枚举中的Java匿名类编译错误

Question

我可以解决这个问题，但我不能理解它，所以我要求一些解释(也许还有一个更好的问题标题)。

请考虑一下:

public class TBGService {
    // TBGObject is an abstract base class which is extended by model classes
    public <T> T doGet(TBGObject model) throws TBGServiceException {
        String uri = model.buildUrl(repository) + model.getObjectKey();
        GetMethod method = new GetMethod(uri); 
        T returned = execute(method, credentials, model.getClass());
        return returned;
    }
}

:

public enum TBGTaskAttributes {
    private TBGTaskAttributes(String id, String type, String label, Object... flags) {
        builder = new TaskAttributeBuilder();
        builder.withId(id).withLabel(label);
        for (Object flag : flags) {
            processFlag(flag);
        }
    }
    public abstract String getValueFromIssue(TBGIssue issue);
    public abstract void setValueInIssue(TBGIssue issue, String value);
}

当我写这段代码来定义一个enum项时:

PROJECT(TaskAttribute.PRODUCT, TaskAttribute.TYPE_SINGLE_SELECT, "Project", new OptionProvider() {
    @Override
    public Set<Entry<String, String>> getOptions(TaskRepository repository) {
        try {
            List<TBGProject> list = TBGService.get(repository)
                .doGet(new TBGProjects()).getProjects();
            [...]
            return map.entrySet();
         } catch (TBGServiceException e) { [...] }
         return null;
    }
}) {
    @Override
    public String getValueFromIssue(TBGIssue issue) {
        return issue.getProjectKey();
    }
    @Override
    public void setValueInIssue(TBGIssue issue, String value) {
        issue.setProjectKey(value);
    }
},
[... other items ...]

我得到编译器错误(也eclipse自动完成不工作):

The method getProjects() is undefined for the type Object

如果我将doGet方法悬停，eclipse会显示它的定义如下:

 <Object> Object TBGService.doGet(TBGObject model)

在其他地方，悬停显示的签名正确为:

 <TBGProjects> TBGProjects TBGService.doGet(TBGObject model)

当参数为new TBGProjects()时。

刚刚更改:

List<TBGProject> list = TBGService.get(repository)
    .doGet(new TBGProjects()).getProjects();

:

TBGProjects projects = TBGService.get(repository).doGet(new TBGProjects());
List<TBGProject> = projects.getProjects();

使它工作。但是这里发生了什么?我错过了什么?

Answer 1

Java根据方法的返回值来推断T的类型。

如果你不把返回值赋给任何东西，Java不知道T应该是什么。

要解决这个问题，您可以将参数更改为T类型，以便Java可以从您传递的参数中推断出T:

public <T extends TBGObject> T doGet(T model) throws TBGServiceException {