title: Gurobi cannot set or print (manually) value of variables
body: <p>I'm writing a program in Java with Gurobi 6.0.2 and I've some trouble about the management of variables, in particular I can't set or print them.</p>
  
  <p>Example:</p>
  
  <pre><code>GRBEnv env = new GRBEnv();
  GRBModel model = new GRBModel(env);
  GRBVar x = model.addVar(0.0, GRB.INFINITY, 0.0, GRB.SEMIINT, "x");
  x.set(GRB.DoubleAttr.X, 10.0);
  System.out.println(x.get(GRB.DoubleAttr.X));
  </code></pre>
  
  <p>NetBeans shows this message:</p>
  
  <pre><code>Exception in thread "main" gurobi.GRBException: Error at GRBVar.set
      at gurobi.GRBVar.set(GRBVar.java:141)
      at core.Optimize.main(Optimize.java:29)
  Java Result: 1
  </code></pre>
  
  <p>This problem happens also when I resolved the relaxation model, so I can't retrive the value of variables.</p>