Originally posted by: open_ball

Hi,

I am receiving the following error.

Exception in thread "main" ilog.cplex.CpxException: CPLEX Error  1006: Error during callback.

        at ilog.cplex.CplexI.CALL(CplexI.java:4969)
        at ilog.cplex.CplexI.setStatus(CplexI.java:6301)
        at ilog.cplex.CplexI.access$500(CplexI.java:120)
        at ilog.cplex.CplexI$SolveHandle.stop(CplexI.java:2941)
        at ilog.cplex.CplexI.solve(CplexI.java:2964)
        at ilog.cplex.IloCplex.solve(IloCplex.java:10254)

I intend to separate the sub problem into multiple problems. Just to explain what I am doing, I will take the TSP example provided by cplex as a reference. In Neighbor class, here are the changes I am making. Please note that I try my best to keep the example provided as small as possible.

        private static final class Neighbor {
              private final IloCplex[] cplex;
              private final IloNumVar[] u;
              private IloObjective[] obj;
              
              public Neighbor(int size) throws IloException {
                 this.cplex = new IloCplex[size];
                 this.u = new IloNumVar[size];
                         for(int i=0; i< size ; i++) {
                                 u[i] = cplex[i].numVar(0.0, Double.POSITIVE_INFINITY,
                                     IloNumVarType.Float);
                                 this.obj[i] = cplex[i].minimize();                     
                             cplex[i].setOut(null);
                             cplex[i].add(u[i]);               
                             cplex[i].add(obj[i]);
                         }
                         
                         //Create dual constraints
                         
                        public IloRange[] separate(IloNumVar[] x, IloNumVar[] estObj, double[] xSol, double[] Z) throws IloException {
                        
                        IloRange[] cut = new IloRange[size];
                        for(int i=0; i< size ; i++) {
                                 cut[i] =null;
                                 cplex[i].remove(obj[i]);                               
                        }
                        for (int i = 0; i < size; i++) { 
                        IloLinearNumExpr objExpr = cplex[i].linearNumExpr();
                        //fill objExpr
                          obj[i] = cplex[i].minimize(objExpr);
                          cplex[i].add(obj[i]);
                          cplex[i].solve();
                         IloCplex.Status status = cplex[i].getStatus();
                         IloLinearNumExpr expr = cplex[i].linearNumExpr();
                         if (status == IloCplex.Status.Optimal) {
                                 if (zMaster[i] > cplex[i].getObjValue() + 0.0001) {                               
                            // create the cut                                     
                               cut[i] = (IloRange) cplex[i].le(estObj[i],expr);
                                
                                 }
                         }else {
                             // An unexpected status occurred -- report it but do nothing.
                             System.err.println("\n!!! Unexpected subproblem solution status: "
                                                + status + "\n");
                           }

These are the changes I made and the problem occurs in the callback function. The place where I receive the exact error is;

              // teardown
                 if (context.inThreadDown()) {                        
                         neighbors[threadNo] = null;                     
                     return;
                  }

Right before the return keyword, I get the error. Can anyone help me out?

It's hard to say what is going wrong exactly. Could it be that you're getting an ArrayIndexOutOfBounds exception from the neighbors array? I suggest that you wrap your code in the callback invoke method in a try/catch block to see if you can get specific information about the exception that is being thrown there.

Originally posted by: open_ball

Hi,

Thanks for the recommendation. I guess the error occurs when I try to define the variable in Neighbor class. In the constructor, after I initialize variable u,

this.u = new IloNumVar[size];

I get into the for loop and and associate each u[i] with one cplex object.

for(int i=0; i< size ; i++) {
         u[i] = cplex[i].numVar(0.0, Double.POSITIVE_INFINITY, IloNumVarType.Float);

This is the part I am receiving the error, but I am not quite sure what I am doing wrong.

Please show us the full exception message that you get from within the invoke method. Your invoke method should look something like this:

public void invoke(IloCplex.Callback.Context context) throws IloException {
   try {
     // Your callback code here.
   } catch (Exception e) {
      System.out.println("Exception from callback: " + e);
      throw e;
   }
}

Originally posted by: open_ball

Hi,

I get Null Pointer exception.

Exception from callback: java.lang.NullPointerException
Exception in thread "main" ilog.cplex.CpxException: CPLEX Error  1006: Error during callback.

        at ilog.cplex.CplexI.CALL(CplexI.java:4969)
        at ilog.cplex.CplexI.setStatus(CplexI.java:6301)
        at ilog.cplex.CplexI.access$500(CplexI.java:120)
        at ilog.cplex.CplexI$SolveHandle.stop(CplexI.java:2941)
        at ilog.cplex.CplexI.solve(CplexI.java:2964)
        at ilog.cplex.IloCplex.solve(IloCplex.java:1025)

When I debug my code, I can see that the exception takes place here at

u[i] = cplex[i].numVar(0.0, Double.POSITIVE_INFINITY,IloNumVarType.Float);

As far as I can tell, you still haven't show us the exception from within the callback, but at least you've given us a bit more information.

My guess is that you haven't initialized the cplex array properly. When you create it with this.cplex = new IloCplex[size]; you get an array of null objects. You have to initialize each of those slots with new IloCplex instances, like so:

for (int i = 0; i < size; i++) {
   cplex[i] = new IloCplex();
}

Originally posted by: open_ball

Hi Ryan,

I did what you suggested within the callback as

public void invoke(IloCplex.Callback.Context context) throws IloException {
   try {
     // Your callback code here.
   } catch (Exception e) {
      System.out.println("Exception from callback: " + e);
      throw e;
   }
}

and just shared the exception message that I received. 

Anyway, you were right. Once I initialize each cplex object individually, it worked.  Would you mind explaining me why  this.cplex = new IloCplex[size]; does not initialize the cplex array properly? I'm sorry if this is more like a Java related question.

You are correct: that's a Java question. When you declare an array of objects in Java, Java allocates space for pointers to the objects and sets the entries of the array to NULL. You then have to assign objects to the array slots (and, if necessary, create them). In your case, inside the loop you would need to set cplex[i] = new IloCplex().