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Array Review
CS 139 – November 28, 2007
Terminology – See html page
Array
 Element
 Subscript
 Instantiation
 Initializer list

Array Model
anArray
int anArray;
anArray = new int [];
0
0
0
0
0
0
Note: the array elements are initialized to 0 since this is a numeric
type and the constructor will initialize numeric fields to 0.
Arrays and loops
anArray
Before:
0
0
0
0
0
0
for (int ii = 0; ii < anArray.length; ii++)
anArray[ii] = ii;
After:
0
1
2
3
4
5
Checkpoint

Do checkpoint 8.1, 8.3, 8.4, 8.6, 8.8
Array length
Arrays are objects. As such, arrays “know
themselves”.
 An array knows how many elements it
contains.
 length is a field of the array (or an
attribute of the array).
 NOTE: no ()…length is a method in the
String class, an attribute or field in the
array class.

length and subscripts
We can use length to determine how
many elements to process in the array.
 The largest subscript is the length – 1.
 So for an array of 20 elements, there are
20 subscripts beginning with 0, so the
highest subscript is 19.
 arrayName.length can be used to control
array processing.

final int ARRAY_SIZE = 10;
char myArray;
myArray = new char[ARRAY_SIZE];
// updating an array – standard for loop
for (int ii = 0; ii < myArray.length; ii++)
myArray = ‘A’;
// reading an array – enhanced for loop
for (char letter : myArray)
System.out.println(letter);
Enhanced for loop limitations
May only be used on a single loop
 May only be used to “read” the elements
of an array.
 May only process the array from 0 –
length – 1; cannot process the array in
reverse.
 Will process all array elements.
 You do not have access to the subscript.

Checkpoint





Do 8.9 (page 454). First draw a model of
the two arrays in question.
Write the statement.
Then write statements to create a new array,
numbers3 which will have the same number
of elements as numbers1 (do not count).
Write a statement that will fill numbers3
with the product of the same position
elements in numbers1 and numbers2.
Do 8.12.
Reassigning Array References

An array reference can be assigned to
another array of the same type.
// Create an array referenced by the numbers variable.
int[] numbers = new int[10];
// Reassign numbers to a new array.
numbers = new int[5];

If the first (ten element) array no longer
has a reference to it, it will be garbage
collected.
Starting Out With Java Control
Structures to Objects
By Tony Gaddis
Copyright © 2005,
Pearson Addison-Wesley. All rights
reserved.
Cha
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8
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#11
Reassigning Array References
int[] numbers = new int[10];
The numbers variable
holds the address of an
int array.
Address
Starting Out With Java Control
Structures to Objects
By Tony Gaddis
Copyright © 2005,
Pearson Addison-Wesley. All rights
reserved.
Cha
pter
8
Slid
e
#12
Reassigning Array References
The numbers variable
holds the address of an
int array.
This array gets marked for
garbage collection
Address
numbers = new int[5];
Starting Out With Java Control
Structures to Objects
By Tony Gaddis
Copyright © 2005,
Pearson Addison-Wesley. All rights
reserved.
Cha
pter
8
Slid
e
#13
Copying Arrays
You cannot copy an array by merely
assigning one reference variable to
another.
 You need to copy the individual
elements of one array to another.

int[] firstArray = {5, 10, 15, 20, 25 };
int[] secondArray = new int[5];
for (int i = 0; i < firstArray.length; i++)
secondArray[i] = firstArray[i];

This code copies each element of
firstArray to the corresponding element
of secondArray.
Starting Out With Java Control
Structures to Objects
By Tony Gaddis
Copyright © 2005,
Pearson Addison-Wesley. All rights
reserved.
Cha
pter
8
Slid
e
#14
Passing Arrays as Arguments
Arrays are objects.
 Their references can be passed to methods
like any other object reference variable.

showArray(numbers);
Address
5 10 15 20 25 30 35 40
Example: PassArray.java
public static void showArray(int[] array)
{
for (int i = 0; i < array.length; i++)
System.out.print(array[i] + " ");
Copyright © 2005,
}
Starting Out With Java Control Pearson Addison-Wesley. All rights
Structures to Objects
By Tony Gaddis
reserved.
Cha
pter
8
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#15
Arrays of Objects
You must use three steps to create an
array of object types.
1. Declare the array
2. Instantiate the array

1. (What are we making when we instantiate the
array?)
3.
Instantiate the objects that the array will
hold
1. This step does not need to happen all at
once…think about Boxes
Checkpoint
Given a Die class, create an array that will
hold 5 Die objects.
 Roll each of the Die objects in the array.
 Display each element of the array.
 Reroll only the 2nd and 3rd elements of
the array.
 Display each element of the array.
