java - 如何解决 "Exception in thread "AWT-EventQueue-0"java.lang.NullPointerException"？

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 3 年前。

我正在开发一个 GUI，它可以让你订购饮料并计算它们的总价。但是
我在第 163 行遇到此错误 (Exception in thread "AWT-EventQueue-0"java.lang.NullPointerException)。我尝试多次尝试更改第 163 行的参数“total = Price.juice("Apple", 1 ， “小的”);”但没有任何效果!

我的代码:

1 类:

import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import javax.swing.ButtonGroup;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JRadioButton;
import javax.swing.JTextField;

public class Assignments1 extends JFrame implements ActionListener{
    private JLabel lblFirst, lblSecond, lblThird, lblInfo;
    private JTextField txtOne;
    private JButton btnAdd, btnOrder;
    private JRadioButton rdJuice, rdWater, rdTea, rdCoffee;
    private ButtonGroup btnGroup;
    private JComboBox comboBox;
    private Prices price;
    private OptionPane optionPane;

    public Assignments1() {

        //set Layout
        setLayout(null);
        gui(); //calling the gui method to show the program.


        btnAdd.addActionListener(this);
        comboBox.addActionListener(this); 

        //set the GUI parameters
        setSize(500, 500);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setResizable(false);
    }

    //design GUI
    private void gui() {

        //initialize global variables
        lblFirst= new JLabel("Select size:");
        lblFirst.setLocation(100, 10);
        lblFirst.setSize(150,50);
        add(lblFirst);

        //creating the JComboBox:
        String bevSize[] = {"Small", "Medium", "Large"}; //array of string contating cities 
        comboBox = new JComboBox(bevSize); //create checkbox 
        comboBox.setSelectedItem("Small");
        comboBox.setLocation(110, 60);
        comboBox.setSize(150, 30); // set the size of frame 
        add(comboBox);
        //comboBox.show(); 

        //////////////////////////////////////////////////////////////////////////////////////////

        lblSecond= new JLabel("Select which type of beverage you want to order:");
        lblSecond.setLocation(100, 120);
        lblSecond.setSize(300,50);
        add(lblSecond);

        // set radio buttons
        btnGroup= new ButtonGroup();

        rdJuice= new JRadioButton("Juice");
        rdJuice.setActionCommand("Juice");
        rdJuice.setLocation(110, 170);
        rdJuice.setSize(70, 50);
        add(rdJuice);

        rdWater= new JRadioButton("Water");
        rdWater.setActionCommand("Water");
        rdWater.setLocation(190, 170);
        rdWater.setSize(70, 50);
        add(rdWater);

        rdTea= new JRadioButton("Tea");
        rdTea.setActionCommand("Tea");
        rdTea.setLocation(280, 170);
        rdTea.setSize(70, 50);
        add(rdTea);

        rdCoffee= new JRadioButton("Coffee");
        rdCoffee.setActionCommand("Coffee");
        rdCoffee.setLocation(350, 170);
        rdCoffee.setSize(70, 50);
        add(rdCoffee);

        //make a group for radio buttons
        btnGroup.add(rdJuice);
        btnGroup.add(rdWater);
        btnGroup.add(rdTea);
        btnGroup.add(rdCoffee);
        rdJuice.setSelected(true);

        /////////////////////////////////////////////////////////////////////

        lblThird= new JLabel("Select how many glasses you want to order:");
        lblThird.setLocation(100, 220);
        lblThird.setSize(300,50);
        add(lblThird);

        // set txtTwo
        txtOne= new JTextField();
        txtOne.setLocation(100, 270);
        txtOne.setSize(300, 30);
        add(txtOne);

        ///////////////////////////////////////////////////////////////////////

        // set Buttons
        btnAdd= new JButton("Add");
        btnAdd.setLocation(100, 310);
        btnAdd.setSize(100, 50);
        add(btnAdd);

        btnOrder= new JButton("Order");
        btnOrder.setLocation(300, 310);
        btnOrder.setSize(100, 50);
        add(btnOrder);

        ////////////////////////////////////////////////////////////////////////

        // register buttons to respond actions

        btnAdd.addActionListener(this);
        btnOrder.addActionListener(this);

        /////////////////////////////////////////////////////////////////////////

        lblInfo= new JLabel();
        lblInfo.setForeground(Color.BLACK);
        lblInfo.setLocation(10,500);
        lblInfo.setSize(200, 50);
        add(lblInfo);
    }

    public void price(ActionEvent e) {

        int total = 0;

        int first = Integer.parseInt(txtOne.getText().trim());
        //rdJuice, rdWater, rdTea, rdCoffee

        String[] options = {"Apple", "Orange", "Pineapple"};
        JComboBox <String> optionList = new JComboBox<>(options);
        optionList.setSelectedIndex(0);
        JOptionPane.showMessageDialog(this, optionList, "Select a fruit",
        JOptionPane.OK_CANCEL_OPTION);

        if(rdJuice.isSelected()) {
            total = price.juice("Apple", 1, "Small");
        }
            else if(rdWater.isSelected()) {
                total = price.water(optionPane.paneWater(), first, comboBox.getSelectedItem());
            }
                else if(rdTea.isSelected()) {
                    total = price.tea(optionPane.paneTea(), first, comboBox.getSelectedItem()); 
                }
                    else if(rdCoffee.isSelected()) {
                        total = price.coffee(optionPane.paneTea(), first, comboBox.getSelectedItem());
                    }

    /*
        catch(NumberFormatException ee) {
            JOptionPane.showMessageDialog(
                    this,"Input Ineger number","Unvalid NUmber",
                    JOptionPane.ERROR_MESSAGE
                    );

        }*/
    }


    public static void main(String[]args) {

        new Assignments1();

    }

    @Override
    public void actionPerformed(ActionEvent arg0) {
        if(arg0.getSource().equals(btnAdd)) 
            price(arg0);
    }
}

第 2 类:

public class Prices {

    public int juice(String kind, int amount, String size) {

        int price = 0;

        if(kind.equals("Apple") && size.equals("Small")) {
            price = 5 * amount;
        }
            else if(kind.equals("Apple") && size.equals("Medium")) {
                price = 10 * amount;
            }
                else if(kind.equals("Apple") && size.equals("Large")) {
                    price = 15 * amount;
                }
        if(kind.equals("Orange") && size.equals("Small")) {
            price = 6 * amount;
        }
            else if(kind.equals("Orange") && size.equals("Medium")) {
                price = 11 * amount;
            }
                else if(kind.equals("Orange") && size.equals("Large")) {
                    price = 16 * amount;
                }
        if(kind.equals("Pineapple") && size.equals("Small")) {
            price = 7 * amount;
        }
            else if(kind.equals("Pineapple") && size.equals("Medium")) {
                price = 12 * amount;
            }
                else if(kind.equals("Pineapple") && size.equals("Large")) {
                    price = 17 * amount;
                }
        return price;
    }

    public int water(boolean kind, int amount, Object size) {

        int price = 0;

        if(kind == true && size.equals("Small")) {
            price = 4 * amount;
        }
            else if(kind == true && size.equals("Medium")) {
                price = 5 * amount;
            }
                else if(kind == true && size.equals("Large")) {
                    price = 6 * amount;
                }
        if(kind == false && size.equals("Small")) {
            price = 2 * amount;
        }
            else if(kind == false && size.equals("Medium")) {
                price = 3 * amount;
            }
                else if(kind == false && size.equals("Large")) {
                    price = 4 * amount;
                }
        return price;
    }

    public int tea(boolean kind, int amount, Object size) {

        int price = 0;

        if(kind == true && size.equals("Small")) {
            price = 4 * amount;
        }
            else if(kind == true && size.equals("Medium")) {
                price = 5 * amount;
            }
                else if(kind == true && size.equals("Large")) {
                    price = 6 * amount;
                }
        if(kind == false && size.equals("Small")) {
            price = 2 * amount;
        }
            else if(kind == false && size.equals("Medium")) {
                price = 3 * amount;
            }
                else if(kind == false && size.equals("Large")) {
                    price = 4 * amount;
                }
        return price;
    }

    public int coffee(boolean kind, int amount, Object size) {

        int price = 0;

        if(kind == true && size.equals("Small")) {
            price = 4 * amount;
        }
            else if(kind == true && size.equals("Medium")) {
                price = 5 * amount;
            }
                else if(kind == true && size.equals("Large")) {
                    price = 6 * amount;
                }
        if(kind == false && size.equals("Small")) {
            price = 2 * amount;
        }
            else if(kind == false && size.equals("Medium")) {
                price = 3 * amount;
            }
                else if(kind == false && size.equals("Large")) {
                    price = 4 * amount;
                }
        return price;
    }
}

我遇到了这个错误:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Assignments1.price(Assignments1.java:163)
    at Assignments1.actionPerformed(Assignments1.java:195)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$500(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

Answer 1

private Prices price;

您的“价格”变量为空。

如果您想访问类中定义的方法，则需要创建它的实例:

private Prices price = new Prices();