Delegating to a Parameter
We'll cover the following
The problem
In the previous example, we wrote Worker
by JavaProgrammer()
, which says that the Manager
instance is delegating to an implicitly created instance of the JavaProgrammer
, but that poses two issues. First, the instances of Manager
class can only route to instances of JavaProgrammer
, not to instances of any other Worker
implementors. Second, an instance of Manager
doesn’t have access to the delegate; that is, if we were to write a method in the Manager
class, we can’t access the delegate from that method.
The solution
It’s easy to fix those limitations by tying the delegate to the parameter passed to the constructor instead of to an implicitly created instance.
// version5/project.kt
class Manager(val staff: Worker) : Worker by staff {
fun meeting() =
println("organizing meeting with ${staff.javaClass.simpleName}")
}
The constructor of the Manager
class receives a parameter named staff
which also serves as a property, due to val
in the declaration. If the val
is removed, staff
will still be a parameter, but not a property of the class. Irrespective of whether or not val
is used, the class can delegate to the parameter staff
.
In the meeting()
method of the Manager
class, we’re able to access staff
since it’s a property of the object. Calls to methods like work()
will go to staff due to delegation. Let’s confirm this behavior by creating a couple of instances of this version of Manager
.
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