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how to change .class file to a text file

I'm trying to make a java program which executes java files and gives output in the text field. I've used Runtime class to compile the .java file .So how do I get the output from that newly made class file.
Runtime.getRuntime().exec("javac Y://CodeSave.java");
Runtime.getRuntime().exec("java Y://CodeSave.class>output.txt");

In the general case: exec returns a Process instance which has accessors (getOutputStream, etc.) for the I/O streams. You read from / write to those streams.
But: In your code you've used >output.txt. That's a shell feature. If you want to do it that way, you need to spawn a shell, not the java tool directly, and have the shell execute that command line. (A search for spawning/execing a shell should find you lots of examples.)

Using Runtime.exec is definitely not the right way to do it, for various reasons. Examples are that both java and javac rely on environment variables which you can't pass that way.
First of all, I'd ask myself if I really needed to do this. Compiling and executing dynamically created code is a huge security risk.
But if you're sure you need to do it, here's what I'd do.
Move your sources to a dedicated temporary folder
Use the ToolProvider api to compile your sources
Use a dynamic throwaway ClassLoader (ByteBuddy may help you there) with a SecurityManager to load and execute your code from within your application

Java main class autocomplete

If I'm trying to run a java program and I don't know the exact name of the Main class, is there any way to use tab completion to figure it out?
java -cp stackoverflow.jar org.<tab>
stackoverflow serverfault stackexchange
java -cp stackoverflow.jjar org.stackoverflow.<tab>
Main IntegrationTest QuestionAnswerConsole
Something along those lines.

Basically you are asking how to configure shell autocompetion to support java. It is possible. Take a look on this discussion: How does bash tab completion work?
I have to say that this is a good idea not only for discovering the main class but also to complete other command line options and a class path. I'd be glad to use such script if you develop it. Good luck.
EDIT
At least on my Ubuntu file less /usr/share/bash-completion/completions/java exists and therefore some completion should work. You are always welcome to improve the script.

The only way to get the Tab Completion that you are talking about is to use a shell that is "Java aware" or a shell script that provides this feature for the java command. git has a similar feature, so I don't think it's completely impossible.
Edit:
According to this question on SU, it is possible to create an autocompletion script for the bash shell. Since the question on SU is slightly different than what you are asking, I don't see a lot of specific details that relate to this question. However, it looks like a good place to start.

I don't know any easy way to get the autocompletion of classes names if they are hidden in a jar file.
On the other hand, you can add a Manifest to your jar to make it auto-executable (ie., you just have to run java -cp ... -jar stackoverflow.jar) !
In the jar archive, add a META-INF folder, and inside that folder, create a MANIFEST.MF file that reads :
Main-Class: org.stackoverflow.Main
(or whatever your main class really is).
Here is some documentation : Setting an application's Entrypoint

Other answers suggest you solutions but as a sidenote, you don't need it if you provide maintainer of the jar file provides a manifest file with path META-INF/MANIFEST.MF. Then java automatically extracts main class from the manifest and you can run it this way:
java -jar stackoverflow.jar

How do I create a Unix load card for a java program

In Unix (or Linux), if I want to run a shell script, I can start the file with #!/bin/sh. With awk, I start the executable file with #!/usr/bin/awk -f and it treats the rest of the file as the program.
How do I do that with a Java program? I tried copying the simple.class to simple, putting #!/export/appl/Mail/java/bin/java at the top and making the file executable, but I get:
69> ./simple
Error: Could not find or load main class ..simple
I know this can be done with an executable shell script, or a C program that execs the java interpreter. Every other interpreter on Unix can be called with a #! load card, surely there's a way to do it with Java.

The most usual way is to have a wrapper for the Java. A shell script that executes the "java -jar yourJar.jar" or equivalent. And then you bundle the shell script and the windows equivalent bat file with your product.
Another option is to have a native launcher. For example you can see the Eclipse project which has gone that way. You download Eclipse and you have a native executable to run. The native executable will launch your Java program.
One more option is to compile Java into native code. For example you can use this commercial tool called Excelsior JET ( http://www.excelsior-usa.com/jet.html ).

The Java class file format doesn't allow text before the header, that's why the Java runtime no longer accepts the .class file after your modification.
On Linux you can use binfmt_misc to support additional executable formats, including .class files. It's basically a way to tell the Linux kernel how to detect executable formats and how to execute them.
This Archilinux Wiki article explains in more detail how to get this effect.

You cannot do it with a Java program. Firstly, the Java program needs to be compiled before execution. Secondly, even if compilation wasn't required, the hash sign is not a comment in Java, so that would be a syntax error.

I've never heard the term "load card". What you have is an "interpreter directive" designated by a shebang. This merely designates which interpreter the shell should invoke on a given script.
As for why C programs can be run directly in the shell, executables recognized by the operating system are passed to the loader. A Java class isn't an executable, at least to the OS anyway. So the shell must know which interpreter to pass control to.
But as you've noticed, the shebang doesn't work. The reason is that the class file is in a specific binary format that the JVM expects. Editing this file will break convention and lead to an error. Therefore, there is no way to do what you've asked.
However, you can create a "shortcut" to the program you want to run by creating an alias or perhaps writing a one-line Shell script to wrap the java command you need. This is the common practice as I understand it.

The other answers explain why you can't do what you are trying to do. However, if your shell is zsh, you can create a suffix alias. For example, to execute .jar files using java:
alias -s .jar="/usr/bin/java -jar"
Then, to execute blarg.jar, you just type ./blarg.jar. Of course, you must chmod +x your .jar file first.

Apart from the wrapper script and binfmt_misc solutions suggested by others, I'd like to suggest a potential solution which doesn't directly answer your question but maybe it solves your actual problem.
Since Scala does have an interpreter that can run code without you having to compile it first, and this code can reference any Java code, if your goal can be summed up as "using Java as a shell scripting language", you could use a .scala file as your starter script (which can include the shebang to be run with scala) from which you can call all your Java classes. This isn't any simpler tha having a bash-based starter script, but it's a good starting point to gradually move to scripting in Scala instead of Java in which case you can get rid of the need to compile to .class file in the first place.

The reason this doesn't work is that Java isn't really an interpreted language, it's partially compiled, partially interpreted.
The .java source code that you'd put the hashbang directive in needs to be compiled to a .class file before the java interpreter can run it. Comments are stripped out by the compiler, so there's no way to push a comment from the .java into the .class file. The .class file is "compiled" output in a specific format, so adding a hashbang directive to the top of it would break the format.
Java isn't really meant to be a scripting language - but some JVM languages are. For example Groovy supports hashbang and so does Clojure.

Cuda-memcheck and JOCL, can a java executable make use of it? (OpenCL)

I love JOCL, Java bindings for OpenCL. I would like to run Cuda-memcheck on an executable from Java, but whenever I make Java applications, they are always just JAR files that point to a Main-Class. Is there a way to create a .exe file like C++ does and feed that to Cuda-memcheck?

This link could be useful: http://jsmooth.sourceforge.net/index.php
You could probably also the jvm executable directly and provide all the arguments necessary to run your java application. If you normally run your java applications through an IDE, check the console output to see what command the IDE uses to launch your app. This command should resemble something like: <path-to-java-exe> [JVM arguments] main_class [application arguments].

You might try looking into one of the Windows ports of the gcc toolchain. I know that gcc has the capability to turn java code into compiled binary, which is what Cuda-memcheck is looking for. If you aren't afraid of a lot of unnecessary output, attaching Cuda-memcheck to the call to the JVM should also work.

How to run Java Programs

I am just starting Java but I know Scheme...How do you run java code using emacs and putty? I made a program and saved it as first.java on emacs and when i try to open it in putty i did load "first.java" but nothing happened so how do you do this and can yo write Java code in Dr.Scheme?

First, you have to name your file the same as the public class that is contained in the file. Then, once your class is written, you need to compile it using javac and run the .class file that is produced using the command java [ClassName].
A few things to note is that you are going to make sure you have a Java Development Kit (JDK) and that your path is set up so that javac and java are on the path.
I would poke around the Java Tutorials, specifically the "Hello World" application example.

To run Test class, you need to compile the file and run it. Here is how you can do it from the command line.
javac Test.java
java Test
Unfortunately, I cannot comment on using DrScheme (which is a Scheme IDE) for developing Java. Do you mean that you want to invoke Test class from within Scheme?

I haven't touched Java in years, but Sun's Java tutorial is probably all you'll need:

Others have already said to make sure your class name matches the java source file name and to run javac on that, and then java on the resulting .class file. Also, make sure your classpath is properly set when you compile and run.