How to access getter methods of another class into main method

I have an abstract class GUIelements, with two (non-static) "getter methods";

public boolean isHighlighted(){
  return highlighted;
 }

public String getText(){
  return text;
 }

I'm trying to convert the number entered with StdDraw into degrees Celcius in the main method. Here's the method:

public static void calculate(){
  String choice = (String)RadioButton.getText();

  if(RadioButton.isHighlighted() && choice.equals("Temperature(C toF)")){

    String text = Text.getText(); 
    double number = Double.parseDouble(text);
    double result = number*1.8 + 32;
   }
  }

RadioButton is a class that draws 3 circles and let's the user select which conversion they want.

My error is telling me that:

Error: Cannot make a static reference to the non-static method getText() from the type GUIelements.

I know that this is right, as I wouldn't be able to access this text (converted into a number) because getText() is non-static. But I have no other idea of how to get this text and subsequently convert it.