java - JButton setIcon nullPointerException

Question

我的代码:

private JButton stopBotting = new JButton();
ImageIcon img = new ImageIcon(getClass().getResource(System.getProperty("user.dir") + "VisualBot/resources/play.png"));
stopBotting.setIcon(img);

add(stopBotting);

完整代码:

package bot;

import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowEvent;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;

public class GuiBotting extends JFrame{

    private int controlWidth = 300;
    private int controlHeight = 78;
    private JButton startBotting = new JButton();
    private JButton stopBotting = new JButton();
    private JButton pauseBotting = new JButton();

    private GuiBotting(){

        setVisible(true);
        setResizable(false);
        setSize(controlWidth, controlHeight);
        setDefaultCloseOperation(DO_NOTHING_ON_CLOSE);
        setLayout(null);

        ImageIcon img = new ImageIcon(getClass().getResource(System.getProperty("user.dir") + "VisualBot/resources/play.png"));
        stopBotting.setIcon(img);

        eventStartBotting startBottingEvent = new eventStartBotting();
        eventStopBotting stopBottingEvent = new eventStopBotting();
        eventPauseBotting pauseBottingEvent = new eventPauseBotting();

        startBotting.addActionListener(startBottingEvent);
        stopBotting.addActionListener(stopBottingEvent);
        pauseBotting.addActionListener(pauseBottingEvent);

        startBotting.setBounds(0, 0, 100, 50);
        stopBotting.setBounds(0, 0, 50, 50);
        pauseBotting.setBounds(50, 0, 50, 50);

        add(stopBotting);
        add(pauseBotting);

        stopBotting.setVisible(false);
        pauseBotting.setVisible(false);

        addWindowListener(new java.awt.event.WindowAdapter() {
            public void windowClosing(WindowEvent winEvt) {
                setVisible(false);
                dispose();

                VisualBot.stopBotting();

                GuiPreparation.openGui();
            }
        });

    }

    public class eventStartBotting implements ActionListener{
        public void actionPerformed(ActionEvent startBottingActionEvenr){
            System.out.println("Starting with botting...");
            VisualBot.startBotting();
            pauseBotting.setVisible(true);
            stopBotting.setVisible(true);
        }
    }

    public class eventStopBotting implements ActionListener{
        public void actionPerformed(ActionEvent stopBottingActionEvent) {

            System.out.println("Stopping with botting...");
            VisualBot.stopBotting();

            dispose(); 

            GuiPreparation.openGui();

        }

    }

    public class eventPauseBotting implements ActionListener{
        public void actionPerformed(ActionEvent pauseBottingActionEvent){
                System.out.println("Pausing botting...");
                VisualBot.pauseBotting();
                stopBotting.setBounds(0, 0, 100, 50);
                stopBotting.setVisible(false);
                pauseBotting.setVisible(false);
        }
    }

    public static void openGui(){
        GuiBotting guiBotting = new GuiBotting();

    }

}

当我打开 gui 时，出现此错误:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at javax.swing.ImageIcon.<init>(Unknown Source)
    at darkorbit.GuiBotting.<init>(GuiBotting.java:31)
    at darkorbit.GuiBotting.openGui(GuiBotting.java:98)
    at darkorbit.GuiPreparation$eventStartBotting.actionPerformed(GuiPreparation.java:255)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$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)
    etc...

我只是不明白，即使我设置了图像的完整路径，我也会得到 nullPointerException。我为此找到了很多答案并尝试了所有答案，但我还没有让它发挥作用。如果重要的话我会使用 Eclipse

也失败了:

package bot;

import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowEvent;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;

public class GuiBotting extends JFrame{

    private int controlWidth = 300;
    private int controlHeight = 78;
    private JButton startBotting = new JButton();
    private JButton stopBotting = new JButton();
    private JButton pauseBotting = new JButton();

    private GuiBotting(){

        setVisible(true);
        setResizable(false);
        setSize(controlWidth, controlHeight);
        setDefaultCloseOperation(DO_NOTHING_ON_CLOSE);
        setLayout(null);

        ImageIcon img = new ImageIcon(getClass().getResource(System.getProperty("user.dir") + "/VisualBot/resources/play.png"));
        stopBotting.setIcon(img);

        eventStartBotting startBottingEvent = new eventStartBotting();
        eventStopBotting stopBottingEvent = new eventStopBotting();
        eventPauseBotting pauseBottingEvent = new eventPauseBotting();

        startBotting.addActionListener(startBottingEvent);
        stopBotting.addActionListener(stopBottingEvent);
        pauseBotting.addActionListener(pauseBottingEvent);

        startBotting.setBounds(0, 0, 100, 50);
        stopBotting.setBounds(0, 0, 50, 50);
        pauseBotting.setBounds(50, 0, 50, 50);

        add(stopBotting);
        add(pauseBotting);

        stopBotting.setVisible(false);
        pauseBotting.setVisible(false);

        addWindowListener(new java.awt.event.WindowAdapter() {
            public void windowClosing(WindowEvent winEvt) {
                setVisible(false);
                dispose();

                VisualBot.stopBotting();

                GuiPreparation.openGui();
            }
        });

    }

    public class eventStartBotting implements ActionListener{
        public void actionPerformed(ActionEvent startBottingActionEvenr){
            System.out.println("Starting with botting...");
            VisualBot.startBotting();
            pauseBotting.setVisible(true);
            stopBotting.setVisible(true);
        }
    }

    public class eventStopBotting implements ActionListener{
        public void actionPerformed(ActionEvent stopBottingActionEvent) {

            System.out.println("Stopping with botting...");
            VisualBot.stopBotting();

            dispose(); 

            GuiPreparation.openGui();

        }

    }

    public class eventPauseBotting implements ActionListener{
        public void actionPerformed(ActionEvent pauseBottingActionEvent){
                System.out.println("Pausing botting...");
                VisualBot.pauseBotting();
                stopBotting.setBounds(0, 0, 100, 50);
                stopBotting.setVisible(false);
                pauseBotting.setVisible(false);
        }
    }

    public static void openGui(){
        GuiBotting guiBotting = new GuiBotting();

    }

}

Answer 1

ImageIcon img = new ImageIcon(getClass().getResource(
    System.getProperty("user.dir") + "VisualBot/resources/play.png"));

user.dir 在这里没有用。

1. 它是一个实际的文件路径，而 getResource(..) 使用的 String 应该是代表一个 String相对于类路径的路径。
2. 它没有以 / 为后缀，因此需要显式添加它才能形成有效路径。

尝试使用:

ImageIcon img = new ImageIcon(getClass().getResource(
    "/VisualBot/resources/play.png"));