我遇到了代码空指针的问题,很难确定原因。我试着用Java和Clojure编写一个Swing应用程序,其中有一个Clojure函数调用了我的JPanel子类中的一个函数,但我一直收到许多异常,让我很烦恼。
我已经尽可能地简化了代码,但仍然出现这些错误。
我已经尽可能地简化了代码,但仍然出现这些错误。
(defn draw-state
"Draws the current state."
[state display]
(do-swing (.drawBlocks display)))
这是整个JPanel子类,虽然我认为唯一相关的部分是drawBlocks函数(最终将替换blocks数组):
public class TDisplay extends JPanel {
private Block[] blocks = new Block[1];
/**
* Create the panel.
*/
public TDisplay() {
setBorder(new BevelBorder(BevelBorder.LOWERED, null, null, null, null));
setBackground(Color.WHITE);
setLayout(new FlowLayout(FlowLayout.CENTER, 5, 5));
blocks[0] = new Block(3, 4, 0);
}
public void paintComponent(Graphics g) {
super.paintComponent(g);
for (Block b : blocks) {
if (b == null) continue;
switch (b.colour) {
case 0:
g.setColor(Color.WHITE);
break;
case 1:
g.setColor(Color.BLACK);
break;
}
g.fillRect(b.x * Block.SIZE, b.y * Block.SIZE, Block.SIZE, Block.SIZE);
}
}
public void drawBlocks() {
System.out.println("Hello world");
}
并且堆栈跟踪:
}
Exception in thread "AWT-EventQueue-0" java.lang.RuntimeException: java.lang.NullPointerException
at clojure.lang.AFn.run(AFn.java:28)
at java.awt.event.InvocationEvent.dispatch(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)
Caused by: java.lang.NullPointerException
at clojure.lang.Reflector.invokeNoArgInstanceMember(Reflector.java:263)
at core$draw_state$fn__401.invoke(core.clj:43)
at clojure.lang.AFn.run(AFn.java:24)
... 8 more
我有一种感觉,觉得自己错过了某些非常显而易见的东西,希望能得到任何帮助。
谢谢!
draw-state
的代码怎么样——有可能传入null
作为display
吗?state
是什么? - Matt Fenwick