Develop Reference

Is there any benefit to upgrading Java 7 compiled code to Java 8?

I have a old application written using Java 7. It runs fine in a Java 8 JRE. I do not plan on rewriting any of the code to make use of Java 8 features. Is there any technical benefit to upgrading the compiled code to the latest Java 8 JDK?
To be clear, the code is currently compiled with Java 7 and already running with the latest Java 8 JRE. It should already benefit from the Java 8 runtime improvements. This question is whether any benefits would be gained by compiling with version 8 and running with Java 8 compiled byte code.
Also, I am not concerned with non-technical benefits such as developer productivity. I think those are important but not the point of this question. I am asking for the sake of production code that has NO development team. It is purely in maintenance mode.

If I understand the question correctly, you want to know if the bytecode produced by javac will be "better" in Java 8 than in Java 7.
The answer is probably not, they constantly fix bugs in the compiler and that sometimes leads to more efficient bytecode. But you will not see any significant speedup from these fixes for Java 8 as far as I can see, the changelog only lists 2 major changes between versions.
The oracle website is terrible and I can't seem to get a list of bugfixes related to javac between versions, but here is a non exhaustive one from OpenJDK. A majority of the ones I can manage to find are fixing errors. So by updating to Java 8, there is a chance it wont compile any more due to javac more correctly following the JLS and there will be very little to no "improvements" to the bytecode.

The main benefit is that Java 8 has the latest bug fixes where as Java 7 isn't being publicly updated.
Also if you are going to run code on an Java 8 JVM, you may as well have just one version of Java installed.
Java 8 might be faster, and it has better support for new features like G1. However, it might be slower for your use case so the only way to know is to test it.
Is there any technical benefit to upgrading the compiled code to the latest Java 8 JDK?
If you are asking whether there is any benefit in re-compiling Java 7 code in a Java 8 compiler, the answer is; almost nothing.
The only subtle difference is that there have been minor differences to the Java API, so there might be very subtle differences the Java 8 compiler might find that the Java 7
Other minor differences are the magic number at the start of the file, possibly the order of the constant pool. The byte code is basically the same, even the support for invokedynamic which was added for lambdas existed in Java 7 but just wasn't used that way.

It could help by creating awareness.
When you switch to Java8, you might find additional warnings being emitted by javac. Example: type inference has been greatly improved with Java8. And that could eliminate the need for #SuppressWarnings annotations in your current code base (and when such annotations are no longer required, the compiler warns about that).
So, even when you don't intend to modify your code base today, switching to Java8 could tell you about such things. Increasing your knowledge can help in making informed decisions.
On the other hand:
I saw some questions here about (rare) situations where Java8 refused to compile Java7 code. So, switching to Java8 also carries a (minimal) risk of running into that kind of problem.
And: even when you don't intend to touch your code base today, there is a certain chance that you change your mind later on. And then, when not paying attention, you might exploit Java8 features. Which could complicate "field updates"; as you now have two versions of your source code to maintain!
Then: in case you have customers running the product using a java7 jre; you have to be really careful about the binary fixes you give to them. We have such a setup; and I have wasted time more than once because I accidentally put a single Java8-compiled class onto a Java7-driven test system. That simply can't happen when your dev and test/customer setup is all Java7.
Long story short: there are a few subtle advantages, and certain risks (where the significance of the risks mainly depend on your overall setup).

I would do for at least these facts.
1) HashMap internals (it is faster under jdk-8)
2) Lots of bugs fixed that might be transparent for you (runtime optimizations) that will make your code faster and better without you actually doing anything.
3) G1 Garbage Collector
EDIT
From a technical point of view this sounds more like something to do with Ahead of Time Compilation or something that a compiler might improve by analyzing the code more. As far as I know such things are not done in java 8 compiler.
From a developer point of view - there are plenty. Increased productivity is the most important one for me.
EDIT 2
I know only two points that matches your second query:
–parameters
to preserve the method parameter names.
-profile
Called Compact Profile Option for a smaller footprint.

If you have no other reasons to recompile your application, then it probably does not make much difference, as stated in the accepted answer.
However, if you have to recompile it even only once, consider this:
Your application source code is compatible with Java 7, and most likely 8 too;
In the eventuality that the code does not compile with Java 8, it will probably not compile either with a Java 8 compiler in Java 7 source compatibility mode (-source 7 with javac);
Your developers and CI will need to run unit and integration tests against a Java 8 runtime to be as close as possible to the production environment. Developers will also need to run the application on the same Java 8 runtime when running it locally;
It is more difficult to compile with a JDK 7 and run with a JRE 8 (in the same build process, or in the same IDE) than doing everything with the same version;
There is no benefit of using -source 7 instead of -source 8 if you compile with a JDK 8 and your target runtime is Java 8;
Using -source 8 guarantees that the developer is using Java 8 (or later) for both compilation and runtime (as it enforces -target 8).
In conclusion, don't recompile it if you don't need to. However, on the first occasion you have to recompile (due to code changes), switch to Java 8. Don't take the risk of having a bug due to environment mismatches, and don't restrict the developers without a good reason.

Is it better practice to write up-to-date or compatible code?

I am a hobbyist java programmer using NetBeans IDE. A problem I face with programming in Java is choosing the correct target JRE version. I understand that most of my friends have at least java 6, but few have a totally up-to-date version with which I compile. I know how to compile for a particular target run-time environment on NetBeans; the problem is that by using an earlier target release of Java, the compiler expects code written in that version.
For a specific example, I occasionally use a Java 8 lambda instead of a new Runnable(){}, however the compiler (or at least NetBeans) doesn't accept this 'modern' code. Another example is the use of the diamond operator, or the use of multiple-exception catch statement, neither of which are supported by Java 6. The language evolves and the compiler evolves, but older versions of Java cannot handle the change. To distribute my code, I am forced to use what some might consider outdated code.
My question is: Is it considered good practice or common practice to write Java for an outdated target version to provide compatibility? Is there some form of 'translator' or cross-compiler for Java code?

My question is: Is it considered good practice or common practice to write Java for an outdated target version to provide compatibility?
It is common practice.
Whether it is "good practice" or not, is highly context dependent. There is a pragmatic trade-off between supporting older applications whose developers don't want (or can't) upgrade, and holding back the development of your application by not making use of new (and presumably beneficial) features.
There is no universal "best" answer.
Is there some form of 'translator' or cross-compiler for Java code?
Effectively, no. For old versions of Java there were "retro-weavers" which allowed you to run "newer" Java on "older" platforms. However, this approach seems to have died out by Java 6.

To answer your first question, it really depends on the standards set by the company/office that you are working for.
From my experience, most of time big companies tries to stay with matured and well established development platforms, they might not give the cutting edge features of the new language, but the companies want the base platforms to be stable and secure first. And if the codebase is large and the project has been in development for a long time, they normally don't bother jumping into the new platform, since that means they might have to invest more resources to rewriting/refactoring a lot of code, which is not always economically lucrative.
I love using those features that you mentioned in my personal projects, but in my office works we are still using JDK6.
As for the second question, I think there are no workable things at this moment that can do the "translation" that you mentioned.

From my experience this isn't a big problem in the industry. Java is used mostly on the server, not so much on the desktop. And to setup a server with the required Java version is not a big deal. For desktop applications I also seen the approach to ship the JRE with the application. It is a little bit ugly to ship a small application (let's say 5 MB) with a 60 MB JRE, but in the "enterprise environment" it doesn't matter and is a practical way to provide the correct JRE. However, I don't like this approach since it leaves open questions, like how to ship security updates to the JRE.
And by the way: Is Java 6 still supported? To use an old JRE is pretty big security issue.

Java 6 VM to call Java 4 VM

I've inherited a very old application that hasn't been upgraded because it depends on a third party library that is dependent on Java 4.
Getting rid of this third party library isn't going to happen in the near future as a critical part of the system is dependent on it.
I want to bring the Java version of the application up to date and am thinking of moving the dependent jar into its own VM and then having some kind of call between the Java 6/7 VM and the Java 4 VM.
First thoughts are to use RMI. Obvious first question is compatibility between VMs when using different Java versions. The third party lib produces byte streams so the returned data won't be affected by serialization. The data passed in can be manipulated into something that can be passed across if compatibility is an issue.
Is this the right way to go?
Are there better ways?

You can do a wrapper as Phillip said in a comment of yours... there are only some minor adjustments that you have to do in your old library
you can use RMI... it's safe! I tested it and it's a good approach... you don't have to change old messy code
you can also use web services but I prefer RMI
If you have to modify something small and you have too many problems to change the JVM (because someone modified some jdk libraries [happened to me]) just leave it to java 4... :)
The problem I had was that I needed a library from jdk 1.5 to use in jdk 1.4 but my solution was to decompile the jdk 1.5 one and compile it back with jdk 1.4... because the old jdk had something modified in it...
The problem I had on decompiled code is that I couldn't find a very good decompiler that knows about casting... and I had some stackoverflow errors (but those are easy to fix)
I hope my answer helps you

Mixing Java 1.4 and 1.6 bytecode in a class hierarchy

The question first, the story will follow:
Is it safe to mix different bytecode version in a class hierarchy? What are the risks?
For a case, Class C extends B, Class B extends Class A. Class A implements Interface I.
My question would involve following example scenarios:
Class A compiled to Java 1.6 bytecode, and have 1.6 features such as generics, etc. The heirs, which are B and C was compiled to 1.4 bytecode.
Interface I compiled to 1.6, while the implementor compiled to 1.4.
Other exotic inheritance scenario involving different version of bytecode.
I have tried as many scenarios I could imagine and it seems to run just fine. However I still feel the urge to ask here as I only know Java at the surface; i know how to code and tweak Java but don't really know what happen under the hood.
Now for those minds who can't help themselves to ask "why would you need to do that???".
I'm in a project to assess the migration of legacy Java 1.4 Swing app, connected to EJB 2 via RMI, to Java 1.6 Swing connected to newer version of App Server running on top of 1.6 also. The J2EE platform will still be 1.4 (EJB 2).
The migration will not be "recompile everything to 1.6", but it will be "code and compile new features to 1.6".
The way they do things is like this:
They only have one path in the CVS, everyone commits there. No tags/branches whatsoever to get the production code.
Whenever a new feature need to be added, they get the JARs from production server, explode them, replace or add new classes as needed, repackage the jars, put them back to server.
Therefore, if they will use Java 6 to compile and using the above method for deployment, there will be a lot of exotic mixes of 1.4 and 1.6 bytecodes.

The JVM byte code is not siginificantly different between Java 1.0 and Java 6. In Java 7 they add one new instruction. Woohoo.
There are so little changes in how the byte code works that
The JVM doesn't support nested classes accessing private members of outer classes, this works through generated code.
The JVM doesn't support runtime checks for generics e.g you cannot new T() where T is a generic.
Basically, they make the JVM smarter and faster but until recently changing the model of how the byte code works has been avoided at all costs.

You can compile with Java 6 but target 1.4 with a compiler setting. We did this for a migration project once. If/when 1.4 disappears, you then change your compiler settings again and target 1.6.
Keeping the target version explicit also means that you can upgrade your SDK without fear of your JAR files becoming unusable to an older JVM.

I am maintaining an environment with mix of 1.4 (old library jars) and 1.5 (my fixes and stuff) classes on Tomcat using Sun JVM 1.5 and it runs fine.
However, for RMI you may be in trouble if client and server has different class version because the server might check the class version (I ran into this problem).
The best way to find out is to do a proof of concept type of project on small scale.
A friendly reminder though, you are digging a pretty big hole for yourself here :-)

These links seem relevant. They document the few edge cases that could break compatibility between 1.4 and 1.5 and between 1.5 and 1.6.
The biggest differences that could cause problems that I can think of is that enum became a keyword, but that would only effect a 1.5+ JVMs when loading an older class file (which doesn't seem to be what you will be doing). The other thing is annotations. The above links seem to suggest everything would be fine, but I would be wary about what would happen if an older JVM loaded up a class with runtime annotations.
Other than that I don't think there have been any bytecode changes between the first version of java and java 6. Meaning the only problems you should encounter are changes to functionality the API or deprecations (listed in the links above).

As long as you aren't using reflection, the only major problem you could have from differing bytecode versions is the ACC_SUPER flag.
In very early versions, invocation of superclass methods was not handled correctly. When they fixed it, they added a new flag to the classfile format, ACC_SUPER to enable it, so that applications relying on the old, broken, behavior were not affected. Naturally, using a class that doesn't contain this flag could cause problems.
However, this is ancient history. Every class compiled in 1.4 and later will have the flag, so this isn't a problem. Bytecode wise, the only major differences between 1.4 and 1.6 are the addition of optional attributes used to store metadata about inner classes, generics, annotations, etc.
However, these don't directly affect the bytecode execution. The only way these have an affect is if you access them through reflection. For instance, java.lang.Class.getDeclaredClasses() will return information from the optional attribute InnerClasses.

Converting Java 1.5 compatible code to Java 1.4

Is there any tool that might help in converting the Java code written using 1.5 to 1.4. This is required because i need to reverse engineer my project using STAR UML. and Star UML is Java 1.4 compatible.
Please Help

Maybe you can try with retroweaver, it works on byte code level though, and I don't know if it's enough for your tool.
To review the changes between Java 1.4 to Java 1.5, the most important is probably generics, then auto boxing and auto unboxing, annotations, variable number of arguments, the foreach loop, and import static.
Generics: At a source code level, while you could probably get rid of generics when used as the collection element type, more difficult will be when a generic type is returned by a method.
Auto boxing/unboxing: will need explicit conversions.
Annotations: since they enable declarative programming, if used they might be difficult to substitute. With Spring, for instance you can define the same behaviour with XML, but it would be hard to convert by hand and also a bad idea.
Variable number of aguments: will have to use arrays.
Foreach loop: have to use iterators explicitely.
import static: just prepend the class name.
It was a pretty good advancement of features from Java 1.4 to 1.5, I wrote this more as a review of the changes, but unless your code is using very little of the new features you'll end losing a lot of time and also you'll reduce the quality of your code, so as sethu advised you probably the easier path is find another tool.

You can do it with NetBeans by selecting the Java platform you want in the properties folder.
Then is going to mark as wrong all the code non compatible with Java 1.4.
(But maybe you should follow the #sethu advice)