Changing class variables by means of a second and third class

Question

I'm working on a personal project and at the same time experimenting with it. I've got 3 classes contained in 3 files: Calculate.java, Geometry.java, and Test.java.

Geometry.java so far only contains some variables I want to work with, the get/set methods along with a constructor method.

package project;  
public class Geometry {  
    public static double length, width;  
    public Geometry() {  
        this.setLength(20);  
        this.setWidth(30);  
    }  
    public void setLength(double length){  
        this.length = length;  
    }  
    public void setWidth(double width){  
        this.width = width;  
    }  
    public double getLength(){  
        return this.length;  
    }  
    public double getWidth(){  
        return this.width;  
    }  
}

Calculate.java has a public variable of type Geometry and a method to work with the variables I created in Geometry.java.

package project;  
import project.Geometry;  
public class Calculate {  
    public static Geometry figure = new Geometry();  
    public static double area;  
    public void calcArea(){  
        this.area = figure.getLength() * figure.getWidth();  
    }  
    public double getArea(){  
        return this.area;  
    }  
}

Finally, in Test.java I'm creating a variable c1 of type Calculate.

package project;  
import project.Calculate;  
public class Test{  
    public static void main(String[] args){  
        Calculate c1 = new Calculate;  
        Calculate c2 = new Calculate;  
        c1.figure.setLength(55);  
        c2.figure.setLength(75);  
        System.out.println("L1: "+c1.figure.getLength()+" L2: "+c2.figure.getLength());  
    }  
}

The console output is: "L1: 75 L2: 75"

My interpretation of the output is that c1.figure and c2.figure are writing data to the same space in memory and thus, when I called c2.figure.setLength(75) it also altered c1.figure.length.

When I first wrote this code I assumed c1.figure and c2.figure would maintain their own separate values but they don't. Is there a way to achieve this (have c1.figure and c2.figure maintain their own values without altering the other)?

PS: First time I post here, I apologize in advance if I messed up the formatting.

Answer 1

public static Geometry figure = new Geometry();

creates one Geometry object regardless of how many Calculate instances you have. Remove the static keyword (which enforces one instance per class) and each Calculate object will contain one Geometry.

The same applies to your area member, and the length and width in the Geometry object. I don't think in this scenario you require static anywhere (indeed, it's rare that I require it at all in small projects).

Here's the tutorial section on instance variables, which is worth reading.