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Scala Var and Val (Variables and Constants)Create variables and constants with var and val. Fix the reassignment to val error.
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Var, variables. Time changes everything. A variable at one point in time equals 10. But then it is reassigned: it now equals 20. Its value is not constant.

Val, meanwhile, is a keyword that indicates constancy. A var can be reassigned (though it does not have to be). A val can never be. A val is something you can rely on.

First example. Let us use var in a simple Scala program. We bind the variable with identifier "animal" to the string literal "cat." Then were assign it (rebind it) to the string "frog."

Tip With this var, we can see the animal means different strings (cat and frog) at different points in time.

Scala program that uses var
// Assign variable to a string literal. var animal = "cat" println(animal) // Reassign the variable. animal = "frog" println(animal)
cat frog

Reassignment to val. This program does not work. We introduce the val keyword, but use it incorrectly. We cannot change a val to point to a different value—it can only be set once.

Scala program that uses val, causes error
val animal = "cat" println(animal) // This will not compile. animal = "frog" println(animal)
error: reassignment to val animal = "frog" ^ one error found

Separate val constants. This program uses val in a correct way. We rewrote our program to have two values, not a variable that is changed in the middle.

Scala program that uses val, separate values
// Use val for a value that cannot be reassigned. val color1 = "magenta" println(color1) // Use a separate val. val color2 = "aqua" println(color2)
magenta aqua

Val type. With val we can specify a type. Here we specify two numbers with initial values of 10. But we specify size2 to be a Double, not an Int. It is printed with a decimal place.

Scala program that uses val with type
// No type is specified, so Int is used. val size1 = 10 // Specify a Double type. val size2: Double = 10 // Print numbers. println(size1) println(size2)
10 10.0

Class fields. With var and val we specify the fields of a class. In Scala these are public. Fields can be accessed outside the class body by default.

Var We use var for width and height, as these can be mutated on Box instances. We assign them to 10 and 20.

Val This is a constant value. It is a String. We cannot assign it, but we can access it and print its value.

String

println

Scala program that uses class, var, val fields
class Box { var width: Int = 0 var height: Int = 0 final val name: String = "Box" } // Create a new Box instance. val box = new Box // Assign width and height var fields. box.width = 10 box.height = 20 // Print fields of box instance. println(box.width) println(box.height) println(box.name)
10 20 Box

A discussion. Imagine we are writing a complex program. A variable X is needed in two places. We reassign it, but make a mistake—the second X should be separate.

And With var, we might end up with a program that is invalid. With val, our program would not compile so we could correct it faster.

A summary. Immutable things are important in Scala. We have List, Map, Set: these are immutable data structures. With val, we have constants, immutable values—these improve program quality.

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