如何从包（java）中的另一个类访问主类中的变量？

Question

I'm doing my homework and I have a hard time finding the problem here. The program I wrote doesn't compile. Please help. Homework details:

"Create a class called Airport with the following fields: Identifier. Coordinates which consist of Latitude and Longitude (Do not create two of each !!). Latitude is positive to indicate is North of the Equator and negative when it's located in the southern hemisphere. Longitude is negative to indicate is West and positive to indicate it's East of the Greenwich median. A magnetic variation which also is indicated negative for West and positive for East. It is OK to have no magnetic variation. Elevation above sea level in feet.

Add a static method that accepts four doubles which are two set off coordinates (double lat1, double long1, double lat2, double long2) and returns the distance in nautical miles using the formula giving in Lab 05. For example San Diego airport has the values ID: SAN, Lat: 32.7335556, Long: -117.1896667, Var: 14, Elev: 16.8' ( http://www.airnav.com/airport/SAN ) The class should have an accessor and mutator methods for each field."

I did most of the work here but I think I need to add constructors here. Please help.

Main class:

package lab06;

import javax.swing.JOptionPane;

public class Lab06 
{
    public static void main(String[] args) {
        double number;       // To hold the number
        String input;        // To hold user input

        //Create two Airport objects.
        Airport firstAirport = new Airport();
        Airport secondAirport = new Airport();

        // Get and store the coordinates for firstAirport.
        input = JOptionPane.showInputDialog("Enter the first Latitude: ");
        number = Double.parseDouble(input);
        firstAirport.setLatitude(number);
        input = JOptionPane.showInputDialog("Enter the first Longitude: ");
        number = Double.parseDouble(input);
        firstAirport.setLongitude(number);
        input = JOptionPane.showInputDialog("Enter the first Elevation: ");
        number = Double.parseDouble(input);
        firstAirport.setElevation(number);

        // Get and store the coordinates for secondAirport.
        input = JOptionPane.showInputDialog("Enter the second Latitude: ");
        number = Double.parseDouble(input);
        secondAirport.setLatitude(number);
        input = JOptionPane.showInputDialog("Enter the second Longitude: ");
        number = Double.parseDouble(input);
        secondAirport.setLongitude(number);
        input = JOptionPane.showInputDialog("Enter the second Elevation: ");
        number = Double.parseDouble(input);
        secondAirport.setElevation(number);
    }

    // The Distance method calculates the distance in nautical miles
    public static void getDistance(String[] args) 
    {
        double R = 3440;
        double dist = Math.sin(firstAirport.getLatitude())
                * Math.sin(secondAirport.getLatitude())
                + Math.cos(secondAirport.getLatitude())
                * Math.cos(firstAirport.getLatitude())
                * Math.cos(firstAirport.getLongitude()
                        - secondAirport.getLongitude());
        dist = Math.acos(dist);
        dist = dist * R;

        // Display result in nautical miles.
        JOptionPane.showMessageDialog(null,
                "The distance in nautical miles is: %.1f\n" + dist);

        System.exit(0);
    }
}

and the Airport class ....

package lab06;

public class Airport 
{
    public double latitude;
    public double longitude;
    public double elevation;

    //The setLatitude method stores a value in the latitude field.
    public void setLatitude(double latitude)
    {
        this.latitude = latitude;
    }
    //The setLongitude method stores a value in the longitude field.
    public void setLongitude(double longitude)
    {
        this.longitude = longitude;
    }
    //The setElevation method stores a value in the elevation field.
    public void setElevation (double elevation)
    {
        this.elevation = elevation;
    }
    //The getLatitude method returns an Airport object's latitude.
    public double getLatitude()
    {
        return latitude;
    }
    //The getLongitude method returns an Airport object's longitude.
    public double getLongitude()
    {
        return longitude;
    }
    //The getElevation method returns an Airport object's elevation.
    public double getElevation()
    {
        return elevation;
    }
}

Answer 1

How do I access variables in the main class from another class in the package (java)?

I presume you are asking about accessing local variables declared in the main >>method<<. The simple answer is that you CAN'T.

But you can pass the value of a variable to another class as a method or constructor argument.

I did most of the work here but I think I need to add constructors here.

Yes. That would be a good idea.

Please help

Hint: read your lecture notes / text book / an online Java tutorial on how to write a constructor.

Answer 2

The way I read the question, the static method should be provided with the data and not know that it is calculating the distances between airports. If the intent was to do that, it would need to take in two Airport objects rather than 4 doubles as described.

Note: As a general rule, avoid making any mutable data statically available (except for caching).

Answer 3

Here is an example of how to pass 3 doubles to Airport :

public class Airport {

    public double latitude;
    public double longitude;
    public double elevation;

    public Airport(double latitude, double longitude, double elevation) {

        this.latitude = latitude;
        this.longitude = longitude;
        this.elevation = elevation;

    }

    //if you need to access variables you add get methods like:
    public double getLatitude(){
       return latitude;
    }

    public static void main( String[] args) {

        Airport ap = new Airport(30.34567, 27.6789,  -140); 
        System.out.println("Airport latitude is "+ ap.getLatitude());
    }

}