Specification does not say that Bignum may have value "-0".
It can be created by the constructor or achieved as the result of
operation (eg -1*0).
Class Bignum declares only one constant ZERO.

=== Here is the minimized test demonstrating the bug ===
import java.lang.Bignum;
class java_lang_Bignum_equals {
  public static void main(String args[]) {
    Bignum b1=new Bignum("-0");
    Bignum b2=new Bignum("0");
    System.out.println(b1.equals(b2));
  }
}
=== Here is the output of the test ===
false