Java Arrays Jussi Pohjolainen Tampere 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

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; ...

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 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]);

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]);

myarray index length = 3 0 1 2 1 12 88

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

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};

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);

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++; }

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++; }

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]); }

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

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

{

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

}

Example 3 int [] 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]); }

int [] 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!

public class CommandLine {

public static void main( String [] args ) {

}

}

Command Line Argument? > java CommandLine Hello World Hello World public class CommandLine { public static void main( String [] args ) { System.out.println(args[0]); System.out.println(args[1]); } }

> java CommandLine Hello World

Hello

World

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)

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 5 10 / 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(&quot;&quot; + a + &quot; / &quot; + b + &quot; = &quot;); System.out.println(result); } }

> java CommandLine 10 5

10 / 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(&quot;&quot; + a + &quot; / &quot; + b + &quot; = &quot;);

System.out.println(result);

}

}