<Pane maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" prefHeight="400.0" prefWidth="900.0" style="-fx-background-color: rgba(0, 100, 100, 0.1);" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1">
<children>
<TitledPane fx:id="title" prefHeight="8.0" prefWidth="${title.parent.width}" text="Yeet" />
<AnchorPane fx:id="main" layoutY="25.0" prefHeight="${main.parent.height - title.height}" prefWidth="${main.parent.width}" />
</children>
</Pane>
问题是，当我尝试编译包含行prefHeight="${main.parent.height - title.height}或main.parent.height-8.00的行时，它们都会导致构建引发错误：
Exception in Application start method
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.sun.javafx.application.LauncherImpl.launchApplicationWithArgs(LauncherImpl.java:389)
at com.sun.javafx.application.LauncherImpl.launchApplication(LauncherImpl.java:328)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at sun.launcher.LauncherHelper$FXHelper.main(LauncherHelper.java:767)
Caused by: java.lang.RuntimeException: Exception in Application start method
at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$154(LauncherImpl.java:182)
at java.lang.Thread.run(Thread.java:748)
Caused by: javafx.fxml.LoadException:
/C:/Users/user/project/sample/main.fxml:23
at javafx.fxml.FXMLLoader.constructLoadException(FXMLLoader.java:2601)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2579)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2441)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3214)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3175)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3148)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3124)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:3104)
at javafx.fxml.FXMLLoader.load(FXMLLoader.java:3097)
at sample.Main.start(Main.java:18)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$161(LauncherImpl.java:863)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$174(PlatformImpl.java:326)
at com.sun.javafx.application.PlatformImpl.lambda$null$172(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$173(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$null$147(WinApplication.java:177)
... 1 more
Caused by: java.lang.NullPointerException
at com.sun.javafx.fxml.expression.Expression.lambda$subtract$2(Expression.java:868)
at com.sun.javafx.fxml.expression.BinaryExpression.evaluate(BinaryExpression.java:55)
at com.sun.javafx.fxml.expression.ExpressionValue.getValue(ExpressionValue.java:192)
at com.sun.javafx.binding.ExpressionHelper.addListener(ExpressionHelper.java:54)
at javafx.beans.value.ObservableValueBase.addListener(ObservableValueBase.java:55)
at com.sun.javafx.fxml.expression.ExpressionValue.addListener(ExpressionValue.java:201)
at javafx.beans.binding.DoubleBinding.bind(DoubleBinding.java:153)
at javafx.beans.property.DoublePropertyBase$2.<init>(DoublePropertyBase.java:186)
at javafx.beans.property.DoublePropertyBase.bind(DoublePropertyBase.java:184)
at javafx.css.StyleableDoubleProperty.bind(StyleableDoubleProperty.java:75)
at javafx.fxml.FXMLLoader$Element.processPropertyAttribute(FXMLLoader.java:319)
at javafx.fxml.FXMLLoader$Element.processInstancePropertyAttributes(FXMLLoader.java:235)
at javafx.fxml.FXMLLoader$ValueElement.processEndElement(FXMLLoader.java:767)
at javafx.fxml.FXMLLoader.processEndElement(FXMLLoader.java:2823)
at javafx.fxml.FXMLLoader.loadImpl(FXMLLoader.java:2532)
... 17 more
Exception running application sample.Main
我不明白原因。如果仅使用main.parent.height，则代码可以正确编译。我还有其他几行在FXML上正确使用算术运算。