I am using a Eclipse, on Mac OS X 11.4.
That means its JDK 1.5.

Unable to compile the explorer demo with the following errors :

oading module 'com.extjs.gxt.samples.resources.Resources'
Module location: file:/Users/alan/workspace/gwttest2/src/com/extjs/gxt/samples/resources/Resources.gwt.xml
Loading inherited module 'com.extjs.gxt.ui.GXT'
Module location: jar:file:/Users/alan/workspace/gwttest2/WebContent/WEB-INF/lib/gxt.jar!/com/extjs/gxt/ui/GXT.gwt.xml
Loading inherited module 'com.google.gwt.i18n.I18N'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/i18n/I18N.gwt.xml
Loading inherited module 'com.google.gwt.http.HTTP'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/http/HTTP.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/core/Core.gwt.xml
Loading inherited module 'com.google.gwt.dev.jjs.intrinsic.Intrinsic'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-dev-mac.jar!/com/google/gwt/dev/jjs/intrinsic/Intrinsic.gwt.xml
Loading inherited module 'com.google.gwt.emul.Emulation'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/emul/Emulation.gwt.xml
Loading inherited module 'com.google.gwt.xml.XML'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/xml/XML.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/UserAgent.gwt.xml
Loading inherited module 'com.google.gwt.json.JSON'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/json/JSON.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.User'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/User.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.RemoteService'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/RemoteService.gwt.xml
Loading inherited module 'com.google.gwt.user.HTTPRequest'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/HTTPRequest.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.DOM'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/DOM.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.HTTPRequest'
Module 'com.google.gwt.user.HTTPRequest' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.History'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/History.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.Popup'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/Popup.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.Form'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/Form.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.TextBox'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/TextBox.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.Focus'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/Focus.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.ImageBundle'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/ImageBundle.gwt.xml
Loading inherited module 'com.google.gwt.user.ClippedImage'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/ClippedImage.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.ClippedImage'
Module 'com.google.gwt.user.ClippedImage' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.RichText'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/RichText.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.i18n.I18N'
Module 'com.google.gwt.i18n.I18N' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.SplitPanel'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/SplitPanel.gwt.xml
Loading inherited module 'com.google.gwt.user.User'
Module 'com.google.gwt.user.User' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.ListBox'
Module location: jar:file:/Users/alan/gwt-mac-1.4.62/gwt-user.jar!/com/google/gwt/user/ListBox.gwt.xml
Loading inherited module 'com.google.gwt.core.Core'
Module 'com.google.gwt.core.Core' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.UserAgent'
Module 'com.google.gwt.user.UserAgent' has already been loaded and will be skipped
Loading inherited module 'com.google.gwt.user.DOM'
Module 'com.google.gwt.user.DOM' has already been loaded and will be skipped
[ERROR] Line 2: Unexpected exception while processing element 'inherits'
java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at com.google.gwt.dev.cfg.ModuleDefSchema$ObjAttrCvt.convertToArg(ModuleDefSchema.java:493)
at com.google.gwt.dev.util.xml.HandlerArgs.convertToArg(HandlerArgs.java:64)
at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:214)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:242)
at org.apache.xerces.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:454)
at org.apache.xerces.impl.XMLNamespaceBinder.startElement(XMLNamespaceBinder.java:571)
at org.apache.xerces.impl.dtd.XMLDTDValidator.startElement(XMLDTDValidator.java:756)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanStartElement(XMLDocumentFragmentScannerImpl.java:752)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(XMLDocumentFragmentScannerImpl.java:1453)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:333)
at org.apache.xerces.parsers.DTDConfiguration.parse(DTDConfiguration.java:524)
at org.apache.xerces.parsers.DTDConfiguration.parse(DTDConfiguration.java:580)
at org.apache.xerces.parsers.XMLParser.parse(XMLParser.java:152)
at org.apache.xerces.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1169)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.parse(ReflectiveParser.java:295)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.access$100(ReflectiveParser.java:48)
at com.google.gwt.dev.util.xml.ReflectiveParser.parse(ReflectiveParser.java:366)
at com.google.gwt.dev.cfg.ModuleDefLoader.nestedLoad(ModuleDefLoader.java:175)
at com.google.gwt.dev.cfg.ModuleDefSchema$BodySchema.__inherits_begin(ModuleDefSchema.java:143)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:223)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:242)
at org.apache.xerces.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:454)
at org.apache.xerces.parsers.AbstractXMLDocumentParser.emptyElement(AbstractXMLDocumentParser.java:217)
at org.apache.xerces.impl.XMLNamespaceBinder.emptyElement(XMLNamespaceBinder.java:594)
at org.apache.xerces.impl.dtd.XMLDTDValidator.emptyElement(XMLDTDValidator.java:777)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanStartElement(XMLDocumentFragmentScannerImpl.java:748)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(XMLDocumentFragmentScannerImpl.java:1453)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:333)
at org.apache.xerces.parsers.DTDConfiguration.parse(DTDConfiguration.java:524)
at org.apache.xerces.parsers.DTDConfiguration.parse(DTDConfiguration.java:580)
at org.apache.xerces.parsers.XMLParser.parse(XMLParser.java:152)
at org.apache.xerces.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1169)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.parse(ReflectiveParser.java:295)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.access$100(ReflectiveParser.java:48)
at com.google.gwt.dev.util.xml.ReflectiveParser.parse(ReflectiveParser.java:366)
at com.google.gwt.dev.cfg.ModuleDefLoader.nestedLoad(ModuleDefLoader.java:175)
at com.google.gwt.dev.cfg.ModuleDefLoader.load(ModuleDefLoader.java:208)
at com.google.gwt.dev.cfg.ModuleDefLoader.loadFromClassPath(ModuleDefLoader.java:81)
at com.google.gwt.dev.GWTCompiler.run(GWTCompiler.java:753)
at com.google.gwt.dev.GWTCompiler.run(GWTCompiler.java:745)
at com.google.gwt.dev.GWTCompiler.main(GWTCompiler.java:213)
[ERROR] Failure while parsing XML
com.google.gwt.core.ext.UnableToCompleteException: (see previous log entries)
at com.google.gwt.dev.util.xml.DefaultSchema.onHandlerException(DefaultSchema.java:56)
at com.google.gwt.dev.util.xml.Schema.onHandlerException(Schema.java:65)
at com.google.gwt.dev.util.xml.Schema.onHandlerException(Schema.java:65)
at com.google.gwt.dev.util.xml.HandlerMethod.invokeBegin(HandlerMethod.java:233)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.startElement(ReflectiveParser.java:242)
at org.apache.xerces.parsers.AbstractSAXParser.startElement(AbstractSAXParser.java:454)
at org.apache.xerces.parsers.AbstractXMLDocumentParser.emptyElement(AbstractXMLDocumentParser.java:217)
at org.apache.xerces.impl.XMLNamespaceBinder.emptyElement(XMLNamespaceBinder.java:594)
at org.apache.xerces.impl.dtd.XMLDTDValidator.emptyElement(XMLDTDValidator.java:777)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanStartElement(XMLDocumentFragmentScannerImpl.java:748)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContentDispatcher.dispatch(XMLDocumentFragmentScannerImpl.java:1453)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:333)
at org.apache.xerces.parsers.DTDConfiguration.parse(DTDConfiguration.java:524)
at org.apache.xerces.parsers.DTDConfiguration.parse(DTDConfiguration.java:580)
at org.apache.xerces.parsers.XMLParser.parse(XMLParser.java:152)
at org.apache.xerces.parsers.AbstractSAXParser.parse(AbstractSAXParser.java:1169)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.parse(ReflectiveParser.java:295)
at com.google.gwt.dev.util.xml.ReflectiveParser$Impl.access$100(ReflectiveParser.java:48)
at com.google.gwt.dev.util.xml.ReflectiveParser.parse(ReflectiveParser.java:366)
at com.google.gwt.dev.cfg.ModuleDefLoader.nestedLoad(ModuleDefLoader.java:175)
at com.google.gwt.dev.cfg.ModuleDefLoader.load(ModuleDefLoader.java:208)
at com.google.gwt.dev.cfg.ModuleDefLoader.loadFromClassPath(ModuleDefLoader.java:81)
at com.google.gwt.dev.GWTCompiler.run(GWTCompiler.java:753)
at com.google.gwt.dev.GWTCompiler.run(GWTCompiler.java:745)
at com.google.gwt.dev.GWTCompiler.main(GWTCompiler.java:213)
[ERROR] Build failed