Cannot make a static reference to the non-static method fxn(int) from the type Two

Since the main method is static and the fxn() method is not, you can’t call the method without first creating a Two object. So either you change the method to:

public static int fxn(int y) {
    y = 5;
    return y;
}

or change the code in main to:

Two two = new Two();
x = two.fxn(x);

Read more on static here in the Java Tutorials