DART somo la 28: Named constructor na constant constructor kwenye OOP

Nini maana ya named constructor

Katika lugha nyingi za kompyuta unaweza kuwa na constructor zaidi ya moja  zenye jina moja kwenye class. Lakini kwenye Dart huwezi kufanya hivi. Ili uweze kutegeneza constructor zaidi ya oja zenye jina moja utatumia named constructor.

Named constructor ni constructor ambayo ina jina sawa na constructor iliyotangulia hivyo inaongezewa keyword namedConstructor.. Kwa mfano hapa nitaleta constructor mbili zenye jina moja.

class Gari {

 //costructor

 Gari() {

 }

 //named constructor

 Gari.namedConstructor(){

 }

}

Hapo kuna construct mbili ambazo zote zinatumia jina moja Gari. Na wakati wa kutengeneza object utatumia hivyo hivyo namedConstructor keyword. Angalia ,fano hapo chini:-

class Gari {

 //costructor

 Gari() {

 print("Tunauza gari");

 }

 //named constructor

 Gari.namedConstructor(){

   print("Gari letu ni jipya");

 }

}

void main(){

Gari();

Gari.namedConstructor();

}

constructor zote zinaweza kushirikiaan properties. Kwa mfano angalia hapo chini nakwenda kutumia properties kwenye constructor zote.

class Gari {

 String? jina;

 String? rangi;

 int? matairi;

 // Constructor

 Gari(this.jina, this.matairi, this.rangi) {

   print("Tunauza gari aina ya ${jina}, ina rangi ${rangi} na matairi ${matairi}");

 }

 // Named constructor

 Gari.namedConstructor(this.jina, this.rangi, this.matairi) {

   print("Gari mpya inaitwa ${jina}, ina idadi ya matairi $matairi na ina rangi $rangi");

 }

}

void main() {

 Gari("oyota Corolla", 6, "Nyekundu");

 Gari.namedConstructor("Toyota Avalon", "nyeusi", 4);

}

Pia constructor method na named constructor zinaweza kutumia method moja kwa kushirikiana. Pia unaweza kuchanganya na default parameter.Angalia mfano hapo chini. 

class gari {

 String? jina;

 String? rangi;

 int? matairi;

 gari(this.jina, this.rangi, this.matairi);

 // here Mobile() is a named constructor

 gari.namedConstructor(this.jina, this.rangi, [this.matairi = 4]);

 void tangazo() {

   print("Jina la gari ni: $jina.");

   print("Rangi ya gar ni: $rangi.");

   print("Idadi ya matairi ni: $matairi");

 }

}

void main() {

 var gari1 = gari("Toyota", "Nyusi", 6);

 gari1.tangazo();

 print(" ");

 var gari2 = gari.namedConstructor...