How to Iterate Through a HashMap to Populate a JSP Dropdown List?

Looping Through a HashMap in JSP

Problem Statement

How to effectively iterate through a HashMap within JSP scriptlets to populate a drop-down list?

Solution

In JSP, traversing a HashMap is identical to regular Java code. One can employ the following approach:

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

Using JSTL for Iterations

However, using JSP scriptlets is discouraged. Instead, it's recommended to leverage JSTL's tag, which allows iterating over Maps. Each iteration returns a Map.Entry object with getKey() and getValue() methods.

For instance:

<%@ taglib prefix="c" uri="jakarta.tags.core" %>

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

Implementing the Select Drop-Down

Adapting the aforementioned code to populate a drop-down list:

<%@ taglib prefix="c" uri="jakarta.tags.core" %>

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

Data Placement in Scope

To make countries accessible from within ${countries}, it needs to be placed in the appropriate scope. This can be achieved through:

• Request scope (Servlet doGet method):

  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);
  }

• Application-wide scope (ServletContextListener contextInitialized method):

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

Additional Resources

• [Iterating over Lists and Maps in JSP using JSTL](https://www.journaldev.com/2680/jsp-iterate-over-list-map-jstl)
• [Iterating over Nested Maps in JSTL](https://stackoverflow.com/questions/11210152/how-to-iterate-over-a-nested-map-in-c-forEach)
• [Iterating an ArrayList within a HashMap using JSTL](https://stackoverflow.com/questions/13002775/how-to-iterate-an-arraylist-inside-a-hashmap-using-jstl)
• [Initializing Servlet and Sharing Application Data on Startup](https://www.mkyong.com/servlet/startup-servlet-contextlistner/)

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