How to compile .java to .class in .jar file

Question

Hey. I have code to make a .jar file, read .java files, and save them as .class files in the .jar file. When I open the .jar file, it says that the magic numbers on the classes are wrong. So I figured that Java must compile the .java files to .class files in bytecode.
Question: What's the code to compile a string from a .java file to a .class file?
Here's my code:
try {
            // Name of jar file to write
            String archiveFile = Name + ".jar";
            Manifest jman = new Manifest();
            try {
                // Create a manifest from a file
                //InputStream fis = new FileInputStream("manifestfile");
                //Manifest manifest = new Manifest(fis);
                // Construct a string version of a manifest
                StringBuffer sbuf = new StringBuffer();
                sbuf.append("Manifest-Version: 1.0\n");
                sbuf.append("Ant-Version: Apache Ant 1.7.1\n");
                sbuf.append("Created-By: 1.5.0_19-137 (Apple Inc.)\n");
                sbuf.append("Main-Class: Main\n");
                sbuf.append("Class-Path: lib/swing-layout-1.0.3.jar\n");
                sbuf.append("X-COMMENT: Main-Class will be added automatically by build\n");
                // Convert the string to a input stream
                InputStream is = new ByteArrayInputStream(sbuf.toString().getBytes("UTF-8"));
                // Create the manifest
                jman = new Manifest(is);
            } catch (IOException e) {
            FileOutputStream stream = new FileOutputStream(archiveFile, true);// archive file is jar file name
            JarOutputStream out = new JarOutputStream(stream, jman);
            out.putNextEntry(new JarEntry("Main.class"));
            StringBuffer sbuf = new StringBuffer();
            sbuf.append(readTextFromJar("Main.txt")));
            out.write(sbuf.toString().getBytes("UTF-8"));
            out.closeEntry();
            out.putNextEntry(new JarEntry("MainScreen.class"));
            sbuf = new StringBuffer();
            sbuf.append(readTextFromJar("MainScreen.txt"));
            out.write(sbuf.toString().getBytes("UTF-8"));
            out.closeEntry();
            out.putNextEntry(new JarEntry("GCanvas.class"));
            sbuf = new StringBuffer();
            sbuf.append(readTextFromJar("GCanvas.txt"));
            out.write(sbuf.toString().getBytes("UTF-8"));
            out.closeEntry();
            out.close();
        } catch (Exception ex) {
            JOptionPane.showMessageDialog(this, ex.toString(), "BUG!", JOptionPane.INFORMATION_MESSAGE);
            ex.printStackTrace();
        }Thanks,
-Gandolf

Answer 1

So I'm guessing that none of you guys have the knowledge on how to compile a java application within a java application. I was thinking that it had something to do with javac... but I'm unsure.
If no one can answer the question, could someone point me to an expert who would know?
-Gandolf