Java – Synchronized HashMap Example

HashMap is a non-synchronized collection class. If we need to perform thread-safe operations on it then we must need to synchronize it explicitly.

Iterator should be used in a synchronized block even if we have synchronized the HashMap explicitly.

[code lang=”java”]
Map map = Collections.synchronizedMap(new HashMap());

// This doesn’t need to be in synchronized block
Set set = map.keySet();
// Synchronizing on map, not on set
synchronized (map) {
// Iterator must be in synchronized block
Iterator iterator = set.iterator();
while (iterator.hasNext()) {


Java – Calculate Difference Between Two Dates Using Joda Time

Take a look @ Joda-Time FAQ

You can use a PeriodFormatter to get the format of your choice.

Try the following sample code.

[code lang=”java”]
DateTime dt = new DateTime();

DateTime twoHoursLater = dt.plusHours(2).plusMinutes(10).plusSeconds(5);

Period period = new Period(dt, twoHoursLater);

PeriodFormatter HHMMSSFormater = new PeriodFormatterBuilder()
.toFormatter(); // produce thread-safe formatter