Java枚举（或int常量）vs c enum

Question

I'm trying to do something that would normally look like this in C:

typedef enum {
    HTTP  =80, 
    TELNET=23, 
    SMTP  =25, 
    SSH   =22, 
    GOPHER=70} TcpPort;

Approach 1

Here is what I have in Java, using enum :

public static enum TcpPort{
    HTTP(80),
    TELNET(23),
    SMTP(25),
    SSH(22),
    GOPHER(70);

    private static final HashMap<Integer,TcpPort> portsByNumber;
    static{
        portsByNumber = new HashMap<Integer,TcpPort>();
        for(TcpPort port : TcpPort.values()){
            portsByNumber.put(port.getValue(),port);
        }
    }
    private final int value;
    TcpPort(int value){
        this.value = value;
    }
    public int getValue(){
        return value;
    }
    public static TcpPort getForValue(int value){
        return portsByNumber.get(value);
    }
}

Approach 1 - Problems

I find that I am having to repeat this pattern in various places, but was wondering: is there a better way? particularly because:

1. this seems convoluted and less elegant, and
2. it also shifts something that would be compile time to run time".

One of the reasons I use this mapping, is because it looks better in switch statements, eg:

switch(tcpPort){
    case HTTP:
        doHttpStuff();
        break;
    case TELNET:
        doTelnetStuff();
        break;
    ....
}

I suppose there are also benefits of stronger type safety with enums.

Approach 2 I am aware I could do:

public static class TcpPort{
    public static final int HTTP   = 80;
    public static final int TELNET = 23;
    public static final int SMTP   = 25;
    public static final int SSH    = 22;
    public static final int GOPHER = 70;
}

but my feeling is that enum s are still better. Is my enum approach above the way to go? or is there another way?

Answer 1

My feeling is that in purposes only for switch statement enum is superfluous in your case, and it is better simply using final static int constants. For instance of memory economy.

Also, Joshua Bloch in his Effective Java recommends using enum s instead of int constants in his Item 30: Use enums instead of int constants . But IMHO it is correct way of enum using for more complicated cases then just replacing c #define construction.

UPDATE : as author mentioned in his comment to this my answer, he is wondering is use if enum is better then int constants in general. In this case such question becomes duplicate (see Java: Enum vs. Int ), and my answer will: in general enum s are better, and why - look at Joshua Bloch's Item 30 , as I mentioned before.

Answer 2

You are mixing two concepts here. #define allows you to use the preprocessor . The compiler will just know the value you defined.

In Java using Enums for just Port Numbers seems unreasonable, as they should be configurable imo.

I would recommend using an interface with factories (surounded by enumeration for default types, allowing 3rd party to add new interfaces [OOP].

Generally speaking i would not use Enumerations for constants, simply because they should be scoped within the proper class.

You can import constants with a static import if you want to increase readability.*

*I would consider static imports bad practice tho.

Answer 3

You could create a couple of classes like this:

package com.ggl.testing;

import java.util.ArrayList;
import java.util.List;

public class TCPPort {

    private List<Port> ports;

    public TCPPort() {
        this.ports = new ArrayList<Port>();
        this.ports.add(new Port("HTTP", 80));
        this.ports.add(new Port("TELNET", 23));
        this.ports.add(new Port("SMTP", 25));
        this.ports.add(new Port("SSH", 22));
        this.ports.add(new Port("GOPHER", 70));
    }

    public Port getPortByName(String portName) {
        for (Port p : ports) {
            if (portName.equals(p.getPortName())) {
                return p;
            }
        }

        return null;
    }

    public Port getPortByNumber(int portNumber) {
        for (Port p : ports) {
            if (portNumber == p.getPortNumber()) {
                return p;
            }
        }

        return null;
    }

    public class Port {

        private final int portNumber;

        private final String portName;

        public Port(String portName, int portNumber) {
            this.portName = portName;
            this.portNumber = portNumber;
        }

        public int getPortNumber() {
            return portNumber;
        }

        public String getPortName() {
            return portName;
        }

    }

}