ChildClass.java

public class ChildClass extends ParentClass {
    // no interface to fooBar
}

Main.java

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class Main {
    public static void main(String[] args) {
        ChildClass c = new ChildClass();
        try {
            Method fooBar = c.getClass().getSuperclass().getDeclaredMethod("fooBar");
            fooBar.setAccessible(true);
            fooBar.invoke(c, true);
        } catch (NoSuchMethodException e) {
            System.out.println("Function does not exist!");
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

Output

Function does not exist!
java.lang.NoSuchMethodException: ParentClass.fooBar()
	at java.lang.Class.getDeclaredMethod(Class.java:2130)
	at Main.main(Main.java:8)

ParentClass.java

public class ParentClass {
    private int fooBar(boolean test) {
        return 1;
    }
}