Transcript Arrays  Introducing Arrays  Declaring Array Variables  Creating Arrays, and Initializing Arrays  Copying Arrays  Multidimensional Arrays.

Arrays
 Introducing Arrays
 Declaring Array
Variables
 Creating Arrays, and Initializing Arrays
 Copying Arrays
 Multidimensional Arrays
Introducing Arrays
Array is a data structure that represents a collection of the
same types of data. Java treats these arrays as objects.
double[] myList = new double[10];
myList
reference
myList[0]
myList[1]
myList[2]
myList[3]
myList[4]
myList[5]
myList[6]
myList[7]
myList[8]
myList[9]
An Array of 10
Elements
of type double
Declaring Array Variables

datatype[] arrayname;
Example:
int[] myList;

datatype arrayname[];
Example:
int myList[];
Creating Arrays
arrayName = new datatype[arraySize];
Example:
myList = new double[10];
Declaring and Creating in One Step

datatype[] arrayname = new
datatype[arraySize];
double[] myList = new double[10];

datatype arrayname[] = new
datatype[arraySize];
double myList[] = new double[10];
Initializing Arrays
 Using
a loop:
for (int i = 0; i < myList.length; i++)
myList[i] = (double)i;
 Declaring,
creating, initializing in one step:
double[] myList = {1.9, 2.9, 3.4, 3.5};
Passing Arrays to Methods
Java uses pass by value to pass parameters to a method.
There are important differences between passing a value of
variables of primitive data types and passing arrays.
 For a parameter of a primitive type value, the actual value
is passed. Changing the value of the local parameter inside
the method does not affect the value of the variable outside
the method.
 For a parameter of an array type, the value of the
parameter contains a reference to an array; this reference is
passed to the method. Any changes to the array that occur
inside the method body will affect the original array that
was passed as the argument.

Copying Arrays?
Before the assignment
list2 = list1;
list1
After the assignment
list2 = list1;
Contents
of list1
list2
list1
Contents
of list1
list2
Contents
of list2
Contents
of list2
Copying Arrays
Using a loop:
int[] sourceArray = {2, 3, 1, 5, 10};
int[] targetArray = new
int[sourceArray.length];
for (int i = 0; i < sourceArrays.length; i++)
targetArray[i] = sourceArray[i];
The arraycopy Utility
arraycopy(sourceArray, src_pos,
targetArray, tar_pos, length);
Example:
System.arraycopy(A1, 0, A2, 0, A1.length);
Multidimensional Arrays

int[][] matrix = new int[10][10];
or
int matrix[][] = new int[10][10];

int[][][] matrix = new int[10][20][30];
for (int i=0; i<matrix.length; i++)
for (int j=0; j<matrix[i].length; j++)
{
matrix[i][j] = (int)(Math.random()*1000);
}
Ragged Arrays
Each row in a two-dimensional array is
itself an array. So, the rows can have
different lengths. Such an array is
known as a ragged array. For example,
int[][] matrix =
{
{1, 2, 3, 4, 5},
{2, 3, 4, 5},
{3, 4, 5},
{4, 5},
{5}
};
Array of arrays
int twoD[][] = new int[4][];
twoD[0] = new int[4];
twoD[1] = new int[3];
twoD[2] = new int[2];
twoD[3] = new int[1];
Array of Objects
Circle[] circleArray = new Circle[10];
An array of objects is actually an array of reference
variables.
 So invoking circleArray[1].findArea()
involves two levels of referencing as shown in the next
figure.
circleArray references to the entire array.
circleArray[1] references to a Circle object.

Array of Objects, cont.
• Invoking circleArray[1].findArea() involves two
levels of referencing as shown in the next figure.
.
circleArray
reference
circleArray[0]
circleArray[1]
Circle object 0
…
Circle object 1
circleArray[9]
Circle object 9
circleArray references to the entire array.
circleArray[1] references to a Circle object
Summarizing the areas of the circles
// Declaration and creation
Circle[] circleArray = new Circle[10];
for(int k=0; k < circleArray.length; k++) {
circleArray[k] = new Circle(Math.random*100);
}
// Print radius and calculate the total area
double totalArea = 0;
for(k=0; k < circleArray.length; k++) {
System.out.println(“k=“+k+”) R=“+circleArray[k].getRadius());
totalArea += circleArray[k].findArea();
}
System.out.println(“Area=”+ totalArea);