How do you define a variable in Kotlin that can only be assigned once?

In Kotlin, a variable that can only be assigned once is defined using the val keyword. When you declare a variable with val, it signifies that the variable is read-only and cannot be altered after its initial assignment. This is analogous to a final variable in some other programming languages, ensuring that once a value is set, it remains constant throughout the scope of the program.

Using val is particularly useful when you want to create immutable data that ensures the integrity of your application by preventing any unintended changes to the variable's value. This enhances code safety and clarity because developers can easily understand which variables are mutable and which are not by looking at the declarations.

In contrast, the var keyword is used for mutable variables that can be reassigned. The const keyword, while present in Kotlin, is specifically used for compile-time constant values, typically for primitive types and Strings that are known ahead of time. Meanwhile, the final keyword is not applicable in Kotlin; it is associated with Java and represents a variable that cannot be changed after it is initialized.