private void posnegActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_posnegActionPerformed
        plusminus = (Double.parseDouble(String.valueOf(display.getText())));
        plusminus = plusminus * (-1);
        display.setText(String.valueOf(plusminus));   // TODO add your handling code here:
    }//GEN-LAST:event_posnegActionPerformed