使用 Map 的方法“getOrDefault()”

Question

I want to get flashAttribute when from one page is redirected to another page and set it to model. And I try to use this code:

Map<String, ?> map = RequestContextUtils.getInputFlashMap(request);
if (map != null) {
    // this is redirect
    model.addAttribute("attr", map.getOrDefault("attr", false));
}

I have an error:

The method getOrDefault(Object, capture#3-of ?) in the type Map is not applicable for the arguments (String, boolean)

What can I do to solve this problem with getOrDefault("attr", false) method?

Answer 1

You need to make an unchecked cast of the map:

Map<String, Object> map =
    (Map<String, Object>) RequestContextUtils.getInputFlashMap(request);

The wildcard in the returned map type is rather wrong, it should have been Map<String, Object> .

In both cases, get(K) returns Object , but the wildcard forbids the methods with ? type as an argument such as V in put(K, V) , which might pollute the type-correctness of the map contents.

The problem is that this also affects methods such as getOrDefault(K, V) , which do not actually modify the map.

Answer 2

对于通配符值，您可以简单地将调用getOrDefault map为您想要的值类型：

    model.addAttribute("attr", ((Map<String, Boolean>) map).getOrDefault("attr", false));