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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.
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) Output 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) Output 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) Output 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) Output 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.

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( Output 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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