public SignatureResult addConstant(ConstantEntity constant) {
        constants.put(constant.getName(), constant);
        constant.setClazz(this);

        SignatureResult result = new SignatureResult();

        if (isInterface() && !constant.isPublic()) {
            result.addConstant(InvalidConstant.error(InvalidConstant.Kind.MUST_BE_PUBLIC_FOR_INTERFACE, constant, null));
        }

        return result;
    }
return empty list if success else list with not valid implemented methods in class

@param _interface
@return
jphp-runtime/src/php/runtime/reflection/ClassEntity.java