Lecture 8: Arrays

You must read the textbook p165-174 or Java Tutorial arrays(and its links)
This section covers these topics:

Declaring a Variable to Refer to an Array
Creating an Array
Accessing an Array Element
Getting the Size of an Array
Array Initializers

Here's a simple program, named ArrayDemo, the program creates an array, puts some values in it, and displays the values.

public class ArrayDemo {
    public static void main(String[] args) {
        int[] anArray;	        // declare an array of integers

        anArray = new int[10];	// create an array of integers

        // assign a value to each array element and print 
        for (int i = 0; i < anArray.length; i++) {
            anArray[i] = i;
            System.out.print(anArray[i] + " ");
        }
        System.out.println();
    }
}

Declaring a Variable to Refer to an Array

Example:

int[] anArray;
float[] anArrayOfFloats;
boolean[] anArrayOfBooleans;
Object[] anArrayOfObjects;
String[] anArrayOfStrings;

What does it do?

It declares an array.
It does not allocate any memory to contain the array elements.
The sample program must assign a value to anArray before the name refers to an array.

Creating an Array

Example:

anArray = new int[10];  // create an array of integers

Syntax:

new elementType[arraySize]

Error message:

If the new statement was ommitted from the sample program, the following error message would display
ArrayDemo.java:4: Variable anArray may not have been initialized.

What it does?

Allocates an array with enough memory for ten integer elements
Assigns the array to the variable

Accessing an Array Element

Example:

anArray[i] = i;
anArrayOfString[0] = "First string";
anArrayOfBoolean[2] = true; // the third element is ture

Note:

In Java, array indices begin at 0 and end at the array length minus 1.
If you don't initiate a particular element in the array, it will be null.
It is recommended to initiate every element in the array.

Getting the Size of an Array

To get the size of an array, we can use

arrayname.length

Example

anArray.length;
anArrayOfString.length;

Array Initializers

boolean[] answers = { true, false, true, true, false};
int[] anIntArray = {1,2,3,4,5};
String[] fruits = {"apple", "orange", "banana", "peach"};

Arrays of Objects

public class ArrayOfStringsDemo {
   public static void main(String[] args) {
      String[] anArray = { "String One", "String Two", "String Three" };

      for (int i = 0; i < anArray.length; i++) {
          System.out.println(anArray[i].toLowerCase());
      }
   }
}

public class ArrayOfStringsDemo2 {
    public static void main(String[] args) {
       String[] anArray = new String[3];
       String[0] = "String One";
       String[1] = "String Two"; 
       String[2] = "String Three";
       for (int i = 0; i < anArray.length; i++) {
           System.out.println(anArray[i].toLowerCase());
       }
    }
}

What's wrong with this program?

 public class BankAccountTest {
     public static void main(String[] args) {
           BankAccount[] accountArray = new BankAccount[3];
            for(int i = 0;  i < accountArray.lenght; i++) {
                  System.out.println(accountArray[i].getName());
             }
     }
}

Answer

public class BankAccountTest {
     public static void main(String[] args) {
           BankAccount[] accountArray = new BankAccount[3];
            accountArray[0] = new BankAccount("Amy", 500000);
            accountArray[1] = new BankAccount("Benny", 3456);
            accountArray[0] = new BankAccount("Charles", 100000); 

            for(int i = 0;  i < accountArray.lenght; i++) {
                  System.out.println(accountArray[i].getName());
             }
     }
}

You have to initiate the elements in the array. Otherwise you will get a null pointer excepetion.

Arrays of Arrays

This example creates an array and uses an initializer to populate it with four sub-arrays.

public class ArrayOfArraysDemo {
    public static void main(String[] args) {
        String[][] cartoons =
        {
           { "Flintstones", "Fred", "Wilma", "Pebbles", "Dino" },
           { "Rubbles", "Barney", "Betty", "Bam Bam" },
           { "Jetsons", "George", "Jane", "Elroy", "Judy", "Rosie", "Astro" },
           { "Scooby Doo Gang", "Scooby Doo", "Shaggy", "Velma", "Fred", "Daphne" }
        };

        for (int i = 0; i < cartoons.length; i++) {
            System.out.print(cartoons[i][0] + ": ");
            for (int j = 1; j < cartoons[i].length; j++) {
                System.out.print(cartoons[i][j] + " ");
            }
            System.out.println();
        }
   }
}

Output

Flintstones: Fred Wilma Pebbles Dino
Rubbles: Barney Betty Bam Bam
Jetsons: George Jane Elroy Judy Rosie Astro
Scooby Doo Gang: Scooby Doo Shaggy Velma Fred Daphne

As with arrays of objects, you must explicitly create the sub-arrays within an array. So if you don't use an initializer, you need to write code like the following, which you can find in: ArrayOfArraysDemo2

public class ArrayOfArraysDemo2 {
  public static void main(String[] args) {
     int[][] aMatrix = new int[4][];

     //populate matrix
     for (int i = 0; i < aMatrix.length; i++) {
         aMatrix[i] = new int[5];	//create sub-array
         for (int j = 0; j < aMatrix[i].length; j++) {
             aMatrix[i][j] = i + j;
         }
     }

     //print matrix
     for (int i = 0; i < aMatrix.length; i++) {
         for (int j = 0; j < aMatrix[i].length; j++) {
              System.out.print(aMatrix[i][j] + " ");
         }
         System.out.println();
     }
  }

Output

0 1 2 3 4
1 2 3 4 5
2 3 4 5 6
3 4 5 6 7

Summary of Arrays

After this class you should know

how to declare an array (e.g. int[] array;)
how to create an array,

you can use new operator (e.g. array = new int[10];)
use an array initializer. (e.g. array = {1,2,3,4,5};)

Once created, the size of the array cannot be changed.
To get the length of the array, use the length attribute. (e.g. array.length)
An element within an array can be accessed by its index. (e.g. array[0], array[1])
Indices begin at 0 and end at the length of the array minus 1.