<dependency>
      <groupId>com.github.javadev</groupId>
      <artifactId>underscore</artifactId>
      <version>1.33</version>
    </dependency>

        import com.github.underscore.lodash.$;

...

        String json = "{\"memory\":[{\"alert\":\"false\",\"value\":\"50%\"}],\"cpu\":"
        + "[{\"alert\":\"true\",\"value\":\"100%\"}],\"hdd\":[{\"alert\":\"false\",\"value\":\"80%\"}]}";

        assertEquals("[memory, cpu, hdd]", $.keys((Map<String, Object>) $.fromJson(json)).toString());