOP w/Java--Wu

3. Arrays Variables are Reference Variables
Chapter 9
Arrays Variables are Reference Variables
byte
short
int
double
long
float
boolean
String
Applet
MessageBox
HiLo
InputBox
etc.
char
primitive
reference
Data Type
A constant data value is represented graphically by a lock icon to convey the fact that the value is locked and cannot be changed.
Primitive variables contain values
Reference variables point at objects
Intro to OOP w/Java--Wu

4. Arrays of Primitive Data Types
Chapter 9
Arrays of Primitive Data Types
Array Declaration
<data type> [ ] <variable> //variation 1
<data type> <variable>[ ] //variation 2
Array Creation
<variable> = new <data type> [ <size> ]
Example
double[] rainfall;
rainfall = new double[12];
Variation 1
double rainfall [ ];
rainfall = new double[12];
Variation 2
As you can declare and create objects in one statement such as
Person p = new Person( );
you can declare and create an array in one statement as
double[ ] rainfall = new double[12];
Strictly speaking, an array is a reference data type and really an object because there is no class called Array in Java. The thumbnail note in page 413 is therefore not 100 percent accurate.
An array is like an object!
Intro to OOP w/Java--Wu

5. Accessing Individual Elements
Chapter 9
Accessing Individual Elements
Individual elements in an array accessed with the indexed expression.
double[] rainfall = new double[12];
rainfall 1 2 3 4 5 6 7 8 9 10 11
rainfall[2]
This indexed expression refers to the element at position #2
Intro to OOP w/Java--Wu

6. Example Programs Zadoot … out to reality … ArrayElements1.java

7. Array Lengths double[] rainfall = new double[12];
Chapter 9
Array Lengths
double[] rainfall = new double[12];
double annualAverage;
double sum = 0.0;
int index;
for (index = 0; index < rainfall.length; index++) {
rainfall[index] = SavitchIn.readLineDouble();
sum += rainfall[index]; }
annualAverage = sum / rainfall.length;
The public constant length returns the capacity of an array.
Notice the public constant length returns the capacity, not the actual number of non-blank values in the array.
Intro to OOP w/Java--Wu

8. Example Programs Phrrud … out to reality … ArrayRain.java
Phrrud … out to reality … ArrayAverages.java

9. Array Bounds Errors
Trying to access an array element that does not exist causes a runtime error
Negative indices
Indices beyond the size
Falop … out to reality … ArrayBoundsError.java

10. Chapter 9
Array Initialization
Like other data types, it is possible to declare and initialize an array at the same time.
int[] number = { 2, 4, 6, 8 };
double[] samplingData = { 2.443, 8.99, 12.3, , 18.2,
9.00, 3.123, , };
String[] monthName = { "January", "February", "March",
"April", "May", "June", "July",
"August", "September", "October",
"November", "December" };
The capacity of the array is set to the number of elements in the list.
number.length samplingData.length monthName.length 4 9 12
Intro to OOP w/Java--Wu

11. Example Programs Groeeet … out to reality … ArrayInit.java
An array variable can be explicitly made to point to no data, using the null value
Spaaocie … out to reality … ArrayNULL.java

12. References are Pointers
A reference variable points to an object
So, arrays are objects, but don't worry about that now
But it does mean you can
Have multiple references to an array
Not copy an array with =
Lose an array

13. Having Two References to an Array
Chapter 9
Having Two References to an Array
A. Variables are allocated in memory.
clemens
twain
B. The reference to the new array is assigned to clemens.
Array in
Memory
C. The reference in clemens is assigned to customer. A
int[] clemens, twain; B
clemens = new int[5]; C
twain = clemens;
Code
State of Memory
Intro to OOP w/Java--Wu

14. Example Program
Dooop … out to reality … ArrayDup.java

15. Cloning an Array An array can be copied using the clone method
Chapter 9
Cloning an Array
An array can be copied using the clone method
It's necessary to cast the clone to the right array type
Babababoom … out to reality … ArrayClone.java
Intro to OOP w/Java--Wu

16. Losing an Array customer A B C Code State of Memory
Chapter 9
Losing an Array
customer
A. The variable is allocated in memory.
B. The reference to the new array is assigned to customer.
Array in
Memory
C. The reference to another array overwrites the reference in customer.
Array in
Memory A
int[] customer; B
customer = new int[5]; C
customer = new int[5];
Code
State of Memory
Intro to OOP w/Java--Wu

17. Garbage Collection
An array that has no references is garbage collected by the java program
Spaaocie … out to reality … ArrayGC.java

18. Two-Dimensional Arrays
Chapter 9
Two-Dimensional Arrays
Two-dimensional arrays are useful in representing tabular information.
Intro to OOP w/Java--Wu

19. Declaring and Creating a 2-D Array
Chapter 9
Declaring and Creating a 2-D Array
Declaration
<data type> [][] <variable> //variation 1
<data type> <variable>[][] //variation 2
Creation
<variable> = new <data type> [<size1>][<size2>]
Example 1 2 3 4
payScaleTable = new double[4][5];
is really a shorthand for
payScaleTable = new double[4][ ];
payScaleTable[0] = new double[5];
payScaleTable[1] = new double[5];
payScaleTable[2] = new double[5];
payScaleTable[3] = new double[5];
which is equivalent to
for (int i = 0; i < 4; i++) {
payScaleTable[i] = new double[5]; }
double[][] payScaleTable;
payScaleTable = new double[4][5]; 1 2 3
Intro to OOP w/Java--Wu

20. Example Programs Ieeei … out to reality … ArrayMatrix.java
Ieeei … out to reality … ArrayCalendar.java
Ieeei … out to reality … ArrayCube.java

21. Multi-dimensional Arrays … NOT
Java does not really have multi-dimensional arrays
Java has arrays of arrays
int[][] data = new int[3][5];
is shorthand for
int[][] data = new int[3][];
data[0] = new int[5];
data[1] = new int[5];
data[2] = new int[5];

22. Multi-dimensional Arrays in RAM
int[][] data = new int[3][5];
Zuuu … out to reality … ArrayRAM.java

23. Irregular Arrays int[][] weirdData = new int[3][];

24. Irregular Arrays int[][] weirdData = new int[3][];
weirdData.length == 3
weirdData[1].length == 4
Jioooul … out to reality … ArrayIrreg1.java

25. Irregular Arrays … null
int[][] data = new int[3][];
data[0] = new int[5];
data[1] = new int[4];
data[2] = null;
Jioooul … out to reality … ArrayIrreg2.java

26. Multi-dimensional Array Initialization
Chapter 9
Multi-dimensional Array Initialization
Like 1D arrays, it is possible to declare and initialize a multi-dimensional array at the same time.
Frong … out to reality … ArrayCubeInit.java
Frong … out to reality … ArrayMDInit.java
Intro to OOP w/Java--Wu

27. Passing Arrays to Methods - 1
Chapter 9
Passing Arrays to Methods - 1
Code A
public int searchMinimum(float[] number)) { … }
minOne = searchMinimum(arrayOne);
At before searchMinimum A
arrayOne
A. Local variable number does not exist before the method execution
When an array is passed to a method, only its reference is passed. A copy of the array is NOT created in the method.
public int searchMinimum(float[] number) {
int indexOfMinimum = 0;
for (int i = 1; i < number.length; i++) {
if (number[i] < number[indexOfMinimum]) { //found a
indexOfMinimum = i; //smaller element }
return indexOfMinimum;
State of Memory
Intro to OOP w/Java--Wu

28. Passing Arrays to Methods - 2
Chapter 9
Passing Arrays to Methods - 2
Code
public int searchMinimum(float[] number)) { … } B
minOne = searchMinimum(arrayOne);
arrayOne
The address is copied at B
number
arrayOne
B. The value of the argument, which is an address, is copied to the parameter.
State of Memory
Intro to OOP w/Java--Wu

29. Passing Arrays to Methods - 3
Chapter 9
Passing Arrays to Methods - 3
Code
public int searchMinimum(float[] number)) { … } C
minOne = searchMinimum(arrayOne);
arrayOne
number
While at inside the method C
C. The array is accessed via number inside the method.
State of Memory
Intro to OOP w/Java--Wu

30. Passing Arrays to Methods - 4
Chapter 9
Passing Arrays to Methods - 4
Code
public int searchMinimum(float[] number)) { … }
minOne = searchMinimum(arrayOne); D
arrayOne
At after searchMinimum D
arrayOne
number
D. The parameter is erased. The argument still points to the same object.
State of Memory
Intro to OOP w/Java--Wu

31. Example Programs Flunot … out to reality … ArrayParamAvg.java
Flunot … out to reality … ArrayParam1.java

32. Returning Arrays
Array variables in methods exist until the method ends, but the array data lives while referenced
An array variable can be returned from a method
The receiving array variable then refers to the array data, and the array persists
Wrrbbrack … out to reality … ArrayReturn.java
Wrrbbrack … out to reality … ArrayParam2.java

33. Local arrays Array variables in methods exist until the method ends
The array data referred to by such an array variable is lost and garbage collected when the method ends
Dessserts … out to reality … ArrayLocalGC.java