上限有限泛型的麻烦

Question

我在使用包含使用泛型的成员的类时遇到了一些麻烦。 请考虑以下示例类：

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Structure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
        }
    }
}

就其本身而言，这是行不通的;

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 <BaseRequest>不是StructImpl <SubRequest>的子类。 所以，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，虽然自动完成工具提示没有（它声明getDependencies应该有（null t）作为其参数）。 我不确定从哪里开始。 我不能在foreach中使用通配符。 如何满足我的StructureUsingObject可以接受具有BaseRequest子类的Structures的要求，同时还能够使用列表中的元素调用getDependencies？

Answer 1

调整您的上一个版本以捕获通配符。

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
    }
}