If I understand the question correctly, you're asking how an object would use a method. Basically just use <objectname>.<methodname>();"
For example:
public class Calculator(){
private int x = 4;
private int y = 5;
public void add(){
System.out.println(x+y);
}
}
In your main method, you would instantiate Calculator by saying
Calculator c = new Calculator();
and you could add by saying
c.add();