How to loop through a HashMap in JSP?

Question

How can I loop through a HashMap in JSP?

<%
    HashMap<String, String> countries = MainUtils.getCountries(l);
%>

<select name="country">
    <% 
        // Here I need to loop through countries.
    %>
</select>

Solution 1

Just the same way as you would do in normal Java code.

for (Map.Entry<String, String> entry : countries.entrySet()) {
    String key = entry.getKey();
    String value = entry.getValue();
    // ...
}

---

However, scriptlets (raw Java code in JSP files, those <% %> things) are considered a poor practice. I recommend to install JSTL (just drop the JAR file in /WEB-INF/lib and declare the needed taglibs in top of JSP). It has a <c:forEach> tag which can iterate over among others Maps. Every iteration will give you a Map.Entry back which in turn has getKey() and getValue() methods.

Here's a basic example:

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<c:forEach items="${map}" var="entry">
    Key = ${entry.key}, value = ${entry.value}<br>
</c:forEach>

Thus your particular issue can be solved as follows:

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<select name="country">
    <c:forEach items="${countries}" var="country">
        <option value="${country.key}">${country.value}</option>
    </c:forEach>
</select>

You need a Servlet or a ServletContextListener to place the ${countries} in the desired scope. If this list is supposed to be request-based, then use the Servlet's doGet():

protected void doGet(HttpServletRequest request, HttpServletResponse response) {
    Map<String, String> countries = MainUtils.getCountries();
    request.setAttribute("countries", countries);
    request.getRequestDispatcher("/WEB-INF/page.jsp").forward(request, response);
}

Or if this list is supposed to be an application-wide constant, then use ServletContextListener's contextInitialized() so that it will be loaded only once and kept in memory:

public void contextInitialized(ServletContextEvent event) {
    Map<String, String> countries = MainUtils.getCountries();
    event.getServletContext().setAttribute("countries", countries);
}

In both cases the countries will be available in EL by ${countries}.

Hope this helps.

See also:

• Iterate over elements of List and Map using JSTL <c:forEach> tag
• How to iterate over a nested map in <c:forEach>
• How to iterate an ArrayList inside a HashMap using JSTL?
• Using special auto start servlet to initialize on startup and share application data

Solution 2

Depending on what you want to accomplish within the loop, iterate over one of these instead:

• countries.keySet()
• countries.entrySet()
• countries.values()

Comments

[BalusC]

By the way, a HashMap is by nature unordered. Are you certain you don't need TreeMap (automagic sort by key) or LinkedHashMap (maintains insertion order)?

[blub]

yes,with this i get alle key set but how can i integrate it into a select field for html?

[BalusC]

@Khue: yes, you can also put attributes in the session. I only don't see why you would like to duplicate application wide data over multiple sessions.

[Khue Vu]

I think in case of session-based info. Thanks for the very nice explanation.

[Taoufik Mohdit]

this is not answering the question which is specific about JSP

[Zibbobz]

I'm sure the answer is obvious but - why are scriptlets considered poor practice? In the absence of JSTL due to maintaining a legacy application, this is my only option.

[BalusC]

@Zibbobz: the text "poor practice" is a link. Click on it.