java_2_13.java

@FXML
 void prepareOperation(ActionEvent event) {
 if (event.getSource() == add) {
             nextOperation = Operation.add;
           } else if (event.getSource() == substract) {
               nextOperation = Operation.substract;
           } else if (event.getSource() == multiply) {
               nextOperation = Operation.multiply;
           } else if (event.getSource() == divide) {
               nextOperation = Operation.divide;
           }
         if (!input.getText().isBlank() && !lastResult.getText().isBlank()) {
         evaluate(); 
     } else if (!input.getText().isBlank()){
         lastResult.setText(input.getText());
         input.setText("");
     } 
 }