diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/PackageInfo.java b/core/src/main/java/com/google/errorprone/bugpatterns/PackageInfo.java
index 03d5cd50e89..5c8f9ca97ce 100644
--- a/core/src/main/java/com/google/errorprone/bugpatterns/PackageInfo.java
+++ b/core/src/main/java/com/google/errorprone/bugpatterns/PackageInfo.java
@@ -24,6 +24,7 @@
 import com.google.errorprone.bugpatterns.BugChecker.CompilationUnitTreeMatcher;
 import com.google.errorprone.matchers.Description;
 import com.sun.source.tree.CompilationUnitTree;
+import java.io.File;
 
 /** A {@link BugChecker}; see the associated {@link BugPattern} annotation for details. */
 @BugPattern(
@@ -37,7 +38,7 @@ public Description matchCompilationUnit(CompilationUnitTree tree, VisitorState s
       return NO_MATCH;
     }
     String name = tree.getSourceFile().getName();
-    int idx = name.lastIndexOf('/');
+    int idx = name.lastIndexOf(File.separatorChar);
     if (idx != -1) {
       name = name.substring(idx + 1);
     }
diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessarilyFullyQualified.java b/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessarilyFullyQualified.java
index ff07de98ab1..115063bc447 100644
--- a/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessarilyFullyQualified.java
+++ b/core/src/main/java/com/google/errorprone/bugpatterns/UnnecessarilyFullyQualified.java
@@ -44,6 +44,7 @@
 import com.sun.tools.javac.code.Symbol.PackageSymbol;
 import com.sun.tools.javac.code.Symbol.TypeSymbol;
 import com.sun.tools.javac.util.Position;
+import java.io.File;
 import java.util.ArrayList;
 import java.util.HashSet;
 import java.util.List;
@@ -189,7 +190,7 @@ public Void visitIdentifier(IdentifierTree identifierTree, Void aVoid) {
 
   private static boolean isPackageInfo(CompilationUnitTree tree) {
     String name = tree.getSourceFile().getName();
-    int idx = name.lastIndexOf('/');
+    int idx = name.lastIndexOf(File.separatorChar);
     if (idx != -1) {
       name = name.substring(idx + 1);
     }