Java Arrays
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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); }}