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---
title: jte Kotlin tips
description: Details specific to Kotlin usage
---
# Kotlin Details
Kotlin entails a few differences. The `jte-kotlin` dependency is needed to enable `.kte` templates that contain
Kotlin code. Keep in mind that Kotlin application code can use either Java (`.jte`) or Kotlin (`.kte`) templates. `.kte`
files are only needed for Kotlin based templates.
## Add required dependencies
This dependency is a compile time dependency to avoid including the Kotlin compiler in the resulting build.
=== "Maven"
```xml linenums="1"
<dependency>
<groupId>gg.jte</groupId>
<artifactId>jte-kotlin</artifactId>
<version>{{ latest-git-tag }}</version>
<scope>compile</scope>
</dependency>
```
=== "Gradle"
```groovy linenums="1"
compileOnly "gg.jte:jte-kotlin:{{ latest-git-tag }}"
```
## Common errors
### Non-null parameters
=== "Non-null (`.kte file`)"
```html linenums="1"
@param name: String
```
=== "Nullable (`.kte file`)"
```html linenums="1"
@param name: String?
```
If a non-null parameter exists in the template, the template will fail to load with an error in the line where the
parameter is being included.
```
Error rendering template:
Failed to render admin/index.kte, error at admin/index.kte:1
(many many lines of stack trace)
Caused by: java.lang.NullPointerException: null cannot be cast to non-null type kotlin.String
```
### Writing a non-supported type to a template
```html linenums="1"
@param myList: List
${myList}
```
This will generate an error like so:
```
gg.jte.TemplateException: Failed to compile template, error at myPage/index.kte:3
jteOutput.writeUserContent(things)
/Users/aUser/app/jte-classes/gg/jte/generated/ondemand/index/JteindexGenerated.kt:13:14
Reason: None of the following functions can be called with the arguments supplied:
public open fun writeUserContent(p0: Content!): Unit defined in gg.jte.html.HtmlTemplateOutput
public open fun writeUserContent(p0: Boolean): Unit defined in gg.jte.html.HtmlTemplateOutput
(etc.)
```
This will be improved in the future.
### Nulls in `@for` expressions
`@for` loops use an iterator. If a variable is null then the following error will appear:
```
Failed to compile template, error at myApp/index.kte:5
for (s in things) {
/Users/aUser/code/cds/jte-classes/gg/jte/generated/ondemand/index/JteindexGenerated.kt:10:13
Reason: Not nullable value required to call an 'iterator()' method on for-loop range
```
A quick fix is to make the parameter have an empty list by default.
```html linenums="1"
@for(things in (things ?: listOf()))
```
|
