你可以这样开始：

Map<String, String> responseMap = requestList.stream().map(requestedItem -> {
            int quantity = productInDbMap.get(requestedItem.getProductId());

            if (quantity >= requestedItem.getRequestedQuantity()) {
                return new AbstractMap.SimpleEntry<>(requestedItem.getProductId(), "order-able");
            } else {
                int availableQuantity = quantity - requestedItem.getRequestedQuantity();
                String s = String.valueOf(availableQuantity);
                return new AbstractMap.SimpleEntry<>(requestedItem.getProductId(), s);
            }
        }).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

一般来说，不要在流或lambda内部修改任何东西，编译器会允许这种情况（如果修改变量，编译器会抱怨），但这是一种不好的做法，也是不安全的。

您可以直接采集到Map：

Map<String,String> responseMap = requestList.stream()
            .collect(Collectors.toMap(ProductInStockRequest::getProductId, requestedItem ->  {
            int quantity = productInDbMap.get(requestedItem.getProductId());

            if (quantity >= requestedItem.getRequestedQuantity()) {
                return"order-able";
            } else {
                int availableQuantity = quantity - requestedItem.getRequestedQuantity();
                return String.valueOf(availableQuantity);
            }
        }));

如果您使用forEach，您的方法将运行良好：

Map<String,String> responseMap = new HashMap<>()
requestList.stream()
            .forEach(requestedItem ->  {
                int quantity = productInDbMap.get(requestedItem.getProductId());

                if (quantity >= requestedItem.getRequestedQuantity()) {
                    responseMap.put(requestedItem.getProductId(), "order-able");

                } else {
                    int availableQuantity = quantity - requestedItem.getRequestedQuantity();
                    String s = String.valueOf(availableQuantity);
                    responseMap.put(requestedItem.getProductId(), s);
                }
        });