Are there any huge differences between objective-c and Java, or iPhone and Android?

I wouldn't waste time trying to find commonality between iOS and Android.

Cross-platform is almost always a waste of time and resources unless the cross-platform capability is central to the apps functioning. That is especially true for platforms such as iPhone and Android which have custom OS and work very tightly with the hardware.

Cross-platform development environments add rather than reduce complexity long term. Yeah, it sounds neat but usually you get 90% what you want easily and then you hit a roadblock that destroys all the savings you made and then starts putting you in the hole. There are simply to many compromises and square pegs jammed into round holes.

Unless your app could in theory work from a generic web page, cross-platform is not for you.

In the specific case of Objective-C and Java, although Java is descended from Objective-C they have no modern interoperability. You can't use code from one on the other.

You should spend the time to learn each platform's specific API. There are no shortcuts.

Tools like XMLVM, Codename One and iSpectrum tell us that you can actually cross-compile Java code (from an Android app or creating one from scratch) to Objective-C code that is running on an iPhone without having to install any virtual machine on the Apple side of things.

I suggest watching Developing iPhone Applications using Java

Unfortunately Apple's license agreement for the iPhone SDK prohibits the porting of the Java virtual machine to the iPhone. In this presentation we introduce an Open Source Java-to-Objective-C cross-compiler as well as a Java-based implementation of the Cocoa library. With the help of these tools, iPhone applications can be written in pure Java. Using the Java version of Cocoa, it is possible to run a Java-based iPhone application as a Java desktop/applet application that can be cross-compiled to run natively on the iPhone. The talk will discuss the challenges of the Java-to-Objective-C cross-compiler as well as the Java-based version of Cocoa. Details are available at http://www.xmlvm.org/

and for more insight a more recent talk about Cross-Compiling Android applications to the iPhone from Frebruary 2010 is available here, which is documented at http://www.xmlvm.org/android/ :

Android is an Open Source platform for mobile devices. Initiated by Google, Android has received much attention. Android applications are developed using Java, although a special compiler converts class files to a proprietary, register-based virtual machine that is used on Android devices to execute applications. Android defines its own API for writing mobile applications. With the help of XMLVM it is possible to cross-compile Java-based Android applications to native iPhone applications.

The Android application is written in Java and makes use of an Android specific API. XMLVM offers a compatibility library, written in Java, that offers the same API as Android, but only makes use of the Java-based API for Cocoa Touch. During the cross-compilation process, both the application and the Android compatibility library are cross-compiled from Java to Objective-C and linked with the Cocoa Touch compatibility library to yield a native iPhone application.

From the 2008 talk about building Java applications for the iPhone http://www.xmlvm.org/iphone/ :

Apple's iPhone has generated huge interest amongst users and developers alike. Like MacOS X, the iPhone development environment is based on Objective-C as the development language and Cocoa for the GUI library. The iPhone SDK license agreement does not permit the development of a virtual machine. Using XMLVM, we circumvent this problem by cross-compiling Java to the iPhone. Just like a Java application can be cross-compiled to AJAX, XMLVM can be used to cross-compile a Java application to Objective-C. The cross-compilation is also accomplished by mimicking a stack-based machine in Objective-C.

If you focus on abstracting away things like system interactions, and stick to the common subset of the languages, you can probably build classes that would require little or no modification to move from one language to the other.

C# does have a better implementation of generics than Java, in that C#'s generics retain strong typing at run-time.

C# also has LINQ, which as far as I know has no equivalent (yet) in Java. LINQ provides a SQL-like query capability, built into the language and fully supported by the .NET frameowrk, that lets you query object collections, and does so in a very functional-language style. LINQ can be extended by query providers to work with other data formats, such as XML and SQL, once the query provider provides an object model that translates to the other format. LINQ also makes parallelization very easy for multi-core work. It's a huge extension to language capability.