When working with Qute templates in Java, developers may face challenges when trying to iterate over data structures like Maps. A common scenario involves needing to iterate through a Map<String, String> and display its contents dynamically. This article will address a specific question raised on Stack Overflow, while providing in-depth analysis, practical examples, and troubleshooting tips to enhance your understanding of Qute's capabilities.
The Question: Iterating Over a Map
A Stack Overflow user posed the following question:
I need to iterate over a
Map<String, String>. I've tried that template syntax:{#for key in myTemplate.myMap.keyset}but it raises:
{myTemplate.myMap.keyset}: Unsupported iterable type found: java.lang.Objectat build time. However, I don't quite understand as a keyset is a set and, as such, it's iterable.
The user is attempting to use the keyset() method of a Map to obtain a set of keys. However, the error indicates that Qute doesn't recognize the returned key set as an iterable object.
Understanding the Issue
The key point here is that Qute templates expect certain types of iterable structures to be compatible with its syntax. The keyset() method from the Java Map interface returns a Set, which should be iterable. However, the error message Unsupported iterable type found: java.lang.Object suggests that there might be a type resolution issue.
Possible Reasons for the Error
-
Type Erasure: In Java, generics are subject to type erasure, which means the generic types are removed at runtime. If the Qute template is trying to interpret the map in a way that loses the type information, it may not treat the keyset correctly.
-
Wrong Context: It might be possible that
myTemplate.myMapis not being recognized as the appropriate type for Qute to understand the key set's generics.
The Correct Approach to Iterate Over a Map in Qute
To iterate over a Map in Qute templates, you'll want to use the correct syntax and methods. Here’s an effective approach:
Using keySet() to Iterate
Instead of accessing the keyset directly, ensure you're casting or properly identifying the type of the map. Here's how you can do it:
{#for entry in myTemplate.myMap}
Key: {entry.key}, Value: {entry.value}
{/for}
Example Implementation
Suppose we have a Java class and a Qute template:
Java Class:
import io.quarkus.qute.TemplateInstance;
import io.quarkus.qute.Template;
import javax.inject.Inject;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
@Path("/map-example")
public class MapResource {
@Inject
Template myTemplate;
@GET
@Produces("text/html")
public TemplateInstance getMap() {
Map<String, String> myMap = Map.of(
"key1", "value1",
"key2", "value2",
"key3", "value3"
);
return myTemplate.data("myMap", myMap);
}
}
Qute Template (myTemplate.qute.html):
<!DOCTYPE html>
<html>
<head>
<title>Map Example</title>
</head>
<body>
<h1>Map Contents</h1>
<ul>
{#for entry in myMap}
<li>Key: {entry.key}, Value: {entry.value}</li>
{/for}
</ul>
</body>
</html>
Practical Example Breakdown
-
Java Backend: The Java class creates a simple REST endpoint that serves a map containing key-value pairs. The
myTemplateis injected and populated with this map data. -
Template Syntax: The Qute template iterates over the map directly. By accessing
entry.keyandentry.value, we can format the output easily.
Troubleshooting Tips
- Check Variable Types: Ensure the map is correctly instantiated and passed to the template with the correct type.
- Debugging: If issues arise, print the type of
myMapin the template to confirm it's being recognized correctly. - Consult Documentation: The official Qute documentation may have updates or notes on specific cases of working with generics.
Conclusion
Iterating over a Map in Qute templates requires careful attention to how types are handled and presented to the template engine. By correctly accessing the map and ensuring that you use the appropriate syntax, you can successfully display its contents. Keep experimenting with different structures and consult community resources like Stack Overflow for ongoing questions and solutions.
References:
By following these guidelines and examples, you can efficiently utilize Qute templates to display key-value pairs from Maps, enhancing the dynamism of your web applications.
