ArrayListWithInterfaceDemo

    public static final class ArrayListWithInterfaceDemo {

        public interface FileExplorerElement {

            ArrayList<FileExplorerElement> subElements = new ArrayList<>();

            String getExt();

            void add(FileExplorerElement subElement);
        }

        public static class FileMP3 implements FileExplorerElement {
            @Override
            public String getExt() {
                return ".mp3";
            }

            @Override
            public void add(FileExplorerElement subElement) {
            }
        }

        public static class FileJPEG implements FileExplorerElement {
            @Override
            public String getExt() {
                return ".zip";
            }

            @Override
            public void add(FileExplorerElement subElement) {
            }
        }

        public static class FolderMedia implements FileExplorerElement {
            @Override
            public String getExt() {
                return ".zip";
            }

            @Override
            public void add(FileExplorerElement subElement) {
                subElements.add(subElement);
            }
        }

        public static class FolderDCIM implements FileExplorerElement {
            @Override
            public String getExt() {
                return "";
            }

            @Override
            public void add(FileExplorerElement subElement) {
                subElements.add(subElement);
            }
        }

        public static class FileZip {
            public String getExt() {
                return ".zip";
            }

            public void addElement(FileExplorerElement subElement) {
            }
        }


        public static class FolderDownload {

            ArrayList<FileExplorerElement> subElements = new ArrayList<>();

            public String getExt() {
                return "";
            }

            public void addElement(FileExplorerElement subElement) {
                subElements.add(subElement);
            }
        }

        public ArrayListWithInterfaceDemo() {
            testMyCode();
        }

        public void testMyCode() {
            FolderDCIM dirDCIM = new FolderDCIM();
            dirDCIM.add(new FileJPEG());
            FolderMedia dirMedia = new FolderMedia();
            dirMedia.add(new FileMP3());
            dirMedia.add(new FileJPEG());
            FolderDCIM dirDownload = new FolderDownload();
            dirDownload.add(new FileMP3());
            dirDownload.add(new FileJPEG());
            dirDownload.add(new FileZip());
            ArrayList<FileExplorerElement> mainDirectory = new ArrayList<>();
            mainDirectory.add(new FolderDCIM());
            mainDirectory.add(new FolderMedia());
            mainDirectory.add(new FolderDownload());
        }

    }