java - JComboBox está lanzando una NullPointerException cuando se le dice addItemListener()

swing

Question

listOfWindTurbines.addItemListener(new dropDownListener()); una NullPointerException en la línea 27 ( listOfWindTurbines.addItemListener(new dropDownListener()); ) cuando intento ejecutar mi programa. ¡Por favor ayuda!

import java.awt.event.*;
import java.awt.*;
import javax.swing.*;
public class PlannerMain {
JFrame frame;
JButton makeMap;
JPanel panel;
JLabel outcome;
JComboBox listOfWindTurbines;
String[] windTurbineSpace = new String[10];
Integer[] windTurbineLengths = new Integer[10];
Integer[] windTurbineWidths = new Integer[10];
JTextField lengthOfRoom, widthOfRoom, widthObjectNeeds, lengthObjectNeeds;
int lengthOfRoomInt, widthOfRoomInt, widthObjectNeedsInt, lengthObjectNeedsInt, largerObjectMeasurement, numberOfItems, numberOfItemsShort;
public static void main(String[] args){
PlannerMain p = new PlannerMain();
}
public PlannerMain(){
windTurbineLengths[0] = 1;
windTurbineWidths[0] = 1;
for(int i = 0;i<=9;i++){
int wNum = i + 1;
windTurbineSpace[i] = "Windturbine "+ wNum;
}
listOfWindTurbines.addItemListener(new dropDownListener());
frame = new JFrame("Minecraft Land Planner");
outcome = new JLabel();
panel = new JPanel();
makeMap = new JButton("Make Map");
lengthOfRoom = new JTextField("Length of Room");
widthOfRoom = new JTextField("Width of Room");
widthObjectNeeds = new JTextField("Width Object Needs");
lengthObjectNeeds = new JTextField("Length Object Needs");
listOfWindTurbines = new JComboBox(windTurbineSpace);
makeMap.addActionListener(new makeMapListener());
frame.setSize(580,550);
frame.add(panel);
panel.add(makeMap);
panel.add(lengthOfRoom);
panel.add(widthOfRoom);
panel.add(lengthObjectNeeds);
panel.add(widthObjectNeeds);
panel.add(listOfWindTurbines);
panel.add(outcome);
frame.setVisible(true);
}
class makeMapListener implements ActionListener{
public void actionPerformed(ActionEvent e) {
lengthOfRoomInt = Integer.parseInt(lengthOfRoom.getText());
widthOfRoomInt = Integer.parseInt(widthOfRoom.getText());
lengthObjectNeedsInt = Integer.parseInt(lengthObjectNeeds.getText());
widthObjectNeedsInt = Integer.parseInt(widthObjectNeeds.getText());
if(lengthObjectNeedsInt<=widthObjectNeedsInt){
largerObjectMeasurement = widthObjectNeedsInt;
}
if(widthObjectNeedsInt<=lengthObjectNeedsInt){
largerObjectMeasurement = lengthObjectNeedsInt;
}
numberOfItems = (lengthOfRoomInt/lengthObjectNeedsInt)*(widthOfRoomInt/widthObjectNeedsInt);
outcome.setText(String.valueOf(numberOfItems));
lengthOfRoom.setSize(30, 20);
widthOfRoom.setSize(30, 20);
widthObjectNeeds.setSize(30, 10);
lengthObjectNeeds.setSize(100, 20);
}
}
class dropDownListener implements ItemListener{
public void itemStateChanged(ItemEvent event) {
if(event.getStateChange() == ItemEvent.SELECTED){
lengthObjectNeeds.setText(Integer.toString(windTurbineLengths[listOfWindTurbines.getSelectedIndex()]));
widthObjectNeeds.setText(Integer.toString(windTurbineLengths[listOfWindTurbines.getSelectedIndex()]));
}
}
}
}

Answer 1

listOfWindTurbines inicializar la variable listOfWindTurbines , por ejemplo:

JComboBox listOfWindTurbines = new JComboBox();