JSTL While loop quick question

Question

Hi,
I have a hashmap that contains the keys "number1", "number2" etc.... I want to loop through as long as the next key is not null, e.g.
// Assume nextKey is the string "number1" and counter is 1
while (myMap.get(nextKey) != null) {
   // do something
   counter++ ;
   nextKey = "number" + counter ;
}How do I perform this boolean while loop? Would I need to use the forEach tag or a custom tag?
Cheers.

Answer 1

The easiest way to do this is to use the <c:forEach> tag providing your map as the 'items' attribute.
Each iteration exposes an object of type Map.Entry which is one key-value pair in your underlying Map object. You can then use the getKey() and getValue() methods on the exposed Map.Entry object.
<c:forEach items = "myMap" var = "eachEntry">
     Key is ${eachEntry.key}
     Value is ${eachEntry.value} <br>
</c:forEach>If you have to supply the keys yourselves (in the above example, both the key and the value were discovered dynamically), then you still use the forEach tag.
However in this scenario what you need is a loop that iterates a fixed number of times equal to the size of the map.
For this you use the jstl function taglib. For example, the below post would output the size of the map.
<%@taglib uri="http://java.sun.com/jsp/jstl/functions"  prefix="fn"%>
${fn:length(myMap)}You use the begin and end attributes of the forEach loop to loop a fixed number of times. The varStatus attribute provides a counter inside the loop. Thus
<c:forEach begin = "0" end = "1" varStatus = "status">
    Counter : ${status.count}
</c:forEach>would cause the loop to iterate twice printing Counter :1 Counter :2.
Finally you need to set an attribute whose value is appended with the counter to evaluate the key.
Combining all of these, your code would be
<c:set var = "key" value = "number"/> //this value will be appended with the counter to evaluate the key
<c:forEach begin="0" end = "${fn:length(myMap)}" varStatus="status">
     <c:set var = "nextKey" value = "${key}${status.count}"/>
     ${myMap[nextKey]}
</c:forEach>Does that help?
ram.