为什么我不能在Java中将对象转换为集合？

Question

我正在为一个类编写一个单元测试，如下所示：

@Test
void testCreateStackResources()
{
    List<StackResource> stackResourceListExpected = new ArrayList<>();
    StackResource stackResource = new StackResource();
    stackResource.setLogicalResourceId("Sample-Logical-ID");
    stackResourceListExpected.add(stackResource);
    ListStackResourcesResult listStackResourcesResult = new ListStackResourcesResult();
    StackResourceSummary stackResourceSummary = new StackResourceSummary();
    stackResourceSummary.setLogicalResourceId("Sample-Logical-ID");
    listStackResourcesResult.setStackResourceSummaries((Collection<StackResourceSummary>) stackResourceSummary); // Problem in this line
    Mockito.when(amazonCloudFormation.listStackResources(Mockito.any(ListStackResourcesRequest.class))).thenReturn(listStackResourcesResult);
    List<StackResource> stackResourceListResult = cloudFormationManager.createStackResources(Mockito.anyString());
    Assert.assertEquals(stackResourceListExpected, stackResourceListResult);
}

现在，当我运行这段代码时，它给我一个错误，即我无法将StackResourceSummary强制转换为Java中的集合。

java.lang.ClassCastException: com.amazonaws.services.cloudformation.model.StackResourceSummary cannot be cast to java.util.Collection

objc[3648]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/bin/java (0x10d19c4c0) and /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/libinstrument.dylib (0x10ea194e0). One of the two will be used. Which one is undefined.

ps：有一个单独的类，名为ListStackResourcesResult，它的setter如下所示：

public void setStackResourceSummaries(java.util.Collection<StackResourceSummary> stackResourceSummaries) {
    if (stackResourceSummaries == null) {
        this.stackResourceSummaries = null;
        return;
    }

    this.stackResourceSummaries = new com.amazonaws.internal.SdkInternalList<StackResourceSummary>(stackResourceSummaries);
}

而我正在尝试使用上面的这种方法。

Answer 1

这是因为StackResourceSummary不扩展或实现任何与集合相关的内容。

http://docs.aws.amazon.com/awsjavasdk/latest/javadoc/com/amazonaws/services/cloudformation/model/stackresourcesummary.html

您需要的是创建一个集合并将StackResourceSummary实例添加到其中。例如如下所示：

List<StackResourceSummary> stackResourceSummaries = new ArrayList<StackResourceSummary>();
stackResourceSummaries.add(stackResourceSummary)

Arrays.asList(stackResourceSummary)

则应使用该集合作为参数。

listStackResourcesResult.setStackResourceSummaries(stackResourceSummaries); // Problem gone in this line