Answers

1. The challenge solved by the double-checked singleton pattern is ensuring that only one class instance is present in the running JVM to avoid possible leaks
2. The usage of the Executors utility that resides in the java.base module and java.util.concurrent package
3. The balking pattern depends on the instance stat
4. The scheduler pattern
5. The producer-consumer pattern is one of the most common concurrent design patterns, with clearly separated and addressed logic