When using the singleton design pattern, do other methods need to use synchronized keyword to ensure thread safety?

xun yan :

I want to ensure that the following class is thread-safe, should I use the synchronized keyword for other methods? Or use a thread-safe data structure to store Email. What should I do?

public class RecycleStation {
    private static volatile RecycleStation uniqueInstance;
    private static List<Email> recycleEmailList ;

    private RecycleStation() {
        recycleEmailList = new ArrayList<>();
    }

    public static RecycleStation getInstance() {
        if (uniqueInstance == null) {
            synchronized (RecycleStation.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new RecycleStation();
                }
            }
        }
        return uniqueInstance;
    }

    public void RecycleEmail(Email email) {
        recycleEmailList.add(email);
    }

    public void deleteEmail(Email email) {
        recycleEmailList.remove(email);
    }

    public void clear() {
        recycleEmailList.clear();
    }

}

igorepst :

First, a Singleton pattern is best implemented as Enum in Java Second, each email manipulation function (clear, Recycle, delete) should be synchronized to ensure thread safety (the link is about an Enum, but the same holds about each and every Sinlgeton implementation):

public synchronized void RecycleEmail(Email email)