我不确定是什么引起了这里的问题，但是，每当我单击要测试的SWT列表中的某个项目时，它都会引发空指针错误。如果有人可以帮助甚至解释如何正确实现SWT列表侦听器，那就太好了。我讨厌将这么多的代码放在一个问题中，但是我无法具体定位错误。
public class Widget {
/**
* Launch the application.
* @param args
*/
private Label FinalPricelabel;
public Widget(Display display) {
uI(display);
}
private void uI(Display display) {
display = Display.getDefault();
Shell shell = new Shell();
shell.setFont(SWTResourceManager.getFont("Segoe UI", 12, SWT.NORMAL));
shell.setBackground(SWTResourceManager.getColor(SWT.COLOR_WIDGET_LIGHT_SHADOW));
shell.setSize(1065, 435);
shell.setText("SWT Application");
Label titlelabel = new Label(shell, SWT.NONE);
titlelabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_WIDGET_DARK_SHADOW));
titlelabel.setForeground(SWTResourceManager.getColor(SWT.COLOR_WHITE));
titlelabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 48, SWT.NORMAL));
titlelabel.setAlignment(SWT.CENTER);
titlelabel.setBounds(0, 0, 1049, 81);
titlelabel.setText("Countertop Price Calculator");
Label constructionlabel = new Label(shell, SWT.NONE);
constructionlabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
constructionlabel.setAlignment(SWT.CENTER);
constructionlabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
constructionlabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
constructionlabel.setBounds(20, 87, 110, 22);
constructionlabel.setText("Construction");
List species = new List(shell, SWT.BORDER);
species.add("African Mahogany");
species.add("Brazilian Cherry(Jatoba)");
species.add("Clear Cherry");
species.add("Character Cherry");
species.add("Douglas Fir");
species.add("Genuine Mahogany");
species.add("Hickory");
species.add("Rustic Hickory");
species.add("Maple");
species.add("Maple with 2 Cherry Strips");
species.add("Mixed Maple Cherry");
species.add("Yellow Pine");
species.add("Knotty Pine");
species.add("Poplar");
species.add("Purpleheart");
species.add("Red Oak");
species.add("Sapele");
species.add("Spanish Cedar");
species.add("White Oak");
species.add("Rustic White Oak");
species.setBounds(173, 115, 110, 68);
Label specieslabel = new Label(shell, SWT.NONE);
specieslabel.setText("Species");
specieslabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
specieslabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
specieslabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
specieslabel.setAlignment(SWT.CENTER);
specieslabel.setBounds(173, 87, 110, 22);
Label edgelabel = new Label(shell, SWT.NONE);
edgelabel.setText("Edge Detail");
edgelabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
edgelabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
edgelabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
edgelabel.setAlignment(SWT.CENTER);
edgelabel.setBounds(325, 87, 110, 22);
List edgedet = new List(shell, SWT.BORDER);
edgedet.add("None +$0");
edgedet.add("1/8\" Radius");
edgedet.add("1/4\" Radius");
edgedet.add("7/16\" Radius");
edgedet.add("1/2\" Radius");
edgedet.add("Beveled Edge");
edgedet.add("Bullnose");
edgedet.add("Ogee Bit");
edgedet.add("Ogee Fillet");
edgedet.add("French Traditional");
edgedet.add("Small Roman Ogee");
edgedet.add("Medium Roman Ogee");
edgedet.add("Large Roman Ogee");
edgedet.add("Small Cove & Bead");
edgedet.add("Large Cove & Bead");
edgedet.add("Wavy Edge");
edgedet.add("Classic Molding");
edgedet.add("Small Classic");
edgedet.add("Large Classic");
edgedet.add("Classic Pattern");
edgedet.add("Classic Cove");
edgedet.add("Tabletop Small Thumbnail");
edgedet.add("Tabletop Large Thumbnail");
edgedet.add("Tabletop Traditional");
edgedet.add("Tabletop Thumbnail & Bead");
edgedet.add("Tabletop Classic");
edgedet.add("Tabletop");
edgedet.setBounds(325, 115, 110, 68);
String tester = "lolcat";
edgedet.addListener(SWT.Selection, event -> onListItemSelect(edgedet));
List texture = new List(shell, SWT.BORDER);
texture.add("None $0");
texture.add("Hand Planed $5");
texture.add("Hand Planed, Time Worn $7.50");
texture.add("Hand Planed, Time Worn(filled) $9");
texture.add("Hand Scraped $5");
texture.add("Wire Brushed $5");
texture.add("Saw Cut(circular saw) $5");
texture.add("Saw Cut(band saw) $5");
texture.add("Chiseled Edges $5");
texture.add("Dings & Dents $2");
texture.add("Worm Holes $2");
texture.add("Distressed(select multiple)");
texture.setBounds(477, 115, 110, 68);
Label texturelabel = new Label(shell, SWT.NONE);
texturelabel.setText("Texture");
texturelabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
texturelabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
texturelabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
texturelabel.setAlignment(SWT.CENTER);
texturelabel.setBounds(477, 87, 110, 22);
Label lblSize = new Label(shell, SWT.NONE);
lblSize.setBackground(SWTResourceManager.getColor(SWT.COLOR_WIDGET_LIGHT_SHADOW));
lblSize.setAlignment(SWT.CENTER);
lblSize.setFont(SWTResourceManager.getFont("Trebuchet MS", 36, SWT.NORMAL));
lblSize.setBounds(782, 87, 110, 68);
lblSize.setText("SIZE");
Label thicknesslabel = new Label(shell, SWT.NONE);
thicknesslabel.setText("Thickness");
thicknesslabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
thicknesslabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
thicknesslabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
thicknesslabel.setAlignment(SWT.CENTER);
thicknesslabel.setBounds(649, 161, 110, 22);
Label depthlabel = new Label(shell, SWT.NONE);
depthlabel.setText("Depth");
depthlabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
depthlabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
depthlabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
depthlabel.setAlignment(SWT.CENTER);
depthlabel.setBounds(782, 161, 110, 22);
Label lengthlabel = new Label(shell, SWT.NONE);
lengthlabel.setText("Length");
lengthlabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
lengthlabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
lengthlabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
lengthlabel.setAlignment(SWT.CENTER);
lengthlabel.setBounds(912, 161, 110, 22);
List thickness = new List(shell, SWT.BORDER);
thickness.setBounds(649, 202, 110, 68);
List construct = new List(shell, SWT.BORDER);
construct.add("Edge Grain");
construct.add("End Grain");
construct.add("Wide Plank");
construct.setBounds(20, 115, 110, 68);
Label finishlabel = new Label(shell, SWT.NONE);
finishlabel.setText("Finish");
finishlabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
finishlabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
finishlabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
finishlabel.setAlignment(SWT.CENTER);
finishlabel.setBounds(20, 209, 110, 22);
List finish = new List(shell, SWT.BORDER);
finish.setBounds(20, 237, 110, 68);
List anglecuts = new List(shell, SWT.BORDER);
anglecuts.setBounds(173, 237, 110, 68);
Label anglecutlabel = new Label(shell, SWT.NONE);
anglecutlabel.setText("Angle Cuts");
anglecutlabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
anglecutlabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
anglecutlabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
anglecutlabel.setAlignment(SWT.CENTER);
anglecutlabel.setBounds(173, 209, 110, 22);
List cnccuts = new List(shell, SWT.BORDER);
cnccuts.setBounds(325, 237, 110, 68);
Label cnclabel = new Label(shell, SWT.NONE);
cnclabel.setText("CNC Cuts");
cnclabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
cnclabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 12, SWT.BOLD));
cnclabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
cnclabel.setAlignment(SWT.CENTER);
cnclabel.setBounds(325, 209, 110, 22);
Spinner depthspinner = new Spinner(shell, SWT.BORDER);
depthspinner.setFont(SWTResourceManager.getFont("Segoe UI", 36, SWT.NORMAL));
depthspinner.setBounds(782, 202, 110, 68);
Spinner lengthspinner = new Spinner(shell, SWT.BORDER);
lengthspinner.setFont(SWTResourceManager.getFont("Segoe UI", 36, SWT.NORMAL));
lengthspinner.setBounds(912, 202, 110, 68);
Label FinalPricelabel = new Label(shell, SWT.NONE);
FinalPricelabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_DARK_GRAY));
FinalPricelabel.setForeground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
FinalPricelabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 42, SWT.NORMAL));
FinalPricelabel.setAlignment(SWT.CENTER);
FinalPricelabel.setBounds(10, 324, 1029, 72);
FinalPricelabel.setText("Final Price: " + tester);
//if texture == "Distressed(select multiple)"
Label distressedlabel = new Label(shell, SWT.NONE);
distressedlabel.setText("Distressing Options");
distressedlabel.setForeground(SWTResourceManager.getColor(192, 192, 192));
distressedlabel.setFont(SWTResourceManager.getFont("Trebuchet MS", 11, SWT.BOLD));
distressedlabel.setBackground(SWTResourceManager.getColor(SWT.COLOR_BLACK));
distressedlabel.setAlignment(SWT.CENTER);
distressedlabel.setBounds(477, 189, 110, 42);
List disstressedoptions = new List(shell, SWT.BORDER | SWT.MULTI);
disstressedoptions.setBounds(477, 237, 110, 68);
//if texture == "Distressed(select multiple)"
shell.open();
shell.layout();
while (!shell.isDisposed()) {
if (!display.readAndDispatch()) {
display.sleep();
}
}
}
private void onListItemSelect(List list) {
String[] items = list.getSelection();
String holder = FinalPricelabel.getText();
FinalPricelabel.setText(holder + items[0]);
}
@SuppressWarnings("unused")
public static void main(String[] args) {
Display display = new Display();
Widget ex = new Widget(display);
display.dispose();
}
}
抛出的错误是
Exception in thread "main" java.lang.NullPointerException
at hardwood_calculator_java.Widget.onListItemSelect(Widget.java:272)
at hardwood_calculator_java.Widget.lambda$0(Widget.java:127)
at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:89)
at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4107)
at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1037)
at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:3924)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3526)
at hardwood_calculator_java.Widget.uI(Widget.java:264)
at hardwood_calculator_java.Widget.<init>(Widget.java:28)
at hardwood_calculator_java.Widget.main(Widget.java:279)
第272行到底是什么？onListItemSelect中的对象之一具有null值，对其进行跟踪，然后查看为什么它现在为null，必须在此行中为该对象272