Arrays and StringsIntroducing ArraysDeclaring ArraysCreating ArraysInitializing ArraysArray of ObjectsCopying ArraysMultidimensional ArraysCommand-Line Parameters

Introducing Arrays

double[] myList = new double[10]

In computer science, an array is a data structure that represents a collection of the same types of data. Java treats these arrays as objects.

An Array of 10 Elementsof type double

Declaring Arrays

datatype[] arrayname;

Example:

int[] myList;

datatype arrayname[];

Example:

int myList[];

Creating Arrays

arrayName = new datatype[arraySize];

Example:myList = new double[10];

An array is considered to be an object. Thus:

myList is really a reference to 10 doubles and a field called length that contains the array’s size. For example, myList.length contains 10.

Declaring and Creatingin One Step

datatype[] arrayname = new datatype[arraySize];

double[] myList = new double[10];

or

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};

Enhanced for statement

Used to iterate through the elements of an array or collection without using a counter.

Form:

for (parameter: arrayName)

statement; Parameter has two parts – a type and an

identifier ArrayName is the array through which to

iterate.

Enhanced for

Example:total = 0;for (int count=0; count < array.length; count ++) total += array [count];

Is the same as:

total = 0;for(int number: array)

total += number;number is actually taking on array[0], array[1],

etc.

Using Arrays in a Gradebook

Objective: Use an array for a grade book used by a professor to store and analyze a set of student grades.

Figure 7.14 in Java book (page 317…) and Figure 7.15 on page 321.

Array of Objects

Declaring and creating:

Circle[] circleArray = new Circle[10];

Initializing:

for (int i=0; i<circleArray.length; i++)

{

circleArray[i] = new Circle();

}

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(sourceArray, 0, targetArray, 0, sourceArray.length);

Multidimensional Arrays

Multidimensional arrays with two dimensions are often used to represent tables of values consisting of information arranged in rows and columns called a two-dimensional array. Example:

int[][] matrix = new int[10][10]; or

int matrix[][] = new int[10][10];

for (int i=0; i<matrix.length; i++) for (int j=0; j<matrix[i].length; j++) { matrix[i][j] = (int)(Math.random()*1000);

}

Nested array initializers

A 2D array can be initialized when it is declared. Example:

int b[][] = { {1, 2},

{3, 4},

{5, 6}};

creates the following array containing the values shown

1 2

3 5

5 6

How does Java treat a 2D array?

A 2D array is thought of as an array of arrays.

For example if b is an array with 3 rows and 2 columns, it is considered to be a one dimensional

array with 3 objects. Each object is an array with 2 elements.b[0] {1, 2}b[1] {3, 4}b[2] {5, 6}

2D arrays with rows of varying lengths

The way Java represents 2D arrays makes them very flexible. Lengths of rows are not required to be the same:

int b[][] = {{1, 2}, {3, 4, 5, 6}};

makes b look like:

b[0] {1, 2}

b[1] {3, 4, 5, 6}

2D Array creation examples

int a[][] = new int[4][2];

int a[][];a = new int [4][2];

int c[][];c = new int[3][]; //create 3 rowsc[0] = new int[3];//create 3 columns for row 0C[1] = new int[2];//create 2 columns for row 1C[2] = new int[4];//create 4 columns for row 2

length field for 2D arrays

For a 2D array declared as:

int B[][]=new int[2][3];

there are several “length” fields B.length contains 2, the number of

rows

B[0].length contains 3, the number of columns in row 0.

B[1].length contains 3, the number of columns in row 1, etc.

Working with 2D arrays

int total = 0;for (int row = 0; row < a.length; row++){

for (int column = 0; column < a[row].length; column++)total += a[row][length];

{Orfor (int rows[] : a) //loop thru rows of a{ //loop thru columns of the current row

for (int oneValue : rows)total += oneValue;

}

Example:

Revisit grade book using 2D arraysFigures 7.18 and 7.19

Command-Line Parameters

class TestMain

{

public static void main(String[] args)

{ ... }

}

java TestMain arg0, arg1, arg2, ..., argn

ProcessingCommand-Line Parameters

In the main method, get the arguments from args[0], args[1], ..., args[n], which corresponds to arg0, arg1, ..., argn in the command line.

Example: Using Command-Line Parameters

Objective: Write a program that will perform binary operations on integers. The program receives three parameters: an operator and two integers.

Java TestCommandParameters + 2 3

Java TestCommandParameters - 2 3

Java TestCommandParameters / 2 3