Why can't I call new from within a static method?

Question

I have some utility functions that I've declared as static, as they are not to be instantiated, only used for utilities. I would like to create a generator method that can generate objects, but in the same static utility context.

public class PeerConnection {

    public class _Heartbeat{
        protected String beat = "HB_000";
        protected String ack  = "HB_001";
        protected String msg  = null;

        protected Date beatTime = null;
        protected Date ackTime  = null;
        protected short missedBeats = 0;
        protected short MAX_MISS = 3;
    }

    public _Heartbeat heartbeat = null;
    //map of heartbeat objects per Peer connection
    public static List<_Heartbeat> Heartbeats = new ArrayList<_Heartbeat>();

    public static void GenerateHeartbeat(){
        Heartbeats.add(new _Heartbeat());
    }

My reasoning, is I want to call this from a SendHeartbeat method:

   private static int SendHeartbeat(PeerConnection peer){
        int acks = 0;
        PeerConnection.GenerateHeartbeat();
        PeerConnection._Heartbeat hb = peer.Heartbeats.get(peer.Heartbeats.size() - 1);
        hb.msg = hb.beat;

        while (acks <= 0 && hb.missedBeats < hb.MAX_MISS){
         [...]
        }
   }

I get the concept of why static works this way, but I'm thinking there has to be a work around for this scenario.

Make the inner class static ... check out this link http://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class — gladiator, Sep 30 '15 at 08:03

score 2 · Answer 1 · answered Sep 30 '15 at 08:07

_Heatbeat is not a static class, so any instance of it is explicitly tied to an instance of the PeerConnection class. I.e. to instantiate _Heartbeat you need an instance of the PeerConnection class.

One option is to make _Heartbeat a static class (i.e. public static class _Heartbeat, which I think is probably what you want.

Another option is to instantiate both together like this new PeerConnection().new _Heartbeat() (I saw this in the java certification exam, but I hate it and never use it so I may not be remembering the syntax correctly).

score 0 · Answer 2 · answered Sep 30 '15 at 08:12

0

You need an actual object of your Peer class to instannciate the inner class as this class belongs to an object. Just simply as that...

answered Sep 30 '15 at 08:12

Ilja KO

1,272
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score 0 · Answer 3 · edited May 23 '17 at 11:44

Documentation : nested classes

I think your question is more about nested classes than static stuff. Give back to Caesar what is Caesar's : A realy good answer to your problem here

So in your case you have two option :

 public static class _Heartbeat{ ... }

or

public static void GenerateHeartbeat(PeerConnection p){
    Heartbeats.add(p.new _Heartbeat());
}

private static int SendHeartbeat(PeerConnection peer){
    int acks = 0;
    PeerConnection.GenerateHeartbeat(peer);
    PeerConnection._Heartbeat hb = peer.Heartbeats.get(peer.Heartbeats.size() - 1);
    hb.msg = hb.beat;

    while (acks <= 0 && hb.missedBeats < hb.MAX_MISS){
     [...]
    }
}

Why can't I call new from within a static method?

3 Answers3