const vs val

const

The const keyword is used to declare properties that are immutable (i.e. read-only properties) at the compile time.

When to use const, required conditions as shown below:

1. Should start from a primitive data type
   ex) Boolean, Int, Long, Char, String
2. Must be at the top level or member of an object or a companion object
3. No custom getter method

class Task (_name: String, _priority: Int = DEFAULT_PRIORITY) {
    companion object {
        const val MIN_PRIORITY = 1
        const val DEFAULT_PRIORITY = 3
        const val MAX_PRIORITY = 5
        const val NAME = "KHAZIX"
        const val TASK_INDEX_NAMES = Array<String>(5){i->"task $i"} // error occurs
    }
}

This code must lead to following error:

Const 'val' has type 'Array<String>'. 
    Only primitives and String are allowed

val

Initialized at runtime. As a result, you may assign a val variable a method or class

class Task(_name: String, _priority: Int = DEFAULT_PRIORITY){
    companion object {
        const val MIN_PRIORITY = 1
        const val DEFAULT_PRIORITY = 3
        const val MAX_PRIORITY = 5
        val currentTimeStamp: LocalDateTime
            get() = LocalDateTime.now()
    }
    
    private fun setNameWithTimestamp(_name : String) = _name + "_" + currentTimeStamp
    private fun validPriority(p: Int) = p.coerceIn(MIN_PRIORITY, MAX_PRIORITY)
}

currentTimestamp declared with val is allocated in runtime. You are able to declare custom get method.

Example code

class Task(_name: String, _priority: Int = DEFAULT_PRIORITY){
    companion object {
        const val MIN_PRIORITY = 1
        const val DEFAULT_PRIORITY = 3
        const val MAX_PRIORITY = 5
        val currentTimeStamp: LocalDateTime
            get() = LocalDateTime.now()
    }
    var currentPriority = validPriority(_priority)
        set(value) {
            field = validPriority(value)
        }

    // custom setter
    var name = setNameWithTimestamp(_name)
        set(value) {
            field = setNameWithTimestamp(value)
        }

    private fun setNameWithTimestamp(_name : String) = _name + "_" + currentTimeStamp
    private fun validPriority(p: Int) = p.coerceIn(MIN_PRIORITY, MAX_PRIORITY)

}

📝Conclusion

Index	const	val
When to use	Variable value to remain constant	Control variable with custom method
Declaration time	Compile	Runtime
Data type	Primitive and String	All