Inside a class, val
is evaluated on initialization while def
is evaluated only when, and every time, the function is called. In the code below you will see that x is evaluated the first time the object is used, but not again when the x member is accessed. In contrast, y is not evaluated when the object is instantiated, but is evaluated every time the member is accessed.
class A(a: Int) {
val x = { println("x is set to something"); a }
def y = { println("y is set to something"); a }
}
// Prints: x is set to something
val a = new A(1)
// Prints: "1"
println(a.x)
// Prints: "1"
println(a.x)
// Prints: "y is set to something" and "1"
println(a.y)
// Prints: "y is set to something" and "1"
println(a.y)