在主 function - dart 中调用 class 方法

Question

I am beginner in dart and I am currently learning about classes and methods. I just want call the method I have created in the interest class to the main function so that it performs the define operations. I don't want to implement any functions or variables in main. I don't know if it is possible I am trying out something; that probably when you want hide some business logic from the public. please help me. I believe there is but I just can't find it.

import 'dart:io';

class Interest {
double principal;
double rate;
int time;

Interest(double aPrincipal, double aRate, int aTime) {
 this.principal = aPrincipal;
 this.rate = aRate;
 this.time = aTime;
}

double interestAmnt() {
 print(
     "This is a program to Calculate the over all amount back for your loan");
 print("What is the Amount of loan you have taken: ");
 this.principal = double.parse(stdin.readLineSync());
 print("What is the interest on your loan: ");
 this.rate = double.parse(stdin.readLineSync());
 print("How long are paying for the long: ");
 this.time = int.parse(stdin.readLineSync());

 return this.principal * (1 + (this.rate * this.time));
}
}

main(List<String> args) {

}

Answer 1

make your function static, like this:

static double interestAmnt()

And in your main, call your function like this:

main(List<String> args) {
  Interest.interestAmnt();
}

Answer 2

There are many ways to do what you like. Dart support top level function but since you prefer creating an object that holds its own business logic, I modified your example to work with some comments:

import 'dart:io';

void main(List<String> args) {
  // you should get your variables here, since it's not the responsibility of the Interest class 
  print('This is a program to Calculate the over all amount back for your loan');
  print('What is the Amount of loan you have taken: ');
  final principal = double.parse(stdin.readLineSync());
  print('What is the interest on your loan: ');
  final rate = double.parse(stdin.readLineSync());
  print('How long are paying for the long: ');
  final time = int.parse(stdin.readLineSync());

  // Now you've all the variables needed to create an interest object, 
  // so create the object which has all the business logic
  final interest = Interest(principal, rate, time);

  // do operations on the created object
  print(interest.interestAmnt());
}

// this is an interest object with one constructor, three class members, and one method.
class Interest {
  // mark these as final since they'll not change in the lifetime of the object
  final double principal;
  final double rate;
  final int time;

  // unlike some other languages, you can do it this way in dart:
  Interest(this.principal, this.rate, this.time);
  // constructor body isn't necessary in this case since we are taking the variables as is
  // take a look at the dart language tour to understand:
  // - how constructors work,
  // - initializer list
  // - constructor body
  // each has its own use case but for your case, this works fine

  // this is a class method that can has access to the instance variables
  // previously, you had this placed inside the constructor body and so it 
  // so it was inaccessible 
  double interestAmnt() {
    return principal * (1 + (rate * time));
  }
}

The main problem in your example you were defining functions in the constructor body that were not accessible by the class itself. A lot of these terms may sound new to you, but overtime you'll get used to it.

Here's a link for the language tour where you can learn more about Dart: https://dart.dev/guides/language/language-tour

Answer 3

I was able to solve the issue with the guidance above but I still believe there is another way out. thanks a lot guys; Thanks very much!

// interest rate

import 'dart:io';

class Interest {
  static double principal;
  static double rate;
  static double time;

  

  static double interestAmnt() {
    print(
        "This is a program to Calculate the over all amount back for your loan");
    print("What is the Amount of loan you have taken: ");
    principal = double.parse(stdin.readLineSync());
    print("What is the interest on your loan: ");
    rate = double.parse(stdin.readLineSync());
    print("How long are paying for the long: ");
    time = double.parse(stdin.readLineSync());
    double res = principal * (1 + (rate * time));
    print(res);

    return res;
  }
}

main(List<String> args) {
  Interest.interestAmnt();
 
}