JAVA: How do you use methods from another class if the class I am trying to use

JAVA:
How do you use methods from another class if the class I am trying to use these methods in, has already extended/inherited from another class? For example, I have a GUI class, public class Grid, that creates a GUI of a 6×7 matrix and has a 2D array 6×7 that is populated whenever buttons on the GUI are pressed. All of the logic to add variables to this 2D matrix are located in another class, class Logic. My question is that since class Grid already extends Application, due to the GUI, how do I reference and use the logic code in class Logic?

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