Here is a good tutorial on Java concurrency : Concurrency
The most common way of way of handling concurrent modifications is by using the synchronized key word, but that is not the only way to do it. There is also methods based on message passing like the actor model and there is atomic variable
The 5 levels of thread safety. It defines objects that are the best/easiest to use in a multi-threaded environment to the worst/hardest.
1. Immutable : The object can't be modified once created. This is the most safe level. No matter how many thread access this object you won't run into any problem
2. Thread-safe : The object contain enough internal synchronization (synchronized method) to be used without worry by multiple threads.
3. Conditionnaly thread-safe : Same as thread-safe except that under some circonstances, you will need to provide external synchronization (outside of that object)
4. Thread-Compatible : Can be use in multi-threaded environment but it is up to the programmer to do the synchronization
5. Thread-hostile : Can't be use in multi-threaded envrionment (Ex.: Thread.stop() - That method was deprecated because it releases the lock when it is called. Hence it could leave an object into an invalid state)
People usually think that the method Collections.synchronizedList(new ArrayList()) will return a thread-safe collection. But that is not true. It is conditionnaly thread-safe. That is true for almost all Java default ''synchronized'' collections. Here is a list of collections that are Thread-safe : Concurrent Collections
Vector are Conditionnally thread-safe. That means that most of the time you don't need to provide external synchronization but in some case you might. One case where you need external synchronization is when iterating over the Vector and modifying it.
Collection.getSynchronizedCollection() - (or something like that) are also conditionnally thread-safe : meaning that you have to provide external synchronization for some operation.
ArrayList is thread compatible : meaning that if you want to use it in a multi-threaded environment you have to provide all the synchronization
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