Question

我确定这是一个非常愚蠢的问题，但我仍然想知道，是否可以动态地转换全局变量cause，换句话说，不使用instanceof运算符？

这个问题的原因是，我觉得instanceof运算符在这里没有做任何好事，它只是静态地转换cause，但在任何一种情况下它都在创建new IOException(cause) < / p>

由于cause的类型为Object，因此我必须将其转换为String或Throwable。

private Object cause; // global variable
//...
if (failed)
    throw cause instanceof String ? new IOException((String) cause) : new IOException((Throwable) cause);

下面是实际的代码片段，其中两个被重写的方法将被异步调用。

public class Command implements ResponseListener {
    private Object cause;

    // ...
    @Override
    public void messageReceived(String message, String status) {
        // ...
        if (!status.equals(MyConstants.COMPLD_MSG)) {
            this.cause = status + " received for " + command.split(":")[0] + message;
            this.failed = true;
        }
        doNotify();
    }

    @Override
    public void exceptionCaught(Throwable cause) {
        this.cause = cause;
        this.failed = true;
        doNotify();
    }

    public void waitForResponse(int cmdTimeout) throws IOException, InterruptedException {
        // ...
        if (failed)
            throw cause instanceof String ? new IOException((String) cause) : new IOException((Throwable) cause);
    }
}

Answer 1

由于String和Throwable的共同超类不可作为IOException的参数，因此您必须将其转换为其中一个，以便确定如何投射，你必须使用instanceof。

Answer 2

Class.cast和Class.isAssignableFrom方法可分别用作演员和instanceof运算符的动态对应方。

Answer 3

为什么原因变量总是 Throwable ？ Throwable 似乎比String更适合表达失败。另外，它避免了你使用＆＃34;丑陋＆＃34; operator instanceof 。

public class Command implements ResponseListener {
    private Throwable cause;

    // ...
    @Override
    public void messageReceived(String message, String status) {
        // ...
        if (!status.equals(MyConstants.COMPLD_MSG)) {
            this.cause = new Throwable(status + " received for " + command.split(":")[0] + message);
            this.failed = true;
        }
        doNotify();
    }

    @Override
    public void exceptionCaught(Throwable cause) {
        this.cause = cause;
        this.failed = true;
        doNotify();
    }

    public void waitForResponse(int cmdTimeout) throws IOException, InterruptedException {
        // ...
        if (failed)
            throw new IOException(cause);
    }
}

以下讨论后更新：

public class Command implements ResponseListener {
    private String cause;

    // ...
    @Override
    public void messageReceived(String message, String status) {
        // ...
        if (!status.equals(MyConstants.COMPLD_MSG)) {
            this.cause = status + " received for " + command.split(":")[0] + message;
            this.failed = true;
        }
        doNotify();
    }

    @Override
    public void exceptionCaught(Throwable cause) {
        if(cause.getMessage().isEmpty()) {
            this.cause = cause.toString();
        }
        else {
            this.cause = cause.getMessage();
        }
        this.failed = true;
        doNotify();
    }

    public void waitForResponse(int cmdTimeout) throws IOException, InterruptedException {
        // ...
        if (failed)
            throw new IOException(cause);
    }
}

Java cast（动态）

3 个答案: