Day 25: Using a implementation of supplier creating and caching a large file (any Class in this case that will initialize a very large object) lazily and thread-safe.

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import java.io.File;
import java.io.FileNotFoundException;
import java.util.Optional;
import java.util.function.Supplier;

class Day25 {
    public static void main(String[] args) throws Exception {
        FileLoader largeFileLoader = new FileLoader();
        File largeFile = largeFileLoader
            .getLargeFile()
            .orElseThrow(FileNotFoundException::new);
        System.out.println(largeFile.getName());
    }

    static class FileLoader {

        private Supplier<Optional<File>> largeFile =
            () -> createAndCacheFile("/home/mohibulhasan/Downloads/data.json");

        public FileLoader() {
            System.out.println("File Loader created");
        }

        public Optional<File> getLargeFile() {
            return largeFile.get();
        }

        private synchronized Optional<File> createAndCacheFile(String path) {
            class LargeFileFactory implements Supplier<Optional<File>> {
                private final File largeFileInstance = new File(path);

                @Override
                public Optional<File> get() {
                    return largeFileInstance.exists()
                        ? Optional.of(largeFileInstance) : Optional.empty();
                }
            }

            if (!(largeFile instanceof LargeFileFactory)) {
                largeFile = new LargeFileFactory();
            }

            return largeFile.get();
        }
    }
}