Accessing private access variable data from different class

Question

I have this Thread inside my Project which runs continously accepting new symbols

public class StreamThread extends Thread {
    private Set<String> allSymbolSet = new HashSet<String>(Arrays.asList("USBC", "TCSD", "PCLJ"));
    private PriorityBlockingQueue<String> priorityBlocking = new PriorityBlockingQueue<String>();

    public void addSymbols(String str) {
        if (str != null) {
            priorityBlocking.add(str);
        }
    }

    public void run() {
        while (true) {
            try {
                while (priorityBlocking.peek() != null) {
                    String symbol = priorityBlocking.poll();
                    allSymbolSet.add(symbol);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

my question is , i want to access the variable allSymbolSet from another class

what will be the best approach to get access to this variable named allSymbolSet from anoter class , for this i have two choices

1. modify the access specifier of allSymbolSet from private to default .

2. Write a get Method which is supoused to return the Set

Please suggest me , what will be the good approach in this case ??

Answer 1

Best approach would be the getter method AND synchronize the access to the object allSymbolSet, something like this:

public Set<String> getAllSymbolSet() {
    synchronized(allSymbolSet) {
        return allSymbolSet;
    }
}

and also synchronize the access to allSymbolSet inside your thread.

Answer 2

A few comments:

• If you make the set non-private, some code could modify it (by mistake or on purpose) which could result in inconsistent behaviour in your StreamThread class. Don't do that.
• Providing a simple getter does not solve the issue above. Prefer returning a copy of your set.
• Make your variables final whenever you can when in a multi-threading environment - it solves many thread safety issues.
• Prefer implementing Runnable than extending Thread
• You will need to synchronize ALL accesses to your set (read and write), for example by using a synchronizedSet or even better by wrapping a ConcurrentHashMap which generally provides better performance.
• instead of peek+poll you can simply take from your queue

So your final class could look like:

public class StreamTask implements Runnable {

    private final Set<String> allSymbolSet;
    private final PriorityBlockingQueue<String> priorityBlocking = new PriorityBlockingQueue<String>();

    public StreamTask() {
         String[] symbols = {"USBC", "TCSD", "PCLJ"};
         //use a thread safe set, for example based on ConcurrentHashMap
         allSymbolSet = Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean> ());
         Collections.addAll(allSymbolSet, symbols);
    }

    public void addSymbols(String str) {
        if (str != null) {
            priorityBlocking.add(str);
        }
    }

    public Set<String> getSymbols() {
        return new HashSet<> (allSymbolSet); //return a copy
    }

    public void run() {
        while (true) {
            try {
                allSymbolSet.add(priorityBlocking.take());
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

Finally, I might be missing something, but that class looks equivalent to the much simpler:

public class StreamTask {

    private final Set<String> allSymbolSet;

    public StreamTask() {
         String[] symbols = {"USBC", "TCSD", "PCLJ"};
         //use a thread safe set, for example based on ConcurrentHashMap
         allSymbolSet = Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean> ());
         Collections.addAll(allSymbolSet, symbols);
    }

    public void addSymbols(String str) {
        if (str != null) {
            allSymbolSet.add(str);
        }
    }

    public Set<String> getSymbols() {
        return new HashSet<> (allSymbolSet); //return a copy
    }
}

Answer 3

Better way is method 2. Writing a getter method. If you want to allow set the values then use a setter later. Then your data will be encapsulated .

Answer 4

Write a get Method which is supposed to return the Set. by using this your private remains private and you also access it from outside using Object of the same class.