Wondering how we can invoke static method g() from non-static method d()

Question

class A {
final int x;
A() {
x = 1;
int f() {
return d(this,this);
int d(A a1, A a2) {
int i = a1.x;
g(a1);
int j = a2.x;
return j - i;
static void g(A a) {
// uses reflection to change a.x to 2
}

Answer 1

public class A {
     final int x;
     A() {
          x = 1;
     int f() throws IllegalArgumentException, SecurityException, IllegalAccessException, NoSuchFieldException {
          return d(this,this);
     int d(A a1, A a2) throws IllegalArgumentException, SecurityException, IllegalAccessException, NoSuchFieldException {
          int i = a1.x;
          g(a1);
          int j = a2.x;
          return j - i;
     static void g(A a) throws IllegalArgumentException, SecurityException, IllegalAccessException, NoSuchFieldException {
          // uses reflection to change a.x to 2
          //a.getClass().getField("x").setInt(a, 5);
          a.getClass().getDeclaredField("x").setInt(a, 5);
     public static void main(String[] args) throws IllegalArgumentException, SecurityException, IllegalAccessException, NoSuchFieldException{
          A a = new A();
          System.out.println(a.f());
throws this exception:
Exception in thread "main" java.lang.IllegalAccessException: Can not set final int field A.x to (int)5
     at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:55)
     at sun.reflect.UnsafeFieldAccessorImpl.throwFinalFieldIllegalAccessException(UnsafeFieldAccessorImpl.java:79)
     at sun.reflect.UnsafeQualifiedIntegerFieldAccessorImpl.setInt(UnsafeQualifiedIntegerFieldAccessorImpl.java:114)
     at java.lang.reflect.Field.setInt(Field.java:802)
     at A.g(A.java:22)
     at A.d(A.java:13)
     at A.f(A.java:9)
     at A.main(A.java:27)