Arrays Java

1
Object Oriented Programming
FIEC ESPOL
Arrays
Creating and Accessing Arrays
• An array can be created using one statement
double[] score = new double[5];
• Or using two statements:
double[] score;double[] score;
score = new double[5];
– The first statement declares score an array of
doubles
– The second statement creates an array and makes
the variable score a name for the array
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Declaring and Creating an Array
• An array is declared and created in almost the
same way that objects are declared and created:
BaseType[] ArrayName = new BaseType[size];
– The size may be given as an expression that
l t t ti i t f levaluates to a nonnegative integer, for example, an
int variable
char[] line = new char[80];
double[] reading = new double[count];
Person[] specimen = new Person[100];
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Three Ways to Use Square Brackets [] with
an Array Name
• Square brackets can be used to create a type
name:
double[] score;
• Square brackets can be used with an integerq g
value as part of the special syntax Java uses to
create a new array:
score = new double[5];
• Square brackets can be used to name an
indexed variable of an array:
max = score[0];
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The length Instance Variable
• An array is considered to be an object
• Every array has exactly one instance variable
named length
d bl [] di d bl [100]
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double[] reading = new double[100];
for (int index = 0; index < reading.length; index++)
{
reading[index] = 42.0;
}
An Array of Characters Is Not a String
• The class String has a constructor that has a
single parameter of type char[]
String s = new String(a);
– The object s will have the same sequence of
characters as the entire array a ("ABC"), but is ancharacters as the entire array a ( ABC ), but is an
independent copy
• Another String constructor uses a subrange of
a character array instead
String s2 = new String(a,0,2);
– Given a as before, the new string object is "AB“
(continued)
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An Array of Characters Is Not a String
• An array of characters does have some
things in common with String objects
– For example, an array of characters can be
output using println
System.out.println(a);
– Given a as before, this would produce the
output
ABC
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Arrays and References
• Like class types, a variable of an array
type holds a reference
– Arrays are objects
A variable of an array type holds the address– A variable of an array type holds the address
of where the array object is stored in memory
– Array types are (usually) considered to be
class types
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Arrays with a Class Base Type
• The base type of an array can be a class type
Date[] holidayList = new Date[20];
• The above example creates 20 indexed
reference variables of type Date. It does notyp
create 20 objects of the class Date
– Each of these indexed variables are automatically
initialized to null
– Any attempt to reference any them at this point would
result in a "null pointer exception" error message
(continued)
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Arrays with a Class Base Type
• Like any other object, each of the indexed variables
requires a separate invocation of a constructor using
new (singly, or perhaps using a for loop) to create an
object to reference
holidayList[0] = new Date();y [ ] ();
. . .
holidayList[19] = new Date();
OR
for (int i = 0; i < holidayList.length; i++)
holidayList[i] = new Date();
• Each of the indexed variables can now be referenced
since each holds the memory address of a Date object
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Arguments for the Method main
• The heading for the main method of a program
has a parameter for an array of String
– It is usually called args by convention
public static void main(String[] args)
– Note that since args is a parameter it could be
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Note that since args is a parameter, it could be
replaced by any other non-keyword identifier
• If a Java program is run without giving an
argument to main, then a default empty array of
strings is automatically provided
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Arguments for the Method main
• If a program requires that the main
method be provided an array of strings
argument, each element must be provided
from the command line when the program
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from the command line when the program
is run
java SomeProgram Hi ! there
– This will set args[0] to "Hi", args[1] to "!",
and args[2] to "there"
– It will also set args.length to 3
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Multidimensional Arrays
• It is sometimes useful to have an array with
more than one index
• Multidimensional arrays are declared and
created in basically the same way as one-
dimensional arrays
Aug 6, 2007 16
y
– You simply use as many square brackets as there are
indices
– Each index must be enclosed in its own brackets
double[][]table = new double[100][10];
int[][][] figure = new int[10][20][30];
Person[][] = new Person[10][100];
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Multidimensional Arrays
• Multidimensional arrays may have any number
of indices, but perhaps the most common
number is two
– Two-dimensional array can be visualized as a two-
dimensional display with the first index giving the row
Aug 6, 2007 17
dimensional display with the first index giving the row,
and the second index giving the column
char[][] a = new char[5][12];
– Note that, like a one-dimensional array, each element
of a multidimensional array is just a variable of the
base type (in this case, char)
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Using the length Instance Variable
• The following program demonstrates how a
nested for loop can be used to process a
two-dimensional array
– Note how each length instance variable is used
int row, column;
for (row = 0; row < page.length; row++)
for (column = 0; column < page[row].length;
column++)
page[row][column] = 'Z';
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Arrays Java

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