如何通过枚举列表“重置”迭代

Question

Enumeration<NetworkInterface> nets
        = NetworkInterface.getNetworkInterfaces();

out.print("List of all network interfaces on this machine:\n");

for (NetworkInterface netint : Collections.list(nets)) {
    out.printf("name:%s (%s)\n", netint.getName(), netint.getDisplayName());
}

I'm doing an assignment that requires me to list a bunch of information using NetworkInterface, but I'm not very familiar with Enumeration, so I'm having trouble.

I went through some javadocs and managed to get this far. The problem is that I need to be able to go through the list multiple times.

For example, I was wanting to use another for loop to cycle through and list all interfaces that were currently up.

Something like this:

for (NetworkInterface netint : Collections.list(nets)) {
    if (netint.isUp()) {
       out.printf("name:%s (%s)\n", netint.getName(), netint.getDisplayName());
    }
}

However, any time I use a for loop after the first one I'm not getting any output. I have a feeling this has to do with my lack of understanding. I can't seem to find an explanation anywhere.

Answer 1

An enumeration can't be reset. The reason your code doesn't work is because your first call to Collections.list(nets) "depleats" the nets enumeration, so the next call to Collections.list(nets) yields an empty collection.

To avoid this collect the interfaces from the nets enumeration and iterate over the resulting list instead:

List<NetworkInteface> list = Collections.list(nets);

// Iteration 1
for (NetworkInterface netint : list) {
    ...
}

// Iteration 2
for (NetworkInterface netint : list) {
    ...
}

Answer 2

You need to either:

• call NetworkInterface.getNetworkInterfaces() afresh before every loop (so that you get a new Enumeration object); or
• store the results of NetworkInterface.getNetworkInterfaces() in a list once , and then just iterate over that list.

To illustrate the second approach:

List<NetworkInterface> nets = Collections.list(
                                 NetworkInterface.getNetworkInterfaces());

for (NetworkInterface netint : nets) {
   ...
}

for (NetworkInterface netint : nets) {
   ...
}

Answer 3

Try Iterator instead of Enumeration. Refer this docs, http://docs.oracle.com/javase/7/docs/api/java/util/Iterator.html