java - 如何从组合框中获取索引

Question

所以，到目前为止，我从来没有真正遇到过编码问题。我正在为一个班级项目制作一个计算器，它只会将与组合框中的单词对应的两个数字相加。我设置了数组，以便我可以使用它们的索引轻松地将它们添加在一起，并将总数显示在我的底部文本框中。我的代码将编译，但我不断收到错误，无论我查找什么，我都找不到我的问题。我不知道我是否正确获取索引，我也不确定从组合框中返回索引的另一种方法。任何帮助将不胜感激，并提前感谢您。

这是我的代码：

import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;
import javafx.scene.control.Button;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.scene.layout.GridPane;
import javafx.geometry.Pos;
import javafx.geometry.Insets;
import javafx.scene.control.TextField;
import javafx.event.EventHandler;
import javafx.event.ActionEvent;


public class Dorm extends Application
{
  private Label iMessage; // displays instructions
  private TextField tDorms; // displays text to choose dormitories
  private TextField tMeals; // display the text to choose a meal plan
  private Label tTotal; // will display the total cost of tution.
  private ComboBox cDormitories; // combo box for dormitories
  private ComboBox cMeals; // combo box for meal plans;
  private VBox mealStuff;// holds objects for the meal plans
  private VBox dormStuff; // holds objects for the dormitories.
  private Button getAnswer; // will perform the multiplication of the chosen dormitory and meal plan.  maybe not needed.
  private GridPane everything; // will hold everything and be sent to scene

  private int mIndex, dIndex;


  // Dormitory Array
 private String[] aDorm = {"Allen Hall $ 1800", "Pike Hall $ 2200", "Farthing Hall $ 2800", "University Suites $ 3000"};

  // Dorm value array
  int[] dormRate = {1800, 2200, 2800, 3000};

  // Meal Plan Array
  private String[] aMeal =
  {
   "7 Meals Per Week $ 600",
   "14 Meals Per Week $ 1100",
   "Unlimited Meals $ 1800" };

  // Meal Plan Value Array
  int[] mealRate = {600, 1100, 1800};

  public static void main (String [] args)
  {
    launch (args);
  }

  @Override
  public void start(Stage primaryStage)
  {
    // Create a label for instructions
    Label iMessage = new Label("Instructions: Please select each option to find out your total cost: ");

    // create the dormitories combobox and controls. then addd them to the VBox
    // comboBox

    ComboBox<String> cDormitories = new ComboBox<>();
    cDormitories.getItems().addAll(aDorm);
    cDormitories.setVisibleRowCount(4);

    // comboBox label
    Label tDorms = new Label("Select a Dormitory");

    // add to VBox
    VBox dormStuff = new VBox(10, cDormitories, tDorms);
    dormStuff.setPadding(new Insets(10));
    dormStuff.setAlignment(Pos.CENTER);

    //create the meal plan combo box, add controls, and add to a VBov.
    ComboBox<String> cMeals = new ComboBox<>();
    cMeals.getItems().addAll(aMeal);
    cMeals.setVisibleRowCount(3);

    // comboBox label
    Label tMeals = new Label("Select a Meal Plan");

    // add to VBox
    VBox mealStuff = new VBox(10, cMeals, tMeals);
    mealStuff.setPadding(new Insets(10));
    mealStuff.setAlignment(Pos.CENTER);

      //create a label for the total
    Label tTotal = new Label("Your total is: "); //+ total );  answer needs to go here

    // create a button
    Button getAnswer = new Button("Get Answer");

    // register event for the button
    getAnswer.setOnAction(new DormComboButton());

    // add both VBoxs to a grid pane
    // create the grid pane
    GridPane everything = new GridPane();
    everything.add(iMessage, 1,0);
    everything.add(dormStuff,0,1);
    everything.add(mealStuff,2,1);
    everything.add(tTotal,1,2);
    everything.add(getAnswer,0,2);

    // Create a scene and display it
    Scene scene = new Scene(everything);
    primaryStage.setScene(scene);
    primaryStage.setTitle("Dorm and Meal Calculator");
    primaryStage.show();
  }


class DormComboButton implements EventHandler<ActionEvent>
{
  @Override
   public void handle(ActionEvent event)
 {
    //get the dorm index
    int dIndex = cDormitories.getSelectionModel().getSelectedIndex(); //suppose to retrieve index from selected item in combo box.

//get the meal index
int mIndex = cMeals.getSelectionModel().getSelectedIndex(); //suppose to retrieve index from selected item in combo box.

    //add those numbers together
    int t = dormRate[dIndex] + mealRate[mIndex]; // should store the math
    String str = Integer.toString(t); // turn math into a string
    //display the results

    tTotal.setText(str); //place math into the label
 }
}

}

[当我按下“获取答案”按钮时收到运行时错误。][1]

最新的堆栈跟踪：

     Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at Dorm$DormComboButton.handle(Dorm.java:120)
at Dorm$DormComboButton.handle(Dorm.java:114)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Node.fireEvent(Node.java:8411)
at javafx.scene.control.Button.fire(Button.java:185)
at com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:182)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:96)
at com.sun.javafx.scene.control.skin.BehaviorSkinBase$1.handle(BehaviorSkinBase.java:89)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:198)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3757)
at javafx.scene.Scene$MouseHandler.access$1500(Scene.java:3485)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1762)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2494)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:394)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$353(GlassViewEventHandler.java:432)
at com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:389)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:431)
at com.sun.glass.ui.View.handleMouseEvent(View.java:555)
at com.sun.glass.ui.View.notifyMouse(View.java:937)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
at java.lang.Thread.run(Unknown Source)

标签： javaarraysjavafxcombobox