问题如下：
有一个ErrorMessageBuilder，实现这个接口的类的示例可以根据传递给它们的异常生成消息，此外，还有一个选项来检查类是否可以处理传递的异常。

interface ErrorMessageBuilder<T extends Exception> {
    String buildMessage(T exception);
    boolean canHandle(Class<? extends Exception> clazz);
}

接下来，有一个类ExceptionHandlerPair，用于创建对（Exception+Handler）

class ExceptionHandlerPair<T extends Exception>{
    private final Class<T> clazz;
    private final ErrorMessageBuilder<? extends Exception> builder;

    public ExceptionHandlerPair(Class<T> clazz, ErrorMessageBuilder<? extends Exception> builder) {
        if(!builder.canHandle(clazz)){
            throw new IllegalArgumentException("Handler can't handle this exception");
        }

        this.clazz = clazz;
        this.builder = builder;
    }

    public boolean canHandle(Class<? extends Exception> clazz) {
        return this.clazz.isAssignableFrom(clazz);
    }

    public ErrorMessageBuilder<? extends Exception> getBuilder() {
        return builder;
    }
}

之后，我尝试创建一个Bilder并创建对。例如，我创建了一个RuntimeExceptionErrorMessageBuilder，它可以处理任何RuntimeException：

class RuntimeExceptionErrorMessageBuilder implements ErrorMessageBuilder<RuntimeException> {
    public String buildMessage(RuntimeException exception) {
        return "Some runtime exception";
    }

    @Override
    public boolean canHandle(Class<? extends Exception> clazz) {
        return RuntimeException.class.isAssignableFrom(clazz);
    }
}

public class SomeClass {
    public static void main(String[] args) {
        var builder = new RuntimeExceptionErrorMessageBuilder();

//        builder.canHandle(NullPointerException.class); // ? -> true
//        builder.canHandle(IllegalArgumentException.class); // ? -> true
//        builder.canHandle(Exception.class); // ? -> false

        var pair = new ExceptionHandlerPair<>(NullPointerException.class, builder);

        if(pair.canHandle(NullPointerException.class)){ // ? -> true
            var message = pair.getBuilder().buildMessage(new NullPointerException()); // <- error Required type: capture of ? extends Exception,  Provided: IllegalArgumentException
            System.out.println(message);
        }

        if(pair.canHandle(IllegalArgumentException.class)){ // ? -> true
            var message = pair.getBuilder().buildMessage(new IllegalArgumentException()); // <- error Required type: capture of ? extends Exception,  Provided: IllegalArgumentException
            System.out.println(message);
        }
    }
}

我们得到一个错误，buildMessage（x）参数给出了一个错误，x参数应该是Exception的后代，但IllegalArgumentException是后代，这是什么问题？
好吧，我想如果这个限制适用于参数，那么让我们添加一个ExceptionWrapper，如下所示：

class ErrorWrapper<T extends Exception>{
    private final T exception;

    public ErrorWrapper(T exception) {
        this.exception = exception;
    }

    public T getException() {
        return exception;
    }
}

之后，使用ErrorWrapper修改ErrorMessageBuilder和RuntimeExceptionErrorMessageBuilder：

interface ErrorMessageBuilder<T extends Exception> {
    String buildMessage(ErrorWrapper<T> exception);
    boolean canHandle(Class<? extends Exception> clazz);
}

class RuntimeExceptionErrorMessageBuilder implements ErrorMessageBuilder<RuntimeException> {
    public String buildMessage(ErrorWrapper<RuntimeException> exception) {
        return "Some runtime exception";
    }

    @Override
    public boolean canHandle(Class<? extends Exception> clazz) {
        return RuntimeException.class.isAssignableFrom(clazz);
    }
}

然后更改main并尝试创建具有更改的消息，但即使我尝试欺骗java也无法工作：错误所需类型：ErrorWrapper〈捕获？扩展异常〉，提供：ErrorWrapper〈capture of？extends Exception〉

public class SomeClassWrapper {
    public static void main(String[] args) {
        var builder = new RuntimeExceptionErrorMessageBuilder();

        ExceptionHandlerPair<NullPointerException> pair = new ExceptionHandlerPair<>(NullPointerException.class, builder);

        if(pair.canHandle(NullPointerException.class)){ // ? -> true
            ErrorWrapper<NullPointerException> npeWrapper = new ErrorWrapper<>(new NullPointerException());

            var message = pair.getBuilder().buildMessage(npeWrapper); // <- error Required type: ErrorWrapper <capture of ? extends Exception>,  Provided: ErrorWrapper<NullPointerException
            System.out.println(message);
        }

        // 
        if(pair.canHandle(NullPointerException.class)){ // ? -> true
            // and here we'll try to swoon over Java!!! :)
            ErrorWrapper<? extends Exception> npeWrapper = new ErrorWrapper<>(new NullPointerException());

            var message = pair.getBuilder().buildMessage(npeWrapper); // <- error Required type: ErrorWrapper <capture of ? extends Exception>,  Provided: ErrorWrapper <capture of ? extends Exception>
            System.out.println(message);
        }
    }
}

如果有人能解释这种行为，我将不胜感激！！！提前感谢！