Java：case表达式必须是常量表达式

Question

I am using enum with values and when I try to use SmartCard.MessageType.SC_CONN.getValue() in the switch case I am getting the error

case expressions must be constant expressions

public class SmartCard {
    public enum MessageType {
        /** 0x00 Acknowledge of message */
        SC_ACKP(0),
        /** 0x01 Connect to the smart card */
        SC_CONN(1),
        /** 0x02 Request ATR attributes of smart card */
        SC_ATTR(2),
        /** 0x03 Send data to smart card */
        SC_SEND(3);

        private int value;

        MessageType(int value) {
            this.value = value;
        }


        public int getValue() {
            return this.value;
        }

    };

    }

public class TCPServer {

public static void main(String args[]) throws Exception {
      }
    private static void handleMessage(int packetType, int dataLen, byte[] receiveMessage, Socket clientSocket,
            SmartCard smartCard) {
        ByteBuffer answerBuffer = null;
        int value = SmartCard.MessageType.SC_CONN.getValue();
        String preferredProtocol = "";
        switch (packetType) {
        case 0:
            break;
        case value:
            break;
        case 2: 

        }

    }
}

Answer 1

在Java中，case语句必须是一个编译时常数，而value不是，而是在运行时分配的。

Answer 2

The second case of the switch statement is written wrongly. value cannot be used here. The value of every case must be constant. Let's see what the JLS says.

Section 14.11

SwitchStatement:
    switch ( Expression ) SwitchBlock

SwitchBlock:
    { SwitchBlockStatementGroupsopt SwitchLabelsopt }

SwitchBlockStatementGroups:
    SwitchBlockStatementGroup
    SwitchBlockStatementGroups SwitchBlockStatementGroup

SwitchBlockStatementGroup:
    SwitchLabels BlockStatements

SwitchLabels:
    SwitchLabel
    SwitchLabels SwitchLabel

SwitchLabel:
    case ConstantExpression :
    case EnumConstantName :
    default :

EnumConstantName:
    Identifier

Look at the definition for a SwitchLabel . The thing after case must either be a ConstantExpression or an EnumConstantName . value is neither of those.

One solution to this problem is to use if...else if...else statements.

if (packetType == 0) {

} else if (packetType == value) {

} else if (packetType == 2) {

}

If you really want to keep the switch statement, do an if check in the default branch.

switch (packetType) {
case 0:
    break;
case 2: 
    break;
default:
    if (packetType == value) { ... }
    break;
}