code: public String getTypeString() {
          if (isPrimitive()) {
              return null;
          }
          if (typeString == null) {
              Class<?> t = getTypeInternal();
              String typeName = t.getName().replace('.', '/');
              String str = (t.isArray()) ? typeName : ("L" + typeName + ';');
              typeString = str.intern();
          }
          return typeString;
      }
docs: Gets the type signature used by the VM to represent the type of this
  field.
  
  @return the signature of this field's class or {@code null} if this
  field's type is primitive.