Remove bridge methods from interfaces, fixes #13

Author: luontola

Diff for: README.md

@@ -122,6 +122,12 @@ the lambda accesses an instance variable).
 Version History
 ---------------
 
+### Upcoming
+
+- Removes from interfaces bridge methods which were generated by JDK 8 e.g.
+  when an interface overrides a method and refines its return type
+  ([Issue #13](https://github.com/orfjackal/retrolambda/issues/13))
+
 ### Retrolambda 1.1.3 (2014-03-25)
 
 - Fixed incompatibility with the Eclipse JDT compiler, version Kepler SR2

Diff for: end-to-end-tests/src/test/java/net/orfjackal/retrolambda/test/DefaultMethodsTest.java

@@ -0,0 +1,38 @@
+// 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 net.orfjackal.retrolambda.test;
+
+import org.junit.Test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.is;
+
+public class DefaultMethodsTest {
+
+    /**
+     * JDK 8 adds a bridge method to an interface when it overrides a method
+     * from the parent interface and refines its return type. This uses Java 8's
+     * default methods feature, which won't work on Java 7 and below, so we have
+     * to remove it for it - this makes the bytecode same as what JDK 7 produces.
+     */
+    @Test
+    public void will_remove_non_abstract_methods_from_interfaces() {
+        class Foo implements Child {
+            @Override
+            public String foo() {
+                return "foo";
+            }
+        }
+        assertThat(new Foo().foo(), is("foo"));
+    }
+
+    public interface Parent {
+        Object foo();
+    }
+
+    public interface Child extends Parent {
+        String foo(); // refined return type
+    }
+}

Diff for: retrolambda/src/main/java/net/orfjackal/retrolambda/LambdaUsageBackporter.java

@@ -36,6 +36,7 @@ private static void resetLambdaClassSequenceNumber() {
 
     private static class MyClassVisitor extends ClassVisitor {
         private final int targetVersion;
+        private int classAccess;
         private String className;
 
         public MyClassVisitor(ClassWriter cw, int targetVersion) {
@@ -49,17 +50,26 @@ public void visit(int version, int access, String name, String signature, String
                 version = targetVersion;
             }
             super.visit(version, access, name, signature, superName, interfaces);
+            this.classAccess = access;
             this.className = name;
         }
 
         @Override
         public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {
+            if (isBridgeMethodOnInterface(access)) {
+                return null;
+            }
             if (LambdaNaming.LAMBDA_IMPL_METHOD.matcher(name).matches()) {
                 access = Flags.makeNonPrivate(access);
             }
             MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
             return new InvokeDynamicInsnConvertingMethodVisitor(api, mv, className);
         }
+
+        private boolean isBridgeMethodOnInterface(int methodAccess) {
+            return Flags.hasFlag(classAccess, Opcodes.ACC_INTERFACE) &&
+                    Flags.hasFlag(methodAccess, Opcodes.ACC_BRIDGE);
+        }
     }
 
     private static class InvokeDynamicInsnConvertingMethodVisitor extends MethodVisitor {