Why doesn't Kotlin perform automatic type-casting?

Question

var a : Double
a = Math.sin(10) // error: the integer literal does not conform to the expected type Double
a = Math.sin(10.0) //This compiles successfully
println(a)         


        

Answer 1

Automatic type casting for numeric types can lead to losing precision. Just consider following java code:

double hoursSinceUnixEra = System.currentTimeMillis()/1000/60/60;

The intention was not to cut the result to full hours, although it compiles without any warning in Java.

val hoursSinceUnixEra = System.currentTimeMillis()/1000/60/60;
someObject.doubleValue = hoursSinceUnixEra

Above Kotlin code won't compile due to unexplicit casting.

Issue of this type can be very hard to find and fix and it's the reason behind this decision. You can still explicitly convert type:

val value = 3
Math.sin(value.toDouble())