Can not use Groovy collections with closures in Jenkinsfile script - jenkins

Question

I want to use groovy closures with collections in Jenkinsfile:
// other parts removed for brevity
steps {
script {
def testList = ["item1", "item2", "item3"]
testList.stream().map{it+".jpeg"}.each{println it}
}
}
// other parts removed for brevity
However, it gives error:
hudson.remoting.ProxyException: groovy.lang.MissingMethodException: No signature of method: java.util.stream.ReferencePipeline$Head.map() is applicable for argument types: (org.jenkinsci.plugins.workflow.cps.CpsClosure2) values: [org.jenkinsci.plugins.workflow.cps.CpsClosure2#248ba046]
Possible solutions: map(java.util.function.Function), max(java.util.Comparator), min(java.util.Comparator), wait(), dump(), grep()
at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onMethodCall(SandboxInterceptor.java:153)
at org.kohsuke.groovy.sandbox.impl.Checker$1.call(Checker.java:161)
at org.kohsuke.groovy.sandbox.impl.Checker.checkedCall(Checker.java:165)
at com.cloudbees.groovy.cps.sandbox.SandboxInvoker.methodCall(SandboxInvoker.java:17)
at WorkflowScript.run(WorkflowScript:49)
at ___cps.transform___(Native Method)
at com.cloudbees.groovy.cps.impl.ContinuationGroup.methodCall(ContinuationGroup.java:86)
...

Answer 1

I am fighting the same problem--constructs in Groovy which I use all the time, like all the handy collection methods, are breaking.
It turns out that in a Jenkinsfile, the code is passed through a transformer called Groovy CPS which enables Jenkins to stop and restart execution of a stage. In the process, stuff gets changed from its original class, and this causes the kind of error you're seeing here, because you're mixing non-CPS code (the Java lib methods) with CPS code (anything inside the Jenkinsfile).
Here's the link to the documentation of this behavior...but I didn't find it very helpful, so I am reverting to doing stuff like I would in Java (pre-lambda) with lots of for loops :(

Answer 2

While it’s not as handy as locally defined closures, you can, in some cases, define methods in the global scope and then rely on them via the Groovy method pointer operator.
void doSomething(final String key, final String value) {
println "${key} → ${value}"
}
pipeline {
stages {
stage('Process data') {
steps {
script {
final Map myData = [
"foo": "bar",
"plop": "yo",
]
myData.each(this.&doSomething)
}
}
}
}
}