import org.checkerframework.checker.nullness.qual.*;

// TODO: Add as test
public class RefineArray {
  public static <T> T[] concat(T @Nullable [] a, T @Nullable [] b) {
    if (a == null) {
      if (b != null) {
        return b;
      } else {
        @SuppressWarnings("unchecked")
        T[] result = (T[]) new Object[0];
        return result;
      }
    } else {
      if (b == null) {
        return a;
      } else {
        @SuppressWarnings("unchecked")
        T[] result = (T[]) new @MonotonicNonNull Object[a.length + b.length];

        System.arraycopy(a, 0, result, 0, a.length);
        System.arraycopy(b, 0, result, a.length, b.length);
        return result;
      }
    }
  }
}