code: public FieldDescriptor getFieldDescriptor(@SlashedClassName String className, String name, String signature, boolean isStatic) {
          FieldDescriptor fieldDescriptor = new FieldDescriptor(className, name, signature, isStatic);
          FieldDescriptor existing = fieldDescriptorMap.get(fieldDescriptor);
          if (existing == null) {
              fieldDescriptorMap.put(fieldDescriptor, fieldDescriptor);
              existing = fieldDescriptor;
          }
          return existing;
      }
docs: Get a FieldDescriptor.
  
  @param className
  the name of the class the field belongs to, in VM format
  (e.g., "java/lang/String")
  @param name
  the name of the field
  @param signature
  the field signature (type)
  @param isStatic
  true if field is static, false if not
  @return FieldDescriptor