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.

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.

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


Java JSP – JSTL Date Format Example

This example show how to format date in JSP using format tag library.

<%@ taglib uri="" prefix="fmt" %>

    <jsp:useBean id="date" class="java.util.Date"/>
    Today is: <fmt:formatDate value="${date}" type="date" pattern="dd-MMM-yyyy"/>


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