我有一个简单的问题。如何通过将参数传递给构造函数来创建新实例?由于GraphicObject可以是任何扩展它的类,我需要知道它是哪个类,在这里我必须使用扩展它的类,所以我正在尝试反射,但到目前为止只有null异常。
GraphicObject decor = null;
try {
decor = decorator.getClass().getDeclaredConstructor(GraphicObject.class).newInstance(mainGroup);
} catch (Exception e) {
System.out.println(e);
}
应该反映的类看起来像这样:
public class DecoratorLeft extends GraphicObjectDecorator {
// jei bus klaida, tai bus del to, kad neuzsaugom per konstruktoriu paduodamos reiksmes i global variable
GraphicObject decoratedGraphic = null;
public DecoratorLeft(GraphicObject decoratedGraphic) {
super(decoratedGraphic);
this.decoratedGraphic = decoratedGraphic;
}
@Override
public void draw(Graphics g, AlignStrategy align) {
alignLeft();
decoratedGraphic.draw(g, align);
//drawAlignedLeft(g, decoratedGraphic, align);
}
}
以下是错误消息:
java.lang.NullPointerException
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at objprog1.Whiteboard.paintComponent(Whiteboard.java:157)
我使用printstactrace和runtimeexception再次修改了try / catch语句:
java.lang.NullPointerException
at objprog1.Whiteboard.paintComponent(Whiteboard.java:179)
at javax.swing.JComponent.paint(JComponent.java:1054)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JSplitPane.paintChildren(JSplitPane.java:1047)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JLayeredPane.paint(JLayeredPane.java:585)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paintToOffscreen(JComponent.java:5228)
at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1482)
at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1413)
at javax.swing.RepaintManager.paint(RepaintManager.java:1206)
at javax.swing.JComponent.paint(JComponent.java:1040)
at java.awt.GraphicsCallback$PaintCallback.run(GraphicsCallback.java:39)
at sun.awt.SunGraphicsCallback.runOneComponent(SunGraphicsCallback.java:78)
at sun.awt.SunGraphicsCallback.runComponents(SunGraphicsCallback.java:115)
at java.awt.Container.paint(Container.java:1967)
at java.awt.Window.paint(Window.java:3867)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:781)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:728)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:677)
at javax.swing.RepaintManager.access$700(RepaintManager.java:59)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1621)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:705)
at java.awt.EventQueue.access$000(EventQueue.java:101)
at java.awt.EventQueue$3.run(EventQueue.java:666)
at java.awt.EventQueue$3.run(EventQueue.java:664)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:675)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
Exception in thread "AWT-EventQueue-0" java.lang.RuntimeException: java.lang.NullPointerException
at objprog1.Whiteboard.paintComponent(Whiteboard.java:182)
at javax.swing.JComponent.paint(JComponent.java:1054)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JSplitPane.paintChildren(JSplitPane.java:1047)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JLayeredPane.paint(JLayeredPane.java:585)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paintToOffscreen(JComponent.java:5228)
at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1482)
at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1413)
at javax.swing.RepaintManager.paint(RepaintManager.java:1206)
at javax.swing.JComponent.paint(JComponent.java:1040)
at java.awt.GraphicsCallback$PaintCallback.run(GraphicsCallback.java:39)
at sun.awt.SunGraphicsCallback.runOneComponent(SunGraphicsCallback.java:78)
at sun.awt.SunGraphicsCallback.runComponents(SunGraphicsCallback.java:115)
at java.awt.Container.paint(Container.java:1967)
at java.awt.Window.paint(Window.java:3867)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:781)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:728)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:677)
at javax.swing.RepaintManager.access$700(RepaintManager.java:59)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1621)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:705)
at java.awt.EventQueue.access$000(EventQueue.java:101)
at java.awt.EventQueue$3.run(EventQueue.java:666)
at java.awt.EventQueue$3.run(EventQueue.java:664)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:675)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:211)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:128)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:117)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:113)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:90)
Caused by: java.lang.NullPointerException
at objprog1.Whiteboard.paintComponent(Whiteboard.java:179)
... 39 more
答案 0 :(得分:0)
我认为你可以通过创建类似的东西来简化:
protected GraphicObject getNewInstance ( GraphicObject decoratedGraphic ){
...
}
在你的GraphicObject
类中,所以每个类都有这个方法,你可以直接从对象中调用它。