[OCPJP]try-with-resources敘述的用法2

Given:

class ImageScanner implements AutoCloseable {

    public void close() throws Exception {
        System.out.print("Scanner closed. ");
    }

    public void scanImage() throws Exception {
        System.out.print("Scan.");
        throw new Exception("Unable to scan. ");
    }
}

class ImagePrinter implements AutoCloseable {

    public void close() throws Exception {
        System.out.print("Printer closed. ");
    }

    public void printImage() {
        System.out.print("Print.");
    }
}

and this code fragment:

try (ImageScanner ir = new ImageScanner();
        ImagePrinter iw = new ImagePrinter()) {
    ir.scanImage();
    iw.printImage();
} catch (Exception e) {
    System.out.print(e.getMessage());
}

What is the result?

A.

B.

C.

D.

題解

程式第32行，呼叫ImageScanner物件的scanImage方法，會先輸出「Scan.」然後再拋出訊息為「Unable to scan. 」的Exception，這個Exception在try-with-resources會被接住，在離開try的區塊進入catch區塊前，會先自動呼叫ImagePrinter物件的close方法，輸出「Printer closed. 」，再自動呼叫ImageScanner物件的close方法，輸出「Scanner closed. 」。因為ImageScanner物件比ImagePrinter物件還要早宣告與實體化出來，基於try-with-resources結構中的資源為先進後出的特性，ImagePrinter物件會比ImageScanner物件還要早被關閉。

在catch區塊中，會輸出「Unable to scan. 」。