how to make a constructor in flutter code example

Example 1: constructor flutter

class Student{
  Student(int enNum){
    print(enNum);
  }
}

main(){
  var myStudent = new Student(15);
}

Example 2: constructoers in flutter

Customer(this.name, this.age, this.location);

Customer.withoutLocation(this.name, this.age);

Customer.empty() {
  name = "";gd
  location = "";
}
cfgdfgdfgdfggdfgdfdfgdfgdggdfgdfgdf