SecureMessage.java

import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.Optional;

public class SecureMessage {

    private final String key;
    private final ObjectMapper objectMapper;

    public SecureMessage(String key, ObjectMapper objectMapper) {
        this.key = key;
        this.objectMapper = objectMapper;
    }

    public Optional<Map> decrypt(String value) {
        return Optional.ofNullable(value)
                .map(this::decryptJSON)
                .map(this::jsonToMap);
    }

    private HashMap jsonToMap(String value) {
        try {
            return objectMapper.readValue(value, HashMap.class);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    // Fake Method instead of Util.decryp
    private String decryptJSON(String value) {
        // should use key to decrypt
        return value;
    }

}

SecureMessageSpec.java

public class SecureMessageSpec {

    private SecureMessage secureMessage;

    @Before
    public void before() {
        secureMessage = new SecureMessage("fake-key", new ObjectMapper());
    }

    @Test
    public void when_value_is_a_json_it_returns_an_optional_map() {
        assertThat(secureMessage.decrypt("{\"foo\": \"bar\"}"),
                is(Optional.of(ImmutableMap.of("foo", "bar"))));
    }

    @Test
    public void when_value_is_null_it_returns_an_empty_optional() {
        assertThat(secureMessage.decrypt(null), is(Optional.empty()));
    }
}