Do not try backporting java.* classes

Fixes #34

Author: luontola

end-to-end-tests/src/test/java/java/lang/Math.java

@@ -0,0 +1,20 @@
+// Copyright © 2013-2014 Esko Luontola <www.orfjackal.net>
+// This software is released under the Apache License 2.0.
+// The license text is at http://www.apache.org/licenses/LICENSE-2.0
+
+package java.lang;
+
+import net.orfjackal.retrolambda.test.ClasspathTest;
+
+/**
+ * @see ClasspathTest#ignores_classes_in_explicit_classpath_that_are_under_the_java_package
+ */
+@SuppressWarnings("UnusedDeclaration")
+public class Math {
+
+    public Math() {
+        // some lambdas to cause Retrolambda try backporting this class
+        Runnable r = () -> {
+        };
+    }
+}

end-to-end-tests/src/test/java/net/orfjackal/retrolambda/test/ClasspathTest.java

@@ -57,4 +57,15 @@ public Runnable foo() {
     public void prefers_classes_in_explicit_classpath_over_classes_in_the_JRE() {
         assertNotNull(getClass().getResource("/com/sun/javafx/application/LauncherImpl$$Lambda$1.class"));
     }
+
+    /**
+     * Classes in the {@code java.*} packages can be loaded only by the bootstrap
+     * class loader, so we must not try to load them with our custom class loader.
+     * This situation arises when backporting Android applications, because android.jar
+     * contains {@code java.*} classes.
+     */
+    @Test
+    public void ignores_classes_in_explicit_classpath_that_are_under_the_java_package() {
+        assertNotNull(getClass().getResource("/java/lang/Math.class"));
+    }
 }

retrolambda/src/main/java/net/orfjackal/retrolambda/interfaces/ClassHierarchyAnalyzer.java

@@ -23,7 +23,12 @@ public class ClassHierarchyAnalyzer implements MethodRelocations {
 
     public void analyze(byte[] bytecode) {
         ClassReader cr = new ClassReader(bytecode);
-        Type clazz = classNameToType(cr.getClassName());
+        String className = cr.getClassName();
+        if (className.startsWith("java/")) {
+            // the JVM disallows user classes in java.* packages, so don't even try backporting them
+            return;
+        }
+        Type clazz = classNameToType(className);
 
         if (Flags.hasFlag(cr.getAccess(), ACC_INTERFACE)) {
             interfaces.add(cr);