import java.net.ServerSocket;

public class TestNetUtil {
    public static void main(String [] args) {

        try {
        ServerSocket socket = new ServerSocket(0);
        int port = socket.getLocalPort();
        socket.close();

        } catch (Exception e)  {
            System.out.println(e.getMessage());
        }
    }
}

执行javac TestNetUtil.java，会生成TestNetUtil.class
然后执行java TestNetUtil