CODEWITHSUNDEEP
kotlin
HelloCW
HelloCW

Reputation: 2255

Why must I declare a type annotation in a class in Kotlin?

The Code A works well, I think both Code B and Code C will work well too, but I failed, why?

In Code B and Code C, I think that the system can deduce the type of mDBHandle

Code A

class LogHandler(val mDBHandle:DBLogHandler=DBLogHandler()) {
}
class DBLogHandler() {
}

Code B

class LogHandler(val mDBHandle=DBLogHandler()) {
}
class DBLogHandler() {
}

Code C

class LogHandler(val mDBHandle:DBLogHandler()) {
}
class DBLogHandler() {
}

Upvotes: 0

Views: 63

Answers (1)

SpaceBison
SpaceBison

Reputation: 3131

Quoting the reference:

Function parameters are defined using Pascal notation, i.e. name: type. Parameters are separated using commas. Each parameter must be explicitly typed

This also applies to constructors.

Let's look at your examples:

class LogHandler(val mDBHandle:DBLogHandler=DBLogHandler())

Here you declare a read-only property of type DBLogHandler and a default value of a new instance (DBLogHandler()).

class LogHandler(val mDBHandle=DBLogHandler())

In this one the parameter (property) type declaration has been omitted, so it's incorrect.

class LogHandler(val mDBHandle:DBLogHandler())

Here it looks like the parameter type has been declared as DBLogHandler(), because the parentheses imply a function (or constructor) call. Therefore, it's not a valid declaration.

If you'd like to declare the mDBHandle property without a default value, you can do it like this:

class LogHandler(val mDBHandle:DBLogHandler)

Upvotes: 3

Related Questions