Yandex test program

Main.java

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import javax.annotation.Nonnull;

public class Main {

    public static void main(String[] args) {
        CacheableDigest digest =
                new CacheableDigest(
                        new DigestImpl());

        // will print "The real method has been called"
        byte[] input = { 1 };
        System.out.println("Case #1");
        digest.doDigest(input);

        // will NOT print message as cache is used
        System.out.println("Case #2");
        digest.doDigest(input);

        // will print "The real method has been called" again, as Array's hashCode method is inherited from Object that
        // uses
        System.out.println("Case #3");
        digest.doDigest(new byte[] { 1 });
    }

    public interface Digest {
        @Nonnull
        byte[] doDigest(@Nonnull byte[] input);
    }

    public static class DigestImpl implements Digest {
        @Nonnull
        @Override
        public byte[] doDigest(@Nonnull byte[] input) {
            // this method does the real job and it's a replacement for abstract doDigest method from the original code
            System.out.println("The real method has been called");
            return input;
        }
    }

    public static class CacheableDigest implements Digest {
        private final Digest orig;
        private final Map<byte[], byte[]> cache;

        public CacheableDigest(@Nonnull Digest orig) {
            this.orig = orig;
            this.cache = new ConcurrentHashMap<>();
        }

        @Nonnull
        @Override
        public byte[] doDigest(@Nonnull byte[] input) {
            return cache.computeIfAbsent(input, orig::doDigest);
        }
    }
};