Java错误:不存在类型变量C、T的实例？扩展SubComponent<T>可以转换为C

Question

我有一个相当复杂的阶级结构：

public interface SubComponent<T> {...}

public interface Component<T, C extends SubComponent<T>> {...}

public class Control<T, I extends Component<T, ? extends SubComponent<T>>> {...}

然后，我有两个类，它们将保存控件和每个组件的当前状态，如下所示：

public class ControlState<T, I extends Component<T, ? extends SubComponent<T>>> {
    // The state keeps a reference to the Control,
    // and a map that holds all the states for each component
    private final Control<T, I> control;
    private final Map<Integer, ComponentState<T, ? extends SubComponent<T>>> components = new TreeMap<>();

    // Has a method to add new components
    public void addComponent(int index) {
        // Here I have error on the control parameter
        ComponentState<T, ? extends SubComponent<T>> state = new ComponentState<>(control, index);
        ...
    }
}

public class ComponentState<T, C extends SubComponent<T>> {
    // The component state also has a reference to the Control
    // and the index to retrieve the Component from a List in the Control
    private final Control<T, ? extends Component<T, C>> control;
    private final int index;

    public ComponentState(Control<T, ? extends Component<T, C>> control, int index) {
        this.control = control;
        this.index = index;
    }
}

在addComponent(int index)方法中，IDE说：

所需类型：Control<T, ? extends Component<T, C>>

提供：Control<T, I>

但是，既然我是：I extends Component<T, ? extends SubComponent<T>>，我不明白问题在哪里，类型应该是兼容的，我做错了什么？

Answer 1

作为解决办法，我决定通过从I类中删除Control信息来简化模型。

因此，结构如下：

public interface SubComponent<T> {...}

public interface Component<T, C extends SubComponent<T>> {...}

public class Control<T> {
    private final List<? extends Component<T, ? extends SubComponent<T>>> components;
}

public class ControlState<T> {
    private final Control<T> control;
    private final Map<Integer, ComponentState<T>> components = new TreeMap<>();

    public void addComponent(int index) {
        ComponentState<T> state = new ComponentState<>(control, index);
        ...
    }
}

public class ComponentState<T> {
    private final Control<T> control;
    private Component<T, ? extends SubComponent<T>> component;
    private int index;
    private final Map<Integer, ? extends SubComponent<T>> subComponents = new TreeMap<>();

    public ComponentState(Control<T> control, int index) {
        this.control = control;
        this.index = index;
    }
}

尽管如此，我还是希望保留这些信息，但我认为我很幸运，这意味着，只要我告诉用户我需要的组件必须是这两个接口的实现，那么在内部，我就拥有了我需要的所有API(由接口公开)，以使系统正常工作。

Answer 2

我认为这应该能恰当地或不恰当地解决你的目标。因此，这是我的想法。

在addComponent()类的ControlState方法中，您试图实例化ComponentState类。现在，在这样做时，可以省略类型参数。在这里，内存分配将像没有类型参数(即非泛型类型)的ComponentState类的对象一样发生，但是，实际对象仍将被称为泛型类型。

此外，您的控件对象也没有实例化(或者为null或正确)。

public class ControlState<T, I extends Component<T, ? extends SubComponent<T>>> {
    // The state keeps a reference to the Control,
    // and a map that holds all the states for each component
    private final Control<T, I> control = null;
    private final Map<Integer, ComponentState<T, ? extends SubComponent<T>>> components = new TreeMap<Integer, ComponentState<T, ? extends SubComponent<T>>>();

    // Has a method to add new components
    public void addComponent(int index) {
        // Here I have error on the control parameter
        ComponentState<T, ? extends SubComponent<T>> state = new ComponentState(control, index);
    }
}

Answer 3

编译器不能统一ComponentState.control字段和ComponentState构造函数的通配符，因此类型错误。

您可以通过物化通配符所代表的类型来修复这个问题，以匹配I在ControlState.state字段中所代表的内容：

public class ComponentState<T, I extends Component<T, ? extends SubComponent<T>>> {
    private final Control<T, I> control;
    private final int index;

    public ComponentState(Control<T, I> control, int index) {
        this.control = control;
        this.index = index;
    }
}

您必须相应地调整components映射的类型。

private Map<Integer, ComponentState<T, I>> components = new TreeMap<>();

现在您可以将ControlState.addComponent实现为

public void addComponent(int index) {
    ComponentState<T, I> state = new ComponentState<>(control, index);
    components.put(index, state);
}