Question

I am making a tool that will write .java files, then (hopefully) compile those files to .class files. All in one process, the user selects a file directory where multiple .java files are written. Now I want the program to compile these Java files.

Answer 1

JavaCompiler is your friend. Check the documentation here

And here an example on how you could use the compiler API

JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
DiagnosticCollector<JavaFileObject> diagnostics = new DiagnosticCollector<>();
StandardJavaFileManager fileManager = compiler.getStandardFileManager(diagnostics, null, null);
Iterable<? extends JavaFileObject> compilationUnits = fileManager.getJavaFileObjectsFromStrings(Arrays.asList("YouFileToCompile.java"));
JavaCompiler.CompilationTask task = compiler.getTask(null, fileManager, diagnostics, null,
        null, compilationUnits);
boolean success = task.call();
fileManager.close();