I wanted to declear several class variables but I keep getting error:
The field spacingX cannot be declared static; static fields can only be declared in static or top level types.

WHY? I read all the examples in processing and the link to java's static fields and methods but even the java's example didn't work.

public class buttons
{
 public int left;
 public int top;
 private static int spacingX, spacingY;
}

Yes, it is an annoying limitation of Java.
Somehow, static variables are similar to global variables, so you can just declare them at sketch level (before setup() or before you class definition. It is ugly and doesn't respect encapsulation (hiding these variables from other classes if needed) but it is simple.
Or you declare your classes in .java files, the classes will be top-level classes instead of nested ones, with more possibilities (but some restrictions, like needing a PApplet reference to use Processing functions).