Just Java

• Why use Inner class
• The use is to increase encapsulation and hide some implementation details. Classes like Semaphore use internal objects for locking, etc.
• Serialization
• Serialization purpose is not persistence, there are DB's for that. Use of serialization is to transfer objects over network. We can write the Java object as byte array in memory, and later again construct the object from byte array in memory.
• ConcurrentModificationException
• arraylist is getting modified by other thread
• Ensure that instance is never garbage collected
• static reference to a singleton, so it won't be eligible for garbage collection until the classloader is eligible for garbage collection
• Java Runtime Memory Management
• MemoryManagement is Garbage Collection and memory allocation
• Local Variables are thread safe, local variables can be used to confine objects to a thread
• Immutable objects are always thread safe. A properly consturcted(this doesn't escapes), and state can't be modified, and may be all fields are final. No requirement for safe publication.