RAMFileSystem ramFS = new RAMFileSystem();
String[] root = ramFS.getRoot();
String[] foo = ramFS.makePath(root, "foo");
String[] a = ramFS.makePath(foo, "A.java");
ramFS.updateResource(a, new Resource("package foo; public class A {}"));
String[] b = ramFS.makePath(foo, "B.java");
ramFS.updateResource(b, new Resource("package foo; public class B {}"));
//
RAMFileSystem output = new RAMFileSystem();
Compiler compiler = Compiler.builder().sourcePath(ramFS).output(output).build();