Generic Classes
In this lesson, you will learn to use generic classes.
We'll cover the following
Generic Classes
Generic classes help restrict the type of values accepted by the class. These supplied values are known as the generic parameter(s).
In the following example, the class FreshProduce
is restricted to accept only the Product
data type. It is acceptable to use FreshProduce
without the <Product>
parameter as Dart will assume it to be of the type Product
. However, if any other data type other than the allowed type is passed, you’ll see a compile-time error.
The FreshProduce
Class
The FreshProduce
class is restricting the type of values that can be supplied to the class. It can only accept input of Product
type when T extends the Product
class. The Product
class is borrowed from previous lessons on generics.
class FreshProduce<T extends Product> {
FreshProduce(int i, double d, String s);
String toString() {
return "Instance of Type: ${T}";
}
}
Passing the Product
type
The FreshProduce
class only accepts the Product
type. Let’s create an instance spinach
of FreshProduce
. As a result, an instance of type Product
is created.
void main() {
//Using `Product` parameter accepted by FreshProduce class
FreshProduce<Product> spinach = FreshProduce<Product>(3, 3.99, "Spinach");
print(spinach.toString());
}
Output:
Instance of Type: Product
Not Passing Product
Type
When Product
is not passed to the FreshProduce
, it assumes that the data is of Product
type.
void main() {
//Passing no
FreshProduce spinach2 = FreshProduce(3, 3.99, "Spinach");
print(spinach2.toString());
}
Output:
Instance of Type: Product
Passing any other type
The FreshProduce
class cannot accept other data types. In the example below a compilation error is thrown when we try to pass an Object
to it.
void main() {
//This code will give compile time error complaining that Object is not of type Product
FreshProduce<Object> spinach3 = FreshProduce<Object>(3, 3.99, "Spinach");
print(spinach3.toString());
}
Output:
main.dart:34:35: Error: Type argument 'Object' doesn't conform to the bound 'Product' of the type variable 'T' on 'FreshProduce'.
- 'Object' is from 'dart:core'.
- 'Product' is from 'main.dart'.
Try changing type arguments so that they conform to the bounds.
FreshProduce<Object> spinach3 = FreshProduce<Object>(3, 3.99, "Spinach");
^
main.dart:16:20: Context: This is the type variable whose bound isn't conformed to.
class FreshProduce<T extends Product> {
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