Arrays in java

An array is a data structure that stores a collection of values of the same type. You access each individual value through an integer index. For example, if a is an array of integers, then a[i] is the ith integer in the array.

You declare an array variable by specifying the array type—which is the element type followed by []—and the array variable name. For example, here is the declaration of an array a of integers:

Code:

int[] a;
 

However, this statement only declares the variable a. It does not yet initialize a with an actual array. You use the new operator to create the array.

Code:


int[] a = new int[100];
 

This statement sets up an array that can hold 100 integers.

NOTE
You can define an array variable either as

Code:

int[] a; or as int a[]; 

Most Java programmers prefer the former style because it neatly separates the type int[] (integer array) from the variable name.

The array entries are numbered from 0 to 99 (and not 1 to 100). Once the array is created, you can fill the entries in an array, for example, by using a loop:

Code:


int[] a = new int[100];
for (int i = 0; i < 100; i++)
   a[i] = i;  // fills the array with 0 to 99

 

practical example Parking Times project using arrays:

The purpose of this program is to reinforce array topics covered in class. Specifically, the assignment is to construct the ParkingTimes class that stores that amount of time spent looking for a parking place each trip to campus and calculates some summary information about these times. Use the test program TestParking.java.

Code:

public class TestParking
{

    public static void main(String[] args)
    {
        ParkingTimes frustration = new ParkingTimes();
        frustration.add(12.3);
        frustration.add(35);
        frustration.add(17);
        frustration.add(5);
        frustration.add(10.3);
        System.out.println ("minimum time spent looking for parking: " + frustration.min());
        System.out.println ("maximum time spent looking for parking: " + frustration.max());
        System.out.println ("average time spent looking for parking: " + frustration.ave());
    }

}
 

This class stores and analyzes the times that ppl spend looking for a parking place.

Code:

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package parkingtimes;

/**
 *
 * @author sam
 */
public class ParkingTimes extends Object{
    
    private double[] times = new double[1000];
    private int num = 0;

    public ParkingTimes() {
    }

    public void add(double t) {
        times[num] = t;
        num++;
    }

    public double ave() {
        if (num <= 0) {
            return 0.0;
        }
        double avg = 0.0;
        for (int i = 0; i < num; i++) {
            avg += times[i];
        }
        return avg / num;
    }

    public double max() {
        if (num <= 0) {
            return 0.0;
        }
        double max = times[0];
        for (int i = 1; i < num; i++) {
            if (times[i] > max) {
                max = times[i];
            }
        }

        return max;
    }

    public double min() {
        if (num <= 0) {
            return 0.0;
        }
        double min = times[0];
        for (int i = 1; i < num; i++) {
            if (times[i] < min) {
                min = times[i];
            }
        }

        return min;
    }
    public void increaseSize(){
        
        double []newarray= new double[times.length*2];
        for(int i=0;i<times.length;i++)
        {
            newarray[i]=times[i];
        }
        times = null;
        times = newarray.clone();
       // System.err.println(times.length);
    }
}