title: JDK-9 won't let me use Strings: "java.lang.String is ambiguous"?
body: <p>A week ago, every project I had was working normally, but when I updated my Eclipse to the Oxygen version and installed JDK-9, things have come strange. 
  When I try to use Strings, after some time (more like every time I save the project) the compiler throws me an error for every String in my project.</p>
  
  <hr>
  
  <h2>String type being recognized as an ambiguous class</h2>
  
  <blockquote>
    <p><a href="https://i.stack.imgur.com/gaKt9.png" rel="noreferrer"><img src="https://i.stack.imgur.com/gaKt9.png" alt="String error"></a>
    <br><br>"The type java.lang.String is ambiguous"</p>
  </blockquote>
  
  <p>So I've tried importing <code>java.lang.String</code> to see if it fix the error: </p>
  
  <h2>After the import</h2>
  
  <blockquote>
    <p><a href="https://i.stack.imgur.com/D0srO.png" rel="noreferrer"><img src="https://i.stack.imgur.com/D0srO.png" alt="String error and import"></a>
    "The package java.lang is accessible from more than one module: java.base, java.base"</p>
  </blockquote>
  
  <h3>Surprise! </h3>
  
  <p>It doesn't :(<br>
  I've tried searching these messages on Google and SO but I found nothing...<br>
  However, the code running in JDK 8 seems to run normally, check it out:  </p>
  
  <h2>Changing to JDK 8 (1.8.0_72)</h2>
  
  <p><a href="https://i.stack.imgur.com/qANxn.png" rel="noreferrer"><img src="https://i.stack.imgur.com/qANxn.png" alt="JDK 8 fixin&#39; it all"></a>
  (Code is not showing any errors before the Apply because I've applied, and open up the compiler preferences again for the screenshot)</p>
  
  <h2>The console error:</h2>
  
  <blockquote>
    <p>Error: Main method not found in class <code>me.mateusakino.ypul.yPuLTester</code>, please define the main method as:<br>
         <code>public static void main(String[] args)</code> 
    or a JavaFX application class must extend <code>javafx.application.Application</code></p>
  </blockquote>
  
  <h1>EDIT 1 - Main class</h1>
  
  <p><a href="https://i.stack.imgur.com/6Ttbi.png" rel="noreferrer"><img src="https://i.stack.imgur.com/6Ttbi.png" alt="enter image description here"></a></p>
  
  <h1>EDIT 5 - Stacktrace of the errors on <em>Edit 4</em> (workspace/.metadata/.log)</h1>
  
  <pre><code>!SESSION 2017-10-13 13:11:39.551 -----------------------------------------------
  eclipse.buildId=4.7.1.M20171009-0410
  java.version=9
  java.vendor=Oracle Corporation
  BootLoader constants: OS=win32, ARCH=x86_64, WS=win32, NL=en_US
  Framework arguments:  -product org.eclipse.epp.package.java.product
  Command-line arguments:  -os win32 -ws win32 -arch x86_64 -product org.eclipse.epp.package.java.product
  
  This is a continuation of log file C:\Users\Home\Desktop\Developer Folders\Java\Oxygen Workspace\.metadata\.bak_0.log
  Created Time: 2017-10-13 13:43:14.245
  
  !ENTRY org.eclipse.core.resources 4 75 2017-10-13 13:43:14.245
  !MESSAGE Errors occurred during the build.
  !SUBENTRY 1 org.eclipse.jdt.core 4 75 2017-10-13 13:43:14.245
  !MESSAGE Errors running builder 'Java Builder' on project 'yPuL'.
  !STACK 0
  java.lang.NullPointerException
      at org.eclipse.jdt.internal.compiler.lookup.ModuleBinding$UnNamedModule.canAccess(ModuleBinding.java:71)
      at org.eclipse.jdt.internal.compiler.lookup.PackageBinding.getTypeOrPackage(PackageBinding.java:230)
      at org.eclipse.jdt.internal.compiler.lookup.Scope.getPackage(Scope.java:2922)
      at org.eclipse.jdt.internal.compiler.ast.QualifiedTypeReference.getTypeBinding(QualifiedTypeReference.java:108)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.getTypeBinding(ArrayQualifiedTypeReference.java:93)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.internalResolveType(TypeReference.java:495)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.internalResolveType(ArrayQualifiedTypeReference.java:107)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:586)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:582)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesWithSuspendedTempErrorHandlingPolicy(SourceTypeBinding.java:1932)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesFor(SourceTypeBinding.java:1835)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.methods(SourceTypeBinding.java:1525)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.internalFaultInTypeForFieldsAndMethods(SourceTypeBinding.java:907)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.faultInTypesForFieldsAndMethods(SourceTypeBinding.java:902)
      at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.faultInTypes(CompilationUnitScope.java:514)
      at org.eclipse.jdt.internal.compiler.Compiler.process(Compiler.java:882)
      at org.eclipse.jdt.internal.compiler.ProcessTaskManager.run(ProcessTaskManager.java:141)
      at java.base/java.lang.Thread.run(Thread.java:844)
  
  !ENTRY org.eclipse.core.resources 4 2 2017-10-13 13:43:24.691
  !MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.core.resources".
  !STACK 0
  java.lang.NullPointerException
  
  !ENTRY org.eclipse.core.resources 4 75 2017-10-13 13:43:24.695
  !MESSAGE Errors occurred during the build.
  !SUBENTRY 1 org.eclipse.jdt.core 4 75 2017-10-13 13:43:24.695
  !MESSAGE Errors running builder 'Java Builder' on project 'yPuL'.
  !STACK 0
  java.lang.NullPointerException
  
  !ENTRY org.eclipse.core.resources 4 2 2017-10-13 13:43:39.104
  !MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.core.resources".
  !STACK 0
  java.lang.NullPointerException
  
  !ENTRY org.eclipse.core.resources 4 75 2017-10-13 13:43:39.110
  !MESSAGE Errors occurred during the build.
  !SUBENTRY 1 org.eclipse.jdt.core 4 75 2017-10-13 13:43:39.110
  !MESSAGE Errors running builder 'Java Builder' on project 'yPuL'.
  !STACK 0
  java.lang.NullPointerException
  
  !ENTRY org.eclipse.core.resources 4 2 2017-10-13 13:44:37.810
  !MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.core.resources".
  !STACK 0
  java.lang.NullPointerException
      at org.eclipse.jdt.internal.compiler.lookup.ModuleBinding$UnNamedModule.canAccess(ModuleBinding.java:71)
      at org.eclipse.jdt.internal.compiler.lookup.PackageBinding.getTypeOrPackage(PackageBinding.java:230)
      at org.eclipse.jdt.internal.compiler.lookup.Scope.getPackage(Scope.java:2922)
      at org.eclipse.jdt.internal.compiler.ast.QualifiedTypeReference.getTypeBinding(QualifiedTypeReference.java:108)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.getTypeBinding(ArrayQualifiedTypeReference.java:93)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.internalResolveType(TypeReference.java:495)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.internalResolveType(ArrayQualifiedTypeReference.java:107)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:586)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:582)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesWithSuspendedTempErrorHandlingPolicy(SourceTypeBinding.java:1932)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesFor(SourceTypeBinding.java:1835)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.methods(SourceTypeBinding.java:1525)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.internalFaultInTypeForFieldsAndMethods(SourceTypeBinding.java:907)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.faultInTypesForFieldsAndMethods(SourceTypeBinding.java:902)
      at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.faultInTypes(CompilationUnitScope.java:514)
      at org.eclipse.jdt.internal.compiler.Compiler.process(Compiler.java:882)
      at org.eclipse.jdt.internal.compiler.ProcessTaskManager.run(ProcessTaskManager.java:141)
      at java.base/java.lang.Thread.run(Thread.java:844)
  
  !ENTRY org.eclipse.core.resources 4 75 2017-10-13 13:44:37.814
  !MESSAGE Errors occurred during the build.
  !SUBENTRY 1 org.eclipse.jdt.core 4 75 2017-10-13 13:44:37.814
  !MESSAGE Errors running builder 'Java Builder' on project 'yPuL'.
  !STACK 0
  java.lang.NullPointerException
      at org.eclipse.jdt.internal.compiler.lookup.ModuleBinding$UnNamedModule.canAccess(ModuleBinding.java:71)
      at org.eclipse.jdt.internal.compiler.lookup.PackageBinding.getTypeOrPackage(PackageBinding.java:230)
      at org.eclipse.jdt.internal.compiler.lookup.Scope.getPackage(Scope.java:2922)
      at org.eclipse.jdt.internal.compiler.ast.QualifiedTypeReference.getTypeBinding(QualifiedTypeReference.java:108)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.getTypeBinding(ArrayQualifiedTypeReference.java:93)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.internalResolveType(TypeReference.java:495)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.internalResolveType(ArrayQualifiedTypeReference.java:107)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:586)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:582)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesWithSuspendedTempErrorHandlingPolicy(SourceTypeBinding.java:1932)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesFor(SourceTypeBinding.java:1835)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.methods(SourceTypeBinding.java:1525)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.internalFaultInTypeForFieldsAndMethods(SourceTypeBinding.java:907)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.faultInTypesForFieldsAndMethods(SourceTypeBinding.java:902)
      at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.faultInTypes(CompilationUnitScope.java:514)
      at org.eclipse.jdt.internal.compiler.Compiler.process(Compiler.java:882)
      at org.eclipse.jdt.internal.compiler.ProcessTaskManager.run(ProcessTaskManager.java:141)
      at java.base/java.lang.Thread.run(Thread.java:844)
  
  !ENTRY org.eclipse.core.resources 4 2 2017-10-13 13:44:53.701
  !MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.core.resources".
  !STACK 0
  java.lang.NullPointerException
      at org.eclipse.jdt.internal.compiler.lookup.ModuleBinding$UnNamedModule.canAccess(ModuleBinding.java:71)
      at org.eclipse.jdt.internal.compiler.lookup.PackageBinding.getTypeOrPackage(PackageBinding.java:230)
      at org.eclipse.jdt.internal.compiler.lookup.Scope.getPackage(Scope.java:2922)
      at org.eclipse.jdt.internal.compiler.ast.QualifiedTypeReference.getTypeBinding(QualifiedTypeReference.java:108)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.getTypeBinding(ArrayQualifiedTypeReference.java:93)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.internalResolveType(TypeReference.java:495)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.internalResolveType(ArrayQualifiedTypeReference.java:107)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:586)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:582)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesWithSuspendedTempErrorHandlingPolicy(SourceTypeBinding.java:1932)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesFor(SourceTypeBinding.java:1835)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.methods(SourceTypeBinding.java:1525)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.internalFaultInTypeForFieldsAndMethods(SourceTypeBinding.java:907)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.faultInTypesForFieldsAndMethods(SourceTypeBinding.java:902)
      at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.faultInTypes(CompilationUnitScope.java:514)
      at org.eclipse.jdt.internal.compiler.Compiler.process(Compiler.java:882)
      at org.eclipse.jdt.internal.compiler.ProcessTaskManager.run(ProcessTaskManager.java:141)
      at java.base/java.lang.Thread.run(Thread.java:844)
  
  !ENTRY org.eclipse.core.resources 4 75 2017-10-13 13:44:53.706
  !MESSAGE Errors occurred during the build.
  !SUBENTRY 1 org.eclipse.jdt.core 4 75 2017-10-13 13:44:53.706
  !MESSAGE Errors running builder 'Java Builder' on project 'yPuL'.
  !STACK 0
  java.lang.NullPointerException
      at org.eclipse.jdt.internal.compiler.lookup.ModuleBinding$UnNamedModule.canAccess(ModuleBinding.java:71)
      at org.eclipse.jdt.internal.compiler.lookup.PackageBinding.getTypeOrPackage(PackageBinding.java:230)
      at org.eclipse.jdt.internal.compiler.lookup.Scope.getPackage(Scope.java:2922)
      at org.eclipse.jdt.internal.compiler.ast.QualifiedTypeReference.getTypeBinding(QualifiedTypeReference.java:108)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.getTypeBinding(ArrayQualifiedTypeReference.java:93)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.internalResolveType(TypeReference.java:495)
      at org.eclipse.jdt.internal.compiler.ast.ArrayQualifiedTypeReference.internalResolveType(ArrayQualifiedTypeReference.java:107)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:586)
      at org.eclipse.jdt.internal.compiler.ast.TypeReference.resolveType(TypeReference.java:582)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesWithSuspendedTempErrorHandlingPolicy(SourceTypeBinding.java:1932)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.resolveTypesFor(SourceTypeBinding.java:1835)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.methods(SourceTypeBinding.java:1525)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.internalFaultInTypeForFieldsAndMethods(SourceTypeBinding.java:907)
      at org.eclipse.jdt.internal.compiler.lookup.SourceTypeBinding.faultInTypesForFieldsAndMethods(SourceTypeBinding.java:902)
      at org.eclipse.jdt.internal.compiler.lookup.CompilationUnitScope.faultInTypes(CompilationUnitScope.java:514)
      at org.eclipse.jdt.internal.compiler.Compiler.process(Compiler.java:882)
      at org.eclipse.jdt.internal.compiler.ProcessTaskManager.run(ProcessTaskManager.java:141)
      at java.base/java.lang.Thread.run(Thread.java:844)
  !SESSION 2017-10-13 13:46:42.392 -----------------------------------------------
  eclipse.buildId=4.7.1.M20171009-0410
  java.version=9
  java.vendor=Oracle Corporation
  BootLoader constants: OS=win32, ARCH=x86_64, WS=win32, NL=en_US
  Framework arguments:  -product org.eclipse.epp.package.java.product
  Command-line arguments:  -os win32 -ws win32 -arch x86_64 -product org.eclipse.epp.package.java.product
  
  !ENTRY org.eclipse.jface 2 0 2017-10-13 13:46:49.852
  !MESSAGE Keybinding conflicts occurred.  They may interfere with normal accelerator operation.
  !SUBENTRY 1 org.eclipse.jface 2 0 2017-10-13 13:46:49.852
  !MESSAGE A conflict occurred for CTRL+TAB:
  Binding(CTRL+TAB,
      ParameterizedCommand(Command(org.eclipse.ui.window.nextEditor,Next Editor,
          Switch to the next editor,
          Category(org.eclipse.ui.category.window,Window,null,true),
          org.eclipse.ui.internal.WorkbenchHandlerServiceHandler@5e8359da,
          ,,true),null),
      org.eclipse.ui.defaultAcceleratorConfiguration,
      org.eclipse.ui.contexts.window,,,system)
  Binding(CTRL+TAB,
      ParameterizedCommand(Command(org.eclipse.ui.window.nextEditor,Next Editor,
          Switch to the next editor,
          Category(org.eclipse.ui.category.window,Window,null,true),
          org.eclipse.ui.internal.WorkbenchHandlerServiceHandler@5e8359da,
          ,,true),null),
      org.eclipse.ui.defaultAcceleratorConfiguration,
      org.eclipse.ui.contexts.window,,,system)
  !SUBENTRY 1 org.eclipse.jface 2 0 2017-10-13 13:46:49.852
  !MESSAGE A conflict occurred for CTRL+SHIFT+TAB:
  Binding(CTRL+SHIFT+TAB,
      ParameterizedCommand(Command(org.eclipse.ui.window.previousEditor,Previous Editor,
          Switch to the previous editor,
          Category(org.eclipse.ui.category.window,Window,null,true),
          org.eclipse.ui.internal.WorkbenchHandlerServiceHandler@36b6802f,
          ,,true),null),
      org.eclipse.ui.defaultAcceleratorConfiguration,
      org.eclipse.ui.contexts.window,,,system)
  Binding(CTRL+SHIFT+TAB,
      ParameterizedCommand(Command(org.eclipse.ui.window.previousEditor,Previous Editor,
          Switch to the previous editor,
          Category(org.eclipse.ui.category.window,Window,null,true),
          org.eclipse.ui.internal.WorkbenchHandlerServiceHandler@36b6802f,
          ,,true),null),
      org.eclipse.ui.defaultAcceleratorConfiguration,
      org.eclipse.ui.contexts.window,,,system)
  
  !ENTRY com.vaadin.designer2 1 0 2017-10-13 13:47:00.882
  !MESSAGE Schedule fetching all notifications
  
  !ENTRY com.vaadin.designer2 1 0 2017-10-13 13:47:02.845
  !MESSAGE Parse notifications
  
  !ENTRY com.vaadin.designer2 1 0 2017-10-13 13:47:03.278
  !MESSAGE Requesting user info (notification settings URL)
  
  !ENTRY com.vaadin.designer2 1 0 2017-10-13 13:47:04.704
  !MESSAGE HTTP Response code :200
  
  !ENTRY org.eclipse.egit.ui 2 0 2017-10-13 13:47:06.014
  !MESSAGE Warning: The environment variable HOME is not set. The following directory will be used to store the Git
  user global configuration and to define the default location to store repositories: 'C:\Users\Home'. If this is
  not correct please set the HOME environment variable and restart Eclipse. Otherwise Git for Windows and
  EGit might behave differently since they see different configuration options.
  This warning can be switched off on the Team &gt; Git &gt; Confirmations and Warnings preference page.
  
  !ENTRY org.eclipse.dltk.core 4 0 2017-10-13 13:47:07.129
  !MESSAGE Unable to find any indexer
  
  !ENTRY org.apache.ivyde.eclipse 1 0 2017-10-13 13:48:10.554
  !MESSAGE starting IvyDE plugin
  
  !ENTRY org.apache.ivyde.eclipse 1 0 2017-10-13 13:48:10.571
  !MESSAGE IvyDE plugin started
  
  !ENTRY org.eclipse.ui 2 2 2017-10-13 13:48:12.395
  !MESSAGE Invalid property category path: org.eclipse.cdt.managedbuilder.ui.properties.Page_head_build (bundle: org.eclipse.cdt.ui, propertyPage: org.eclipse.cdt.managedbuilder.ui.properties.BuildLogPreferencePage)
  
  !ENTRY org.eclipse.jface 4 2 2017-10-13 13:48:45.611
  !MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.jface".
  !STACK 0
  java.lang.IllegalArgumentException
      at org.eclipse.core.resources.ProjectScope.&lt;init&gt;(ProjectScope.java:58)
      at com.vaadin.designer2.eclipse.preferences.VisualDesignerPreferences.getProjectPreferences(VisualDesignerPreferences.java:680)
      at com.vaadin.designer2.eclipse.preferences.VisualDesignerPreferences.getDefaultDesigner(VisualDesignerPreferences.java:245)
      at com.vaadin.designer2.eclipse.preferences.DesignerProjectPreferencePage.createDesignerSelection(DesignerProjectPreferencePage.java:179)
      at com.vaadin.designer2.eclipse.preferences.DesignerProjectPreferencePage.createContents(DesignerProjectPreferencePage.java:109)
      at org.eclipse.jface.preference.PreferencePage.createControl(PreferencePage.java:241)
      at org.eclipse.jface.preference.PreferenceDialog.createPageControl(PreferenceDialog.java:1426)
      at org.eclipse.jface.preference.PreferenceDialog$8.run(PreferenceDialog.java:1193)
      at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
      at org.eclipse.ui.internal.JFaceUtil.lambda$0(JFaceUtil.java:44)
      at org.eclipse.jface.util.SafeRunnable.run(SafeRunnable.java:173)
      at org.eclipse.jface.preference.PreferenceDialog.showPage(PreferenceDialog.java:1185)
      at org.eclipse.ui.internal.dialogs.FilteredPreferenceDialog.showPage(FilteredPreferenceDialog.java:591)
      at org.eclipse.jface.preference.PreferenceDialog$5.lambda$0(PreferenceDialog.java:657)
      at org.eclipse.swt.custom.BusyIndicator.showWhile(BusyIndicator.java:70)
      at org.eclipse.jface.preference.PreferenceDialog$5.selectionChanged(PreferenceDialog.java:654)
      at org.eclipse.jface.viewers.StructuredViewer$3.run(StructuredViewer.java:872)
      at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
      at org.eclipse.ui.internal.JFaceUtil.lambda$0(JFaceUtil.java:44)
      at org.eclipse.jface.util.SafeRunnable.run(SafeRunnable.java:173)
      at org.eclipse.jface.viewers.StructuredViewer.firePostSelectionChanged(StructuredViewer.java:869)
      at org.eclipse.jface.viewers.StructuredViewer.handlePostSelect(StructuredViewer.java:1238)
      at org.eclipse.jface.viewers.StructuredViewer.lambda$0(StructuredViewer.java:1261)
      at org.eclipse.swt.events.SelectionListener$1.widgetSelected(SelectionListener.java:81)
      at org.eclipse.jface.util.OpenStrategy.firePostSelectionEvent(OpenStrategy.java:261)
      at org.eclipse.jface.util.OpenStrategy.access$5(OpenStrategy.java:256)
      at org.eclipse.jface.util.OpenStrategy$1.lambda$1(OpenStrategy.java:426)
      at org.eclipse.swt.widgets.RunnableLock.run(RunnableLock.java:37)
      at org.eclipse.swt.widgets.Synchronizer.runAsyncMessages(Synchronizer.java:182)
      at org.eclipse.swt.widgets.Display.runAsyncMessages(Display.java:4213)
      at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3820)
      at org.eclipse.jface.window.Window.runEventLoop(Window.java:818)
      at org.eclipse.jface.window.Window.open(Window.java:794)
      at org.eclipse.ui.dialogs.PropertyDialogAction.run(PropertyDialogAction.java:157)
      at org.eclipse.jface.action.Action.runWithEvent(Action.java:473)
      at org.eclipse.jface.action.ActionContributionItem.handleWidgetSelection(ActionContributionItem.java:565)
      at org.eclipse.jface.action.ActionContributionItem.lambda$4(ActionContributionItem.java:397)
      at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:86)
      at org.eclipse.swt.widgets.Display.sendEvent(Display.java:4428)
      at org.eclipse.swt.widgets.Widget.sendEvent(Widget.java:1079)
      at org.eclipse.swt.widgets.Display.runDeferredEvents(Display.java:4238)
      at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:3817)
      at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine$5.run(PartRenderingEngine.java:1150)
      at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:336)
      at org.eclipse.e4.ui.internal.workbench.swt.PartRenderingEngine.run(PartRenderingEngine.java:1039)
      at org.eclipse.e4.ui.internal.workbench.E4Workbench.createAndRunUI(E4Workbench.java:153)
      at org.eclipse.ui.internal.Workbench.lambda$3(Workbench.java:680)
      at org.eclipse.core.databinding.observable.Realm.runWithDefault(Realm.java:336)
      at org.eclipse.ui.internal.Workbench.createAndRunWorkbench(Workbench.java:594)
      at org.eclipse.ui.PlatformUI.createAndRunWorkbench(PlatformUI.java:148)
      at org.eclipse.ui.internal.ide.application.IDEApplication.start(IDEApplication.java:151)
      at org.eclipse.equinox.internal.app.EclipseAppHandle.run(EclipseAppHandle.java:196)
      at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.runApplication(EclipseAppLauncher.java:134)
      at org.eclipse.core.runtime.internal.adaptor.EclipseAppLauncher.start(EclipseAppLauncher.java:104)
      at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:388)
      at org.eclipse.core.runtime.adaptor.EclipseStarter.run(EclipseStarter.java:243)
      at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
      at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.base/java.lang.reflect.Method.invoke(Method.java:564)
      at org.eclipse.equinox.launcher.Main.invokeFramework(Main.java:653)
      at org.eclipse.equinox.launcher.Main.basicRun(Main.java:590)
      at org.eclipse.equinox.launcher.Main.run(Main.java:1499)
      at org.eclipse.equinox.launcher.Main.main(Main.java:1472)
  </code></pre>
  
  <h1>EDIT 7:</h1>
  
  <p>I could <a href="https://i.stack.imgur.com/gCO4i.png" rel="noreferrer">reproduce</a> it in another workspace (Temp Workspace), and here is the <code>.classpath</code> file asked from @howlger:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;classpath&gt;
      &lt;classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER"/&gt;
      &lt;classpathentry kind="src" path="src"/&gt;
      &lt;classpathentry kind="output" path="bin"/&gt;
  &lt;/classpath&gt;
  </code></pre>
  
  <h1>EDIT 8 - Eclipse report (Asked by <a href="https://stackoverflow.com/questions/46731161/jdk-9-wont-let-me-use-strings-java-lang-string-is-ambiguous?noredirect=1#comment80420889_46731161">Howlger</a>):</h1>
  
  <p><a href="https://bugs.eclipse.org/bugs/show_bug.cgi?id=526020" rel="noreferrer">https://bugs.eclipse.org/bugs/show_bug.cgi?id=526020</a></p>
  
  <h1>Details:</h1>
  
  <ul>
  <li>OS: Windows 10 Home (64bit)</li>
  <li>JDK 9.0</li>
  <li>Initial Eclipse: Oxygen Release (4.7.0) Build id: 20170620-1800</li>
  <li>Current Eclipse: Oxygen.1a Release (4.7.1a) Build id: 20171005-1200</li>
  <li><strong>EDIT 4:</strong> Project settings:
  
  <ul>
  <li>Referenced Libraries: <a href="http://jregex.sourceforge.net/" rel="noreferrer">jRegex</a></li>
  <li>Encoding: UTF-8</li>
  <li><a href="https://i.stack.imgur.com/A6daJ.png" rel="noreferrer">Compiler Settings</a></li>
  <li><a href="https://i.stack.imgur.com/V0u8n.png" rel="noreferrer">Execution Envieroments</a></li>
  </ul></li>
  <li><strong>EDIT 4:</strong> Java 9 Version: build 9+181</li>
  <li><strong>EDIT 4:</strong> Java HotSpot(TM) 64-Bit Server VM (build 9+181, mixed mode)</li>
  <li><strong>EDIT 6:</strong> <a href="https://i.stack.imgur.com/KjOcA.png" rel="noreferrer">All Eclipse's Plugins</a></li>
  <li>If any other information is needed, please ask in the comment section...</li>
  </ul>
  
  <h1>What I've tried:</h1>
  
  <ul>
  <li>Restarting the IDE</li>
  <li>Searching if there is more than one String in the Java Proposals/Completion (There was only one, <code>java.lang</code>'s).</li>
  <li>Checked the JRE System Library for more than one <code>java.base</code> package (There was only one, from <code>jrt-fs.jar</code>).</li>
  <li><strong>EDIT 2</strong>: adding modules through eclipse.ini</li>
  <li><strong>EDIT 2</strong>: updating eclipse to the version: 4.7.1a (<a href="https://i.stack.imgur.com/WwcUt.png" rel="noreferrer">Too many images in this post already</a>)</li>
  <li><strong>EDIT 3</strong>: checked for double JDKs in the project and/or Eclipse preferences (<a href="https://i.stack.imgur.com/W9WWk.png" rel="noreferrer">Image link</a>)</li>
  <li><strong>EDIT 4</strong>: Rebuilding java index (didn't work), enabling <code>New Java index</code> and rebuilding again: worked for around 3 minutes, but then this error appeared and the glitches came back again: <a href="https://i.stack.imgur.com/xoPmt.png" rel="noreferrer">java.lang.ClassCastException</a> while building</li>
  <li><strong>EDIT 4</strong>: Removing JDK 9 from default library and adding it back again, the same thing from above, but this time it has thrown the error: <a href="https://i.stack.imgur.com/As8pq.png" rel="noreferrer">java.lang.NullPointerException</a> to the workspace, and the String error is still happening, even after restarting the IDE... (BTW, <a href="https://i.stack.imgur.com/HPqlr.png" rel="noreferrer">this warning</a> appeared to the package keyword, not sure if it was already here before...)</li>
  </ul>