Values are Objects

In Scala all values are objects. An object is a grouping of data and operations on that data. For example, 2 is an object. The data is the integer 2, and the operations on that data are familiar operations like +, -, and so on. We call operations of an object the object's methods.

Method Calls

We interact with objects by calling or invoking methods. For example, we can get the uppercase version of a String by calling its toUpperCase method.

"Titan!".toUpperCase
// res0: String = "TITAN!"

Some methods accept parameters or arguments, which control how the method works. The take method, for example, takes characters from a String. We must pass a parameter to take to specify how many characters we want.

"Gilgamesh went abroad in the world".take(3)
// res1: String = "Gil"
"Gilgamesh went abroad in the world".take(9)
// res2: String = "Gilgamesh"

A method call is an expression, and thus evaluates to an object. This means we can chain method calls together to make more complex programs:

"Titan!".toUpperCase.toLowerCase
// res3: String = "titan!"

Some methods have more than one parameter. When this is the case we separate parameters with a comma. In the example below we use substring method, which copies a portion of a String. It has two parameters, which specify the start and end positions that we copy.

"Gilgamesh went abroad in the world".substring(10, 14)
// res4: String = "went"

Sometimes there are two or more methods that have the same name but a different number of parameters, or parameters of different types. We call these overloaded methods. For example, the substring method that we saw above has two overloads: one where we specify only where want to start the substring from, and one where we specify both the start and the end.

"Gilgamesh went abroad in the world".substring(10)
// res5: String = "went abroad in the world"
"Gilgamesh went abroad in the world".substring(10, 14)
// res6: String = "went"

anExpression.methodName(param1, ...)

anExpression.methodName

Operators

We have said that all values are objects, and we call methods with the syntax object.methodName(parameter). How then do we explain expressions like 1 + 2?

In Scala, any expression written a.b(c) can be written a b c. So these are equivalent:

1 + 2
// res7: Int = 3
1.+(2)
// res8: Int = 3

This first way of calling a method is known as operator style. It is considered good style to only use operator style when the method is widely understand to be written that way. For example, we write 1 + 2 because everyone understands that notation better than 1.+(2). However, it is bad style to write "Gilgamesh" substring 4 instead of "Gilgamesh".substring(4) as substring is not widely known to be an operator.