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kotlinvariablesvariable-assignmentreadonlyval

In Kotlin, Why can the value of a val integer be reassigned by the inc() method?

Consider the following,

val x: Int = 0

val variables cannot be changed so doing x += 1 wouldn't work

The compiler says Val cannot be reassigned

why then does x.inc() work fine

doesn't x.inc() reassign the value from 0 to 1

Solution

  • x.inc() does not increment the variable x. Instead, it returns a value that is one more than the value of x. It does not change x.

    val x = 0
x.inc() // 1 is retuned, but discarded here
print(x) // still 0!

    As its documentation says:

    Returns this value incremented by one.

    That might seem like a very useless method. Well, this is what Kotlin uses to implement operator overloading for the postfix/prefix ++ operator.

    When you do a++, for example, the following happens:

    • Store the initial value of a to a temporary storage a0.
    • Assign the result of a0.inc() to a.
    • Return a0 as the result of the expression.

    Here you can see how inc's return value is used.