diff --git a/.gitignore b/.gitignore
index 11bbeceb..efff076b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -39,3 +39,7 @@ Package.resolved
 */**/*.o
 */**/*.swiftdeps
 */**/*.swiftdeps~
+
+*/**/*.index-build
+.index-build
+.build-vscode
diff --git a/Samples/JavaKitSampleApp/Sources/JavaKitExample/JavaKitExample.swift b/Samples/JavaKitSampleApp/Sources/JavaKitExample/JavaKitExample.swift
index f074fee6..7e2e178b 100644
--- a/Samples/JavaKitSampleApp/Sources/JavaKitExample/JavaKitExample.swift
+++ b/Samples/JavaKitSampleApp/Sources/JavaKitExample/JavaKitExample.swift
@@ -21,6 +21,21 @@ enum SwiftWrappedError: Error {
 
 @JavaImplementation("com.example.swift.HelloSwift")
 extension HelloSwift: HelloSwiftNativeMethods {
+
+  @JavaMethod
+  func compute(_ a: JavaInteger?, _ b: JavaInteger?) -> JavaInteger? {
+    guard let a else { fatalError("parameter 'a' must not be null!") }
+    guard let b else { fatalError("parameter 'b' must not be null!") }
+
+    print("[swift] a = \(a.intValue())")
+    print("[swift] b = \(b.intValue())")
+    print("[swift] a + b = \(a.intValue() + b.intValue())")
+    let result = JavaInteger(a.intValue() + b.intValue())
+    let x = JavaObjectHolder(object: result.javaThis, environment: try! JavaVirtualMachine.shared().environment())
+    print("[swift] Integer(a + b) = \(result)")
+    return result
+  }
+
   @JavaMethod
   func sayHello(_ i: Int32, _ j: Int32) -> Int32 {
     print("Hello from Swift!")
diff --git a/Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/HelloSwift.java b/Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/HelloSwift.java
index d7ea7f22..ea29587b 100644
--- a/Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/HelloSwift.java
+++ b/Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/HelloSwift.java
@@ -30,6 +30,9 @@ public HelloSwift() {
     }
 
     native int sayHello(int x, int y);
+
+    native Integer compute(Integer x, Integer y);
+
     native String throwMessageFromSwift(String message) throws Exception;
 
     // To be called back by the native code
diff --git a/Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/JavaKitSampleMain.java b/Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/JavaKitSampleMain.java
index 9036b7a5..3e85dc6a 100644
--- a/Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/JavaKitSampleMain.java
+++ b/Samples/JavaKitSampleApp/Sources/JavaKitExample/com/example/swift/JavaKitSampleMain.java
@@ -20,7 +20,16 @@
  */
 public class JavaKitSampleMain {
     public static void main(String[] args) {
-        int result = new HelloSubclass("Swift").sayHello(17, 25);
-        System.out.println("sayHello(17, 25) = " + result);
+        var subclass = new HelloSubclass("Swift");
+
+        int intResult = subclass.sayHello(17, 25);
+        System.out.println("sayHello(17, 25) = " + intResult);
+
+        Integer integerResult = subclass.compute(16, 25);
+        System.out.println("compute(17, 25) = " + integerResult);
+
+        if (integerResult == null) {
+            throw AssertionError("integerResult was null!")
+        }
     }
 }
diff --git a/Samples/SwiftKitSampleApp/com_example_swift_HelloJava2Swift.h b/Samples/SwiftKitSampleApp/com_example_swift_HelloJava2Swift.h
new file mode 100644
index 00000000..19a6a52d
--- /dev/null
+++ b/Samples/SwiftKitSampleApp/com_example_swift_HelloJava2Swift.h
@@ -0,0 +1,37 @@
+/* DO NOT EDIT THIS FILE - it is machine generated */
+#include <jni.h>
+/* Header for class com_example_swift_HelloJava2Swift */
+
+#ifndef _Included_com_example_swift_HelloJava2Swift
+#define _Included_com_example_swift_HelloJava2Swift
+#ifdef __cplusplus
+extern "C" {
+#endif
+/*
+ * Class:     com_example_swift_HelloJava2Swift
+ * Method:    jniWriteString
+ * Signature: (Ljava/lang/String;)J
+ */
+JNIEXPORT jlong JNICALL Java_com_example_swift_HelloJava2Swift_jniWriteString
+  (JNIEnv *, jclass, jstring);
+
+/*
+ * Class:     com_example_swift_HelloJava2Swift
+ * Method:    jniGetInt
+ * Signature: ()J
+ */
+JNIEXPORT jlong JNICALL Java_com_example_swift_HelloJava2Swift_jniGetInt
+  (JNIEnv *, jclass);
+
+/*
+ * Class:     com_example_swift_HelloJava2Swift
+ * Method:    add
+ * Signature: (Ljava/lang/Integer;Ljava/lang/Integer;)J
+ */
+JNIEXPORT jlong JNICALL Java_com_example_swift_HelloJava2Swift_add
+  (JNIEnv *, jclass, jobject, jobject);
+
+#ifdef __cplusplus
+}
+#endif
+#endif
diff --git a/Sources/Java2SwiftLib/JavaClassTranslator.swift b/Sources/Java2SwiftLib/JavaClassTranslator.swift
index bfd1657f..65d1951d 100644
--- a/Sources/Java2SwiftLib/JavaClassTranslator.swift
+++ b/Sources/Java2SwiftLib/JavaClassTranslator.swift
@@ -546,7 +546,8 @@ extension JavaClassTranslator {
     let resultType = try translator.getSwiftTypeNameAsString(
       javaMethod.getGenericReturnType()!,
       preferValueTypes: true,
-      outerOptional: .implicitlyUnwrappedOptional
+      // outerOptional: .implicitlyUnwrappedOptional // FIXME: why? it's allowed to return null objects hmmm
+      outerOptional: .optional // FIXME: why? it's allowed to return null objects hmmm
     )
 
     // FIXME: cleanup the checking here
@@ -555,6 +556,7 @@ extension JavaClassTranslator {
     } else {
       resultTypeStr = ""
     }
+    assert(!resultTypeStr.contains("!"), "contained !: '\(resultTypeStr)'")
 
     let throwsStr = javaMethod.throwsCheckedException ? "throws" : ""
     let swiftMethodName = javaMethod.getName().escapedSwiftName
diff --git a/Sources/JavaKit/BridgedValues/JavaValue+Integer.swift b/Sources/JavaKit/BridgedValues/JavaValue+Integer.swift
new file mode 100644
index 00000000..e2cf3633
--- /dev/null
+++ b/Sources/JavaKit/BridgedValues/JavaValue+Integer.swift
@@ -0,0 +1,99 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import JavaTypes
+
+// TODO: extension JavaValue: CustomStringConvertible where JNIType == jobject? {
+extension JavaInteger: CustomStringConvertible {
+    public var description: String {
+        "JavaKit.\(Self.self)(\(toString()))"
+    }
+}
+
+extension JavaInteger: JavaValue {
+  public typealias JNIType = jobject?
+
+  public static var jvalueKeyPath: WritableKeyPath<jvalue, JNIType> { \.l }
+
+  public static var javaType: JavaType {
+    .class(package: "java.lang", name: "Integer")
+  }
+
+  // FIXME: cannot implement in extension, need to fix source generator
+//  public required init(fromJNI value: JNIType, in environment: JNIEnvironment) {
+//    fatalError()
+//  }
+
+
+  public func getJNIValue(in environment: JNIEnvironment) -> JNIType {
+    fatalError()
+  }
+
+  public static func jniMethodCall(in environment: JNIEnvironment) -> JNIMethodCall<JNIType> {
+    environment.interface.CallObjectMethodA
+  }
+
+  public static func jniFieldGet(in environment: JNIEnvironment) -> JNIFieldGet<JNIType> {
+    environment.interface.GetObjectField
+  }
+
+  public static func jniFieldSet(in environment: JNIEnvironment) -> JNIFieldSet<JNIType> {
+    environment.interface.SetObjectField
+  }
+
+  public static func jniStaticMethodCall(in environment: JNIEnvironment) -> JNIStaticMethodCall<JNIType> {
+    environment.interface.CallStaticObjectMethodA
+  }
+
+  public static func jniStaticFieldGet(in environment: JNIEnvironment) -> JNIStaticFieldGet<JNIType> {
+    environment.interface.GetStaticObjectField
+  }
+
+  public static func jniStaticFieldSet(in environment: JNIEnvironment) -> JNIStaticFieldSet<JNIType> {
+    environment.interface.SetStaticObjectField
+  }
+
+  public static func jniNewArray(in environment: JNIEnvironment) -> JNINewArray {
+    return { environment, size in
+      // FIXME: Introduce a JavaString class that we can use for this.
+      let clazz = environment.interface.FindClass(environment, "java/lang/Integer")
+      return environment.interface.NewObjectArray(environment, size, clazz, nil)
+    }
+  }
+
+  public static func jniGetArrayRegion(in environment: JNIEnvironment) -> JNIGetArrayRegion<JNIType> {
+    return { environment, array, start, length, outPointer in
+      let buffer = UnsafeMutableBufferPointer(start: outPointer, count: Int(length))
+      for i in 0..<length {
+        buffer.initializeElement(
+          at: Int(i),
+          to: environment.interface.GetObjectArrayElement(environment, array, Int32(start + i))
+        )
+      }
+    }
+  }
+
+  public static func jniSetArrayRegion(in environment: JNIEnvironment) -> JNISetArrayRegion<JNIType> {
+    return { environment, array, start, length, outPointer in
+      let buffer = UnsafeBufferPointer(start: outPointer, count: Int(length))
+      for i in start..<start + length {
+        environment.interface.SetObjectArrayElement(environment, array, i, buffer[Int(i)])
+      }
+    }
+  }
+
+  public static var jniPlaceholderValue: jobject? {
+    nil
+  }
+}
diff --git a/Sources/JavaKit/JavaObject+MethodCalls.swift b/Sources/JavaKit/JavaObject+MethodCalls.swift
index 9f02a58d..0bb0028d 100644
--- a/Sources/JavaKit/JavaObject+MethodCalls.swift
+++ b/Sources/JavaKit/JavaObject+MethodCalls.swift
@@ -187,6 +187,7 @@ extension AnyJavaObject {
       parameterTypes: repeat (each Param).self,
       resultType: Result.self
     )
+    print("[swift][jni] Call for result \(Result.self)")
     return try javaMethodCall(
       method: methodID,
       args: repeat each arguments
diff --git a/Sources/JavaKit/JavaObjectHolder.swift b/Sources/JavaKit/JavaObjectHolder.swift
index 173991c9..3fa55d68 100644
--- a/Sources/JavaKit/JavaObjectHolder.swift
+++ b/Sources/JavaKit/JavaObjectHolder.swift
@@ -17,7 +17,8 @@ import JavaRuntime
 /// Stores a reference to a Java object, managing it as a global reference so
 /// that the Java virtual machine will not move or deallocate the object
 /// while this instance is live.
-public class JavaObjectHolder {
+public final class JavaObjectHolder {
+  // FIXME: thread safety!
   public private(set) var object: jobject?
   public let environment: JNIEnvironment
 
@@ -38,6 +39,6 @@ public class JavaObjectHolder {
   }
 
   deinit {
-    self.forget()
+    // self.forget()
   }
 }
diff --git a/Sources/JavaKit/generated/JavaInteger.swift b/Sources/JavaKit/generated/JavaInteger.swift
index e5cd5fc1..48ec0364 100644
--- a/Sources/JavaKit/generated/JavaInteger.swift
+++ b/Sources/JavaKit/generated/JavaInteger.swift
@@ -3,6 +3,12 @@ import JavaRuntime
 
 @JavaClass("java.lang.Integer")
 open class JavaInteger: JavaNumber {
+
+  // FIXME: move ot extension?
+  public required init(fromJNI value: JNIType, in environment: JNIEnvironment) {
+    fatalError()
+  }
+
   @JavaMethod
   @_nonoverride public convenience init(_ arg0: Int32, environment: JNIEnvironment? = nil)