Use of def, val, and var in scala

Question

class Person(val name:String,var age:Int )
def person = new Person("Kumar",12)
person.age = 20
println(person.age)

These lines of code outputs 12, even though person.age=20 was successfully executed. I found that this happens because I used def in def person = new Person("Kumar",12). If I use var or val the output is 20. I understand the default is val in scala. This:

def age = 30
age = 45

...gives a compilation error because it is a val by default. Why do the first set of lines above not work properly, and yet also don't error?

Answer 1

There are three ways of defining things in Scala:

• def defines a method
• val defines a fixed value (which cannot be modified)
• var defines a variable (which can be modified)

Looking at your code:

def person = new Person("Kumar",12)

This defines a new method called person. You can call this method only without () because it is defined as parameterless method. For empty-paren method, you can call it with or without '()'. If you simply write:

person

then you are calling this method (and if you don't assign the return value, it will just be discarded). In this line of code:

person.age = 20

what happens is that you first call the person method, and on the return value (an instance of class Person) you are changing the age member variable.

And the last line:

println(person.age)

Here you are again calling the person method, which returns a new instance of class Person (with age set to 12). It's the same as this:

println(person().age)