Question

我不明白我在做错了什么。我收到以下错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at Student.paint(Student.java:19)
    at School$ButtonHandler.actionPerformed(School.java:127)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
    at java.awt.Component.processMouseEvent(Component.java:6038)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3260)
    at java.awt.Component.processEvent(Component.java:5803)
    at java.awt.Container.processEvent(Container.java:2058)
    at java.awt.Component.dispatchEventImpl(Component.java:4410)
    at java.awt.Container.dispatchEventImpl(Container.java:2116)
    at java.awt.Component.dispatchEvent(Component.java:4240)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
    at java.awt.Container.dispatchEventImpl(Container.java:2102)
    at java.awt.Window.dispatchEventImpl(Window.java:2429)
    at java.awt.Component.dispatchEvent(Component.java:4240)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)

以下代码应该为用户使用文本字段输入的等级绘制矩形：

public class Student extends Person {

    static Graphics g;

    public static void paint(Graphics g, int gradelevel) {
        if (gradelevel == 9) {
            g.setColor(Color.green);
            g.fillOval(100, 100, 50, 50);
        } else if (gradelevel == 10) {
            g.setColor(Color.cyan);
            g.fillOval(200, 200, 100, 100);
        } else if (gradelevel == 11) {
            g.setColor(Color.red);
            g.fillOval(200, 200, 100, 100);
        } else if (gradelevel == 12) {
            g.setColor(Color.blue);
            g.fillOval(200, 200, 100, 100);
        }
    }

}

Answer 1

你宣布了

 static Graphics g;

且g从未初始化。所以当你致电g.setColor(Color.green);，NullPointerException

时

刚才意识到代码中有两个g。

 public static void paint(Graphics g, int gradelevel)

因此，在调用paint方法时，您可能会传递未初始化的static Graphics g;。

如需更准确的答案，请显示您如何调用paint方法。

Answer 2

对于图形应用程序，您需要extends一个容器（例如JFrame，JPanel）而不是覆盖paintComponent(Graphics g)方法。请看下面的代码示例。否则，声明一个未初始化的Graphics类将抛出NPE。

public class Student extends JPanel {

    @Override
    protected void paintComponent(Graphics g) {
        g.setColor(Color.green);
        g.fillOval(100, 100, 50, 50);
        ..............
    }
}

需要帮助才能更正以下代码

2 个答案: