How do use Global varibales in a function argument?

So my issue is that I cannot use my global variables (a,b,c) in my arguments. I need to be able to use them in my boolean function and double function. What am I doing wrong? How can I fix this?

 public class triareamain extends javax.swing.JFrame {
double a, b, c; 
public void DisplayError() {
side1input.setText("Error");
side2input.setText("Type");
side3input.setText("+ Integers");
}
public double areaCal(double a, double b, double c) {
double s = (a + b + c) / 2;
double area = Math.sqrt(s * (s - a) * (s - b) * (s - c));
return area;
}
public static boolean isValid(double a, double b, double c) {
if (a > b + c || b > a + c || c > a + b) {
return true;
} else {
return false;
}
}
private void calculatebuttonActionPerformed(java.awt.event.ActionEvent evt) {                                                
try {
a = Double.valueOf(side1input.getText());
b = Double.valueOf(side2input.getText());
c = Double.valueOf(side3input.getText());
boolean area = isValid();
if (area == false) {
double finalarea = areaCal();
} else {
DisplayError();
}
} catch (NumberFormatException e) {
side1input.setText("Error");
side2input.setText("Type");
side3input.setText("+ Integers");
}

a, b, and c are not global variables. Java doesn't have that concept. They are fields of class triareamain.

However you also created parameters of the same name, so those names are shadowing the fields.

If you wanted the areaCal() method to use the fields directly, remove the parameters:

public double areaCal() {

If you want the method to use the parameters, then pass values in the call:

double finalarea = areaCal(a, b, c);

If you keep the parameters, I highly recommend that you rename either the fields or the parameters. Shadowing of variable names is very confusing to the programmer, and will in high probability be the cause of bugs.