Because this
refers to the object instance. There is no object instance in a call of a static method. But of course you can access your static field (only the static ones!). Just use
class Sub {
static int y;
public static void foo() {
y = 10;
}
}
If you want to make sure you get the static field y
and not some local variable with the same name, use the class name to specify:
class Sub {
static int y;
public static void foo(int y) {
Sub.y = y;
}
}