I run this script and got exception error

[Anamika Rai]

class Person {
String name
String toString() { name } 
}
def John = new Person(name:'John') 
def Ram = new Person(name:'Ram') 
def p = John 
def gs = "Name: ${p}" 
assert gs == 'Name: John' 
p = Ram 
assert gs == 'Name: John' 
john.name = 'Ram' 
assert gs == 'Name: Ram'

[PD Sheehan]

You defined John with a capital J

But you try to set john.name with a lowercase j