java - 上限泛型的问题

Question

我在处理包含使用泛型的成员的类时遇到了一些问题。考虑以下示例类:

class BaseRequest {
    // Content goes here
}

class SubRequest extends BaseRequest {
    // IFace methods go here, etc.
}

// Simple datastructure interface
interface Structure<T> {

    List<T> getElementsList();


    Collection<T> getDependencies(T t);
}

// Simple structure implementation, details unimportant
class StructureImpl<T> implements Structure<T> {

    @Override
    public List<T> getElementsList() {
        // Implementation details not important
        return null;
    }


    @Override
    public Collection<T> getDependencies(T t) {
        // implementation details not important
        return null;
    }
}

然后我有以下使用结构的类

class StructureUsingObject {
    final Structure<BaseRequest> structure;


    public StructureUsingObject(Structure<BaseRequest> structure) {
        this.structure = structure;
    }


    public void foo() {
        for (BaseRequest request : structure.getElementsList()) {
            Collection<BaseRequest> requests = this.structure.getDependencies(request);
            // do things
        }
    }
}

这本身是行不通的；

public void foo() {
    StructureImpl<BaseRequest> baseImpl = new StructureImpl<BaseRequest>();
    StructureUsingObject baseStruct = new StructureUsingObject(baseImpl);

    StructureImpl<SubRequest> subImpl = new StructureImpl<SubRequest>();
    // Compile error on this line
    StructureUsingObject subStruct = new StructureUsingObject(subImpl);
}

这是有道理的； StructureImpl 实际上并不是 StructureImpl 的子类。因此，我尝试修改 StructureUsingObject 以使用通配符:

    class StructureUsingObject {
    final Structure<? extends BaseRequest> structure;


    public StructureUsingObject(Structure<? extends BaseRequest> structure) {
        this.structure = structure;
    }


    public void foo() {
        for (BaseRequest request : structure.getElementsList()) {
            // Compile error on this line
            Collection<BaseRequest> requests = this.structure.getDependencies(request);

            // To clarify, a compile error would happen here as well; the issue isn't with the return type
            this.structure.getDependencies(request);

            // do things
        }
    }
}

这也不起作用；

The method getDependencies(capture#4-of ? extends BaseRequest) in the type Structure<capture#4-of ? extends BaseRequest> is not applicable for the arguments (BaseRequest)

好吧，这也是有道理的，尽管自动完成工具提示没有(它指出 getDependency 应该将 (null t) 作为其参数)。我不太确定从这里该去哪里。我无法在 foreach 中使用通配符。如何满足我的 StructureUsingObject 可以接受具有 BaseRequest 子类的结构的要求，同时还能够使用列表中的元素调用 getDependency？

Answer 1

调整您的最新版本以捕获通配符。

public void foo() {
    bar(this.structure);
}

private <T extends BaseRequest> void bar(Structure<T> s) {
    for (T request : s.getElementsList()) {
        Collection<T> requests = s.getDependencies(request);

        // do things
    }
}