Wildcard Type Mismatch (int) 编译错误，但不是 (String)

Question

我已经阅读了几个与我的类似的问题，但我仍然不明白发生了什么，或者为什么它只发生在 (int) 而不是其他更复杂的类型上：

    HashMap<?, ?> returncp = startAndWaitForJob("DSP_WF_" + operation, cp);

    DSPResponseDTO dto = (DSPResponseDTO) returncp.get("RESPONSE_OBJECT");
    String respDesc = (String) returncp.get("statusInfoResponseDescription");
    int respCode = (int) returncp.get("statusInfoResponseCode");

编译错误：

[javac]         int respCode = (int) returncp.get("statusInfoResponseCode");
[javac]                                             ^
[javac]   required: int
[javac]   found:    CAP#1
[javac]   where CAP#1 is a fresh type-variable:
[javac]     CAP#1 extends Object from capture of ?

已阅读的问题：

incompatible types and fresh type-variable

Bounded-wildcard related compiler error

Java: Wildcard Types Mismatch Results in Compilation Error

Answer 1

我不知道你的地图里有什么类型，但是这里

String respCode = (int) returncp.get("statusInfoResponseCode");

您将 get 调用的结果转换为 int 并将其分配给 String。

如果是Integer，使用

Integer respCode = (Integer) returncp.get("statusInfoResponseCode");

Answer 2

首先是什么CAP#1

我找到了这个 quote:

What on earth does "capture#337 of ?" mean? When the compiler encounters a variable with a wildcard in its type, such as the box parameter of rebox(), it knows that there must have been some T for which box is a Box. It does not know what type T represents, but it can create a placeholder for that type to refer to the type that T must be. That placeholder is called the capture of that particular wildcard. In this case, the compiler has assigned the name "capture#337 of ?" to the wildcard in the type of box. Each occurrence of a wildcard in each variable declaration gets a different capture, so in the generic declaration foo(Pair x, Pair y), the compiler would assign a different name to the capture of each of the four wildcards because there is no relationship between any of the unknown type parameters.

现在让我们回到消息CAP#1 extends Object from capture of ?

它认为 CAP#1 是 Object 的一个实例，更像是 class 扩展 class Object.[=25 的一个实例=]

现在需要想象泛型在编译时被转换为强制转换。这意味着：

HashMap<?, ?> map = new HashMap();
int test = (int) map.get("");

大致相当于：

HashMap<?, ?> map = new HashMap();
int test = (int) (Object) map.get("");

不可能（或编译器不允许）将 Object 转换为 'int'。当然，这个Object是用来表示简单的

但是完全可以将 Object 转换为 'String' 或 'Integer'。

所以你可以做的是：

HashMap<?, Integer> returncp = startAndWaitForJob("DSP_WF_" + operation, cp);

在那种情况下，您甚至不需要投任何东西 (see)。

但如果由于任何原因无法做到：

int respCode = ((Integer) returncp.get("statusInfoResponseCode")).intValue();

Wildcard Type Mismatch (int) 编译错误，但不是 (String)

Wildcard Type Mismatch (int) compilation error, but not (String)

java

wildcard