diff --git a/classpath/java/lang/Boolean.java b/classpath/java/lang/Boolean.java
new file mode 100644
index 0000000000..cfecde87ad
--- /dev/null
+++ b/classpath/java/lang/Boolean.java
@@ -0,0 +1,13 @@
+package java.lang;
+
+public final class Boolean {
+  private final boolean value;
+
+  public Boolean(boolean value) {
+    this.value = value;
+  }
+
+  public boolean booleanValue() {
+    return value;
+  }
+}
diff --git a/classpath/java/lang/Byte.java b/classpath/java/lang/Byte.java
new file mode 100644
index 0000000000..03250631a1
--- /dev/null
+++ b/classpath/java/lang/Byte.java
@@ -0,0 +1,13 @@
+package java.lang;
+
+public final class Byte {
+  private final byte value;
+
+  public Byte(byte value) {
+    this.value = value;
+  }
+
+  public byte byteValue() {
+    return value;
+  }
+}
diff --git a/classpath/java/lang/Character.java b/classpath/java/lang/Character.java
new file mode 100644
index 0000000000..3a14f10e21
--- /dev/null
+++ b/classpath/java/lang/Character.java
@@ -0,0 +1,13 @@
+package java.lang;
+
+public final class Character {
+  private final char value;
+
+  public Character(char value) {
+    this.value = value;
+  }
+
+  public char charValue() {
+    return value;
+  }
+}
diff --git a/classpath/java/lang/Double.java b/classpath/java/lang/Double.java
new file mode 100644
index 0000000000..8765d9541e
--- /dev/null
+++ b/classpath/java/lang/Double.java
@@ -0,0 +1,13 @@
+package java.lang;
+
+public final class Double {
+  private final double value;
+
+  public Double(double value) {
+    this.value = value;
+  }
+
+  public double doubleValue() {
+    return value;
+  }
+}
diff --git a/classpath/java/lang/Float.java b/classpath/java/lang/Float.java
new file mode 100644
index 0000000000..3e8d6b5eef
--- /dev/null
+++ b/classpath/java/lang/Float.java
@@ -0,0 +1,13 @@
+package java.lang;
+
+public final class Float {
+  private final float value;
+
+  public Float(float value) {
+    this.value = value;
+  }
+
+  public float floatValue() {
+    return value;
+  }
+}
diff --git a/classpath/java/lang/Integer.java b/classpath/java/lang/Integer.java
new file mode 100644
index 0000000000..68eb18b006
--- /dev/null
+++ b/classpath/java/lang/Integer.java
@@ -0,0 +1,13 @@
+package java.lang;
+
+public final class Integer {
+  private final int value;
+
+  public Integer(int value) {
+    this.value = value;
+  }
+
+  public int intValue() {
+    return value;
+  }
+}
diff --git a/classpath/java/lang/Long.java b/classpath/java/lang/Long.java
index c4040168ff..ed0c74f157 100644
--- a/classpath/java/lang/Long.java
+++ b/classpath/java/lang/Long.java
@@ -1,6 +1,6 @@
 package java.lang;
 
-public class Long {
+public final class Long {
   private final long value;
 
   public Long(long value) {
diff --git a/classpath/java/lang/Short.java b/classpath/java/lang/Short.java
new file mode 100644
index 0000000000..214cc0efe0
--- /dev/null
+++ b/classpath/java/lang/Short.java
@@ -0,0 +1,13 @@
+package java.lang;
+
+public final class Short {
+  private final short value;
+
+  public Short(short value) {
+    this.value = value;
+  }
+
+  public short shortValue() {
+    return value;
+  }
+}
diff --git a/classpath/java/lang/StringBuilder.java b/classpath/java/lang/StringBuilder.java
index 4e4524ea18..a734bab1a6 100644
--- a/classpath/java/lang/StringBuilder.java
+++ b/classpath/java/lang/StringBuilder.java
@@ -10,14 +10,16 @@ public class StringBuilder {
     return this;
   }
 
+  public StringBuilder append(Object o) {
+    return append(o == null ? "null" : o.toString());
+  }
+
   public StringBuilder append(int v) {
-    append(String.valueOf(v));
-    return this;
+    return append(String.valueOf(v));
   }
 
   public StringBuilder append(long v) {
-    append(String.valueOf(v));
-    return this;
+    return append(String.valueOf(v));
   }
 
   public String toString() {
diff --git a/classpath/java/lang/System.java b/classpath/java/lang/System.java
index 30f4de747c..af5df4b24e 100644
--- a/classpath/java/lang/System.java
+++ b/classpath/java/lang/System.java
@@ -15,6 +15,8 @@ public abstract class System {
 
   public static native String getProperty(String name);
 
+  public static native void gc();
+
   public static class Output {
     public synchronized native void print(String s);
 
diff --git a/classpath/java/lang/Void.java b/classpath/java/lang/Void.java
new file mode 100644
index 0000000000..b1400cbd7f
--- /dev/null
+++ b/classpath/java/lang/Void.java
@@ -0,0 +1,5 @@
+package java.lang;
+
+public final class Void {
+
+}
diff --git a/classpath/java/lang/ref/Reference.java b/classpath/java/lang/ref/Reference.java
index f195306a16..48451617c3 100644
--- a/classpath/java/lang/ref/Reference.java
+++ b/classpath/java/lang/ref/Reference.java
@@ -4,7 +4,7 @@ public abstract class Reference<T> {
   private T target;
   private ReferenceQueue<? super T> queue;
   private Object vmNext;
-  Reference<? extends T> next;
+  Reference next;
 
   protected Reference(T target, ReferenceQueue<? super T> queue) {
     this.target = target;
@@ -16,7 +16,7 @@ public abstract class Reference<T> {
   }
 
   public void clear() {
-    target = 0;
+    target = null;
   }
 
   public boolean isEnqueued() {
diff --git a/classpath/java/lang/ref/ReferenceQueue.java b/classpath/java/lang/ref/ReferenceQueue.java
index 87ded829a1..dcf92cd42e 100644
--- a/classpath/java/lang/ref/ReferenceQueue.java
+++ b/classpath/java/lang/ref/ReferenceQueue.java
@@ -1,6 +1,6 @@
 package java.lang.ref;
 
-public abstract class ReferenceQueue<T> {
+public class ReferenceQueue<T> {
   private Reference<? extends T> front;
   private Reference<? extends T> rear;
 
diff --git a/makefile b/makefile
index 114281bdb8..ea327eb334 100644
--- a/makefile
+++ b/makefile
@@ -16,7 +16,7 @@ src = src
 classpath = classpath
 test = test
 
-input = $(cls)/Exceptions.class
+input = $(cls)/References.class
 
 cxx = g++
 cc = gcc
diff --git a/src/builtin.cpp b/src/builtin.cpp
index ffef0725dd..29358142a3 100644
--- a/src/builtin.cpp
+++ b/src/builtin.cpp
@@ -106,6 +106,14 @@ arraycopy(Thread* t, jobject src, jint srcOffset, jobject dst, jint dstOffset,
   t->exception = makeArrayStoreException(t);
 }
 
+void
+gc(Thread* t)
+{
+  ENTER(t, Thread::ExclusiveState);
+
+  collect(t, Heap::MajorCollection);
+}
+
 jobject
 trace(Thread* t, jint skipCount)
 {
@@ -217,6 +225,8 @@ populate(Thread* t, object map)
       reinterpret_cast<void*>(arraycopy) },
     { "Java_java_lang_System_loadLibrary",
       reinterpret_cast<void*>(loadLibrary) },
+    { "Java_java_lang_System_gc",
+      reinterpret_cast<void*>(gc) },
 
     { "Java_java_lang_Thread_start",
       reinterpret_cast<void*>(start) },
diff --git a/src/heap.cpp b/src/heap.cpp
index 85d68ddcd6..0f055b65db 100644
--- a/src/heap.cpp
+++ b/src/heap.cpp
@@ -1438,7 +1438,9 @@ makeHeap(System* system)
     virtual Status status(void* p) {
       p = mask(p);
 
-      if (c.nextGen1.contains(p)) {
+      if (p == 0) {
+        return Null;
+      } else if (c.nextGen1.contains(p)) {
         return Reachable;
       } else if (c.nextGen2.contains(p)
                  or (c.gen2.contains(p)
diff --git a/src/heap.h b/src/heap.h
index f8f25ba8b6..00b7ae45eb 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -13,6 +13,7 @@ class Heap {
   };
 
   enum Status {
+    Null,
     Reachable,
     Unreachable,
     Tenured
diff --git a/src/machine.cpp b/src/machine.cpp
index 6346c21a67..0c45c4b693 100644
--- a/src/machine.cpp
+++ b/src/machine.cpp
@@ -142,7 +142,9 @@ referenceTargetUnreachable(Thread* t, object* p, Heap::Visitor* v)
   v->visit(p);
   jreferenceTarget(t, *p) = 0;
 
-  if (t->vm->heap->status(jreferenceQueue(t, *p)) != Heap::Unreachable) {
+  if (jreferenceQueue(t, *p)
+      and t->vm->heap->status(jreferenceQueue(t, *p)) != Heap::Unreachable)
+  {
     // queue is reachable - add the reference
 
     v->visit(&jreferenceQueue(t, *p));
@@ -161,6 +163,17 @@ referenceTargetUnreachable(Thread* t, object* p, Heap::Visitor* v)
   }
 }
 
+void
+referenceUnreachable(Thread* t, object* p, Heap::Visitor* v)
+{
+  if (jreferenceQueue(t, *p)
+      and t->vm->heap->status(jreferenceQueue(t, *p)) != Heap::Unreachable)
+  {
+    // queue is reachable - add the reference
+    referenceTargetUnreachable(t, p, v);    
+  }
+}
+
 void
 referenceTargetReachable(Thread* t, object* p, Heap::Visitor* v)
 {
@@ -222,12 +235,12 @@ postVisit(Thread* t, Heap::Visitor* v)
 
   for (object* p = &(m->weakReferences); *p;) {
     if (m->heap->status(*p) == Heap::Unreachable) {
-      // reference is unreachable - remove it from the list
+      // reference is unreachable
 
+      referenceUnreachable(t, p, v);
       *p = jreferenceNext(t, *p);
     } else if (m->heap->status(jreferenceTarget(t, *p)) == Heap::Unreachable) {
-      // target is unreachable - clear the reference, remove it from
-      // the list, and enqueue it if it has a live reference queue
+      // target is unreachable
 
       referenceTargetUnreachable(t, p, v);
       *p = jreferenceNext(t, *p);
@@ -279,19 +292,19 @@ postVisit(Thread* t, Heap::Visitor* v)
 
     for (object* p = &(m->tenuredWeakReferences); *p;) {
       if (m->heap->status(*p) == Heap::Unreachable) {
-        // reference is unreachable - remove it from the list
+        // reference is unreachable
 
+        referenceUnreachable(t, p, v);
         *p = jreferenceNext(t, *p);
       } else if (m->heap->status(jreferenceTarget(t, *p))
                  == Heap::Unreachable)
       {
-        // target is unreachable - clear the reference, remove it from
-        // the list, and enqueue it if it has a live reference queue
+        // target is unreachable
 
         referenceTargetUnreachable(t, p, v);
         *p = jreferenceNext(t, *p);
       } else {
-        // target is reachable
+        // both reference and target are reachable
 
         referenceTargetReachable(t, p, v);
         p = &jreferenceNext(t, *p);
@@ -1106,7 +1119,9 @@ updateBootstrapClass(Thread* t, object bootstrapClass, object class_)
   // verify that the classes have the same layout
   expect(t, classSuper(t, bootstrapClass) == classSuper(t, class_));
   expect(t, classFixedSize(t, bootstrapClass) == classFixedSize(t, class_));
-  expect(t, (classObjectMask(t, bootstrapClass) == 0
+  expect(t,
+         (classVmFlags(t, bootstrapClass) & ReferenceFlag)
+         or (classObjectMask(t, bootstrapClass) == 0
              and classObjectMask(t, class_) == 0)
          or intArrayEqual(t, classObjectMask(t, bootstrapClass),
                           classObjectMask(t, class_)));
@@ -1243,7 +1258,7 @@ Machine::Machine(System* system, Heap* heap, ClassFinder* classFinder):
   stateLock(0),
   heapLock(0),
   classLock(0),
-  finalizerLock(0),
+  referenceLock(0),
   libraries(0),
   classMap(0),
   bootstrapClassMap(0),
@@ -1262,7 +1277,7 @@ Machine::Machine(System* system, Heap* heap, ClassFinder* classFinder):
   if (not system->success(system->make(&stateLock)) or
       not system->success(system->make(&heapLock)) or
       not system->success(system->make(&classLock)) or
-      not system->success(system->make(&finalizerLock)))
+      not system->success(system->make(&referenceLock)))
   {
     system->abort();
   }
@@ -1274,7 +1289,7 @@ Machine::dispose()
   stateLock->dispose();
   heapLock->dispose();
   classLock->dispose();
-  finalizerLock->dispose();
+  referenceLock->dispose();
 
   if (libraries) {
     libraries->dispose();
@@ -1339,6 +1354,13 @@ Thread::Thread(Machine* m, Allocator* allocator, object javaThread,
 
     m->unsafe = false;
 
+    classVmFlags(t, arrayBody(t, m->types, Machine::JreferenceType))
+      |= ReferenceFlag;
+    classVmFlags(t, arrayBody(t, m->types, Machine::WeakReferenceType))
+      |= ReferenceFlag | WeakReferenceFlag;
+    classVmFlags(t, arrayBody(t, m->types, Machine::PhantomReferenceType))
+      |= ReferenceFlag | WeakReferenceFlag;
+
     m->bootstrapClassMap = makeHashMap(this, NormalMap, 0, 0);
 
 #include "type-java-initializations.cpp"
@@ -2090,7 +2112,7 @@ addFinalizer(Thread* t, object target, void (*finalize)(Thread*, object))
 {
   PROTECT(t, target);
 
-  ACQUIRE(t, t->vm->finalizerLock);
+  ACQUIRE(t, t->vm->referenceLock);
 
   t->vm->finalizers = makeFinalizer
     (t, 0, reinterpret_cast<void*>(finalize), t->vm->finalizers);
diff --git a/src/machine.h b/src/machine.h
index d2d3c82a60..cf2bf9d015 100644
--- a/src/machine.h
+++ b/src/machine.h
@@ -61,6 +61,9 @@ enum MapType {
 const int NativeLine = -1;
 const int UnknownLine = -2;
 
+const unsigned ReferenceFlag = 1 << 0;
+const unsigned WeakReferenceFlag = 1 << 1;
+
 class Thread;
 
 typedef Thread JNIEnv;
@@ -1106,7 +1109,7 @@ class Machine {
   System::Monitor* stateLock;
   System::Monitor* heapLock;
   System::Monitor* classLock;
-  System::Monitor* finalizerLock;
+  System::Monitor* referenceLock;
   System::Library* libraries;
   object classMap;
   object bootstrapClassMap;
diff --git a/src/run.cpp b/src/run.cpp
index 36c073ff97..f33d9e8c36 100644
--- a/src/run.cpp
+++ b/src/run.cpp
@@ -76,6 +76,18 @@ popFrame(Thread* t)
   }
 }
 
+void
+registerWeakReference(Thread* t, object r)
+{
+  PROTECT(t, r);
+
+  ACQUIRE(t, t->vm->referenceLock);
+
+  // jreferenceNext(t, r)
+  cast<object>(r, 3 * BytesPerWord) = t->vm->weakReferences;
+  t->vm->weakReferences = r;
+}
+
 inline object
 make(Thread* t, object class_)
 {
@@ -86,6 +98,10 @@ make(Thread* t, object class_)
   memset(static_cast<object*>(instance) + 1, 0,
          sizeInBytes - sizeof(object));
 
+  if (UNLIKELY(classVmFlags(t, class_) & WeakReferenceFlag)) {
+    registerWeakReference(t, instance);
+  }
+
   return instance;
 }
 
@@ -1085,8 +1101,8 @@ run(Thread* t)
     uint8_t offset1 = codeBody(t, code, ip++);
     uint8_t offset2 = codeBody(t, code, ip++);
 
-    int32_t b = popInt(t);
-    int32_t a = popInt(t);
+    object b = popObject(t);
+    object a = popObject(t);
     
     if (a == b) {
       ip = (ip - 3) + static_cast<int16_t>(((offset1 << 8) | offset2));
@@ -1097,8 +1113,8 @@ run(Thread* t)
     uint8_t offset1 = codeBody(t, code, ip++);
     uint8_t offset2 = codeBody(t, code, ip++);
 
-    int32_t b = popInt(t);
-    int32_t a = popInt(t);
+    object b = popObject(t);
+    object a = popObject(t);
     
     if (a != b) {
       ip = (ip - 3) + static_cast<int16_t>(((offset1 << 8) | offset2));
diff --git a/test/References.java b/test/References.java
new file mode 100644
index 0000000000..fcf473813c
--- /dev/null
+++ b/test/References.java
@@ -0,0 +1,36 @@
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.Reference;
+import java.lang.ref.WeakReference;
+import java.lang.ref.PhantomReference;
+
+public class References {
+  public static void main(String[] args) {
+    Object a = new Object();
+    Object b = new Object();
+    Object c = new Object();
+    Object d = new Object();
+
+    ReferenceQueue q = new ReferenceQueue();
+
+    Reference ar = new WeakReference(a);
+    Reference br = new WeakReference(b, q);
+    Reference cr = new WeakReference(c, q);
+    Reference dr = new PhantomReference(d, q);
+
+    a = b = c = d = cr = null;
+    
+    System.out.println("a: " + ar.get());
+    System.out.println("b: " + br.get());
+    System.out.println("d: " + dr.get());
+
+    System.gc();
+
+    System.out.println("a: " + ar.get());
+    System.out.println("b: " + br.get());
+    System.out.println("d: " + dr.get());
+
+    for (Reference r = q.poll(); r != null; r = q.poll()) {
+      System.out.println("polled: " + r.get());      
+    }
+  }
+}