scala class constructor parameters

Question

What's the difference between:

class Person(name: String, age: Int) {   def say = "My name is " + name + ", age " + age } 

and

class Person(val name: String, val age: Int) {    def say = "My name is " + name + ", age " + age } 

Can I declare parameters as vars, and change their values later? For instance,

class Person(var name: String, var age: Int) {    age = happyBirthday(5)    def happyBirthday(n: Int) {     println("happy " + n + " birthday")     n   } } 

Answer 1

For the first part the answer is scope:

scala> class Person(name: String, age: Int) {      |   def say = "My name is " + name + ", age " + age      | }  scala> val x = new Person("Hitman", 40)  scala> x.name <console>:10: error: value name is not a member of Person               x.name 

If you prefix parameters with val, var they will be visible from outside of class, otherwise, they will be private, as you can see in code above.

And yes, you can change value of the var, just like usually.

Answer 2

This

class Person(val name: String, val age: Int) 

makes the fields available externally to users of the class e.g. you can later do

val p = new Person("Bob", 23) val n = p.name 

If you specify the args as var, then the scoping is the same as for val, but the fields are mutable.