code: protected double computeDelta(Evaluation currentEvaluation, Evaluation previousEvaluation){
          if(problem.isMinimizing()){
              // minimization: return decrease
              return previousEvaluation.getValue() - currentEvaluation.getValue();
          } else {
              // maximization: return increase
              return currentEvaluation.getValue() - previousEvaluation.getValue();
          }
      }
docs: Computes the amount of improvement of <code>currentEvaluation</code> over <code>previousEvaluation</code>,
  taking into account whether evaluations are being maximized or minimized. Positive deltas indicate improvement.
  In case of maximization the amount of increase is returned, which is equal to
  <pre> currentEvaluation - previousEvaluation </pre>
  while the amount of decrease, equal to
  <pre> previousEvaluation - currentEvaluation </pre>
  is returned in case of minimization.
  
  @param currentEvaluation evaluation to be compared with previous evaluation
  @param previousEvaluation previous evaluation
  @return amount of improvement of current evaluation over previous evaluation