Maintaining same synchronized array-list between multi-threads in java

Question

I am working with threads as a fledgling. So I need some help.

For certain work I need a single array-list, which value will be shared by all threads. I want something like this, main() class will provide the array-list to the threads in time of thread creation. Threads will add values to the array-list and a change made by a thread will be reflected in every copy of that array-list and importantly this have to be done in a synchronized fashion.

For example, main() class has given two threads the array-list. Then first thread added a value on slot 1 of the array, second thread while adding will see the change and when it will add,it will add in the second position. When main will give the array list to a new thread all this changes done previously will be readily included, and it will start adding from the third or later positions. Another thing is, only one thread can make a change at a time or it should be synchronized.

How can I do this in java? Can any one help me?

Pass the same List to all threads, and make sure every access is synchronized. Or better, encapsulate the list in a class that delegates to the list, and ensures the synchronization, and pass an instance of that class to all threads. Start coding, and come back with what you tried if you face a concrete problem. Post the description of the concrete problem, and the code. — JB Nizet
– JB Nizet, Commented Jan 24, 2016 at 9:17
Note that you need to make sure that the following can be done by a single thread without other threads "interrupting": 1. Find the next free index 2. Set a value at this index. Synchronisation is one way of achieving this. — David Soroko
– David Soroko, Commented Jan 24, 2016 at 11:13
I encapsulated the list in a class and passed it to calling threads. It works. For synchronization I used singelton class with synchronized access to the methods. Thanx @JBNizet — Mahin
– Mahin, Commented Jan 27, 2016 at 13:57
For proper synchronization I used singelton class with synchronized access to the methods. Thanx @DavidSoroko — Mahin
– Mahin, Commented Jan 27, 2016 at 13:58

Heri · Accepted Answer · 2016-01-24 11:00:21Z

1

The collections framework offers convenient wrappers for the synchronizing:

        List<TypeOfItem> list = Collections.synchronizedList( new ArrayList<>() );

You can pass around such a list for adding, removing, reading etc. by different threads. Each access will be synchronized.

answered Jan 24, 2016 at 11:00

Heri

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4 Comments

David Soroko Over a year ago

This doesn't help in the scenarion when a thread needs to find the minimal unassigned index and populate it atomically.

Heri Over a year ago

Why do you want to do that? The ArrayList maintains these things for you. Just use add() and the item is added to the list.

David Soroko Over a year ago

Because thread2 may have decided that a[1] is free and about to write the value 2. At the same time thread3 will also determine that a[1] is free and about to write the value 3. You end up with a race condition where the same code sometimes produces a[1]=2 and sometimes a[1]=3

Mahin Over a year ago

I encapsulated the list in a class and passed it to calling threads. It works. For synchronization I used singelton class with synchronized access to the methods. Thanx @Heri

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