Java Arrays

Jussi PohjolainenTampere University of Applied Sciences

Types

• Java has two kind of types– Primitive Types• int, byte, short, long, double, float, boolean, char

– Class Types• Everything else, for example

– String, Scanner, Arrays, Vector, JButton, JCheckBox

Problem?

• Implement an application that will calculate 100 students exam average.

• Variables needed?– int studentA;– int studentB;– int studentC;– int studentD;– ...

Several Variables at Once?

• Array comes to the rescue!• Just a list of variables• Declare the array

– int [] array;

• Initialize the array and set it's size– array = new array[3];

• Store values into array– array[0] = 2;– array[1] = 3;– array[2] = 7;

Array

int [] myarray = new int[3];myarray[0] = 1;myarray[1] = 12;myarray[2] = 88;System.out.println(myarray[0]);System.out.println(myarray[1]);System.out.println(myarray[2]);

0 1 21 12 88

myarray

index

length = 3

Indexes

• The values can be retrieved from the array using index.

• The first value is found from index 0 and the last from length – 1– int [] myarray = new int[LENGTH];– myarray[0] = 1; // first one

– myarray[LENGTH-1] = 22; // last one

Initializing the Array with {}

int [] myarray = new int[3];myarray[0] = 2;myarray[1] = 8;myarray[2] = 12;

<=>

int [] myarray = {2,8,12};

Common Problems

int [] myarray1 = {2,8,12};int [] myarray2 = new int[3];

// Output?System.out.println(myarray1[3]);// Output?System.out.println(myarray2[0]));// Output?System.out.println(myarray1);

Example 1

int [] myarray = {2,8,12};int i = 0;while(i < 3){ System.out.println(myarray[i]);

i++;}

Example 2

int [] myarray = {2,8,12};int i = 0;while(i < myarray.length){ System.out.println(myarray[i]);

i++;}

Example 3

int [] myarray = {2,8,12};

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

{ System.out.println(myarray[i]);

}

Example 3int [] myarray = new int[3];

for(int i = 0; i < myarray.length; i++){ myarray[i] = myscanner.nextInt();}

for(int i = myarray.length-1; i >= 0; i++){ System.out.println(myarray[i]);}

COMMAND LINE ARGUMENTS

Command Line Argument?

public class CommandLine { public static void main(String [] args) {

}}

Declaration of an array!

Command Line Argument?

> java CommandLine Hello WorldHelloWorld

public class CommandLine { public static void main(String [] args) {

System.out.println(args[0]); System.out.println(args[1]);

}}

About Command Line Argument

• Command line argument is user input from the command line

• Argument array is initialized automatically for you (size and content)

What can go wrong?> java CommandLine 10 510 / 5 = 2

public class CommandLine { public static void main(String [] args) { int a = Integer.parseInt(args[0]); int b = Integer.parseInt(args[1]); int result = a / b; System.out.print("" + a + " / " + b + " = ");

System.out.println(result); }}