java - Java/Java Card中的"Switch Case"和"If...else.."区别

Question

请看一下下面的 Java Card 程序。它们在逻辑上看似相等，但在实践中有不同的输出:

使用 switch ... case 的简单传入 APDU 命令分析器:

package testPack;

import javacard.framework.APDU;
import javacard.framework.Applet;
import javacard.framework.ISO7816;
import javacard.framework.ISOException;

public class TestApp extends Applet {

    private TestApp() {
    }

    public static void install(byte bArray[], short bOffset, byte bLength) throws ISOException {
        new TestApp().register();
    }

    public void process(APDU arg0) throws ISOException {
        if (selectingApplet()) {
            return;
        }

        byte[] buffer = arg0.getBuffer();
        byte ins = buffer[ISO7816.OFFSET_INS];

        switch (ins) {
        case (byte) 0x80:
            ISOException.throwIt((short) 0x6901);
            break;
        default:
            ISOException.throwIt((short) 0x6902);
            break;
        }
    }
}

使用 if ... else ... 的相同程序:

package testPack;

import javacard.framework.APDU;
import javacard.framework.Applet;
import javacard.framework.ISO7816;
import javacard.framework.ISOException;

public class TestApp extends Applet {

    private TestApp() {
    }

    public static void install(byte bArray[], short bOffset, byte bLength) throws ISOException {
        new TestApp().register();
    }

    public void process(APDU arg0) throws ISOException {
        if (selectingApplet()) {
            return;
        }

        byte[] buffer = arg0.getBuffer();
        byte ins = buffer[ISO7816.OFFSET_INS];

        if (ins == (byte) 0x80) {
            ISOException.throwIt((short) 0x6901);
        } else {
            ISOException.throwIt((short) 0x6902);
        }
    }
}

正如我们所知，在 Java Card 中，APDU 缓冲区 header 字段被视为有符号字节。所以我认为响应 00 80 00 00 00 APDU 命令，我必须为上述两个程序接收 69 02。

但这是结果:

第一个程序:

Download Cap begin...
Download Cap successful.
Install Applet begin...
Install Applet successful.
Select Applet begin...
Select Applet successful.
Send: 00 80 00 00 00
Recv: 69 01

第二个程序:

Download Cap begin...
Download Cap successful.
Install Applet begin...
Install Applet successful.
Select Applet begin...
Select Applet successful.
Send: 00 80 00 00 00
Recv: 69 02

为什么响应不同？

Answer 1

java没有错。这段代码:

byte ins = (byte) 0x80;

switch (ins) {
    case (byte) 0x80:
        System.out.println("switch matched");
        break;
    default:
        System.out.println("switch missed");
}

if (ins == (byte) 0x80) {
    System.out.println("if matched");
} else {
    System.out.println("if missed");
}

产生:

switch matched
if matched

问题出在其他地方。

---

可能值得注意的是 (byte) 0x80 导致 -128。