Web Article - Fast, Effective, Double-Checked Locking in Android and Java Apps

public class Foo {
    private volatile Map< String, String > data;

    public Foo() { }

    public void setData(String key, String value) {
        if (data == null) {
            synchronized (this) {
                if (data == null)
                    data = new ConcurrentHashMap < String, String >();
            }
        }

        data.put(key, value);
    }
}

"There are two important things here. The first is adding the volatile modifier to the data field. This will instruct the compiler and eventually the Dalvik VM to ensure reads and writes are performed in a happened-before order (see here for more details). In other words, writes to the field always happen before reads (without this keyword, the compiler or JIT optimizer may decide to reverse them). Next, we add another null check for data outside the synchronized block. This will ensure once data has been allocated, we never synchronize again. However, if data is indeed null, we synchronize and then double-check if data is null just to make sure another thread hasn’t assigned a value between the two checks. If we lost the race, we’ll just continue normally and exit the synchronized block."

Source: Dzone