Jenkins pipeline groovy script to get user and pass of an existing Jenkins credential

jenkins-pipeline-script-getuserpass

pipeline {
    agent none
    stages {
        stage('First') {
            steps{
                script{
                    withCredentials([usernamePassword(credentialsId: 'credentialId', usernameVariable: 'username', passwordVariable: 'token')]) {
                        print 'token.collect { it }=' + token.collect { it }
                        print 'username.collect { it }=' + username.collect { it }
                    }
                }
            }
        }
    }
}