• dart不允许匿名类，你完全可以把它放在主体之外。

import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  MyWidget({Key? key}) : super(key: key);

  @override
  State<MyWidget> createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  void test() {
    Celular cellphoneOfRafa = Celular('Azul', 5, 0.500, 5.7);
    Celular cellphoneOfZe = Celular('Preto', 6, 0.700, 7.7);
    print(cellphoneOfRafa.toString());
    print(cellphoneOfZe.toString());
    double resultado = cellphoneOfRafa.valorDoCelular(1000);
    print(resultado);
  }

  @override
  Widget build(BuildContext context) {
    test();
    return Scaffold();
  }
}

class Celular {
  final String cor;
  final int qtdPros;
  final double tamanho;
  final double peso;

  Celular(this.cor, this.qtdPros, this.peso, this.tamanho);

  String toString() {
    return 'cor $cor, Quantidade de Processadores ${qtdPros}';
  }

  double valorDoCelular(double valor) {
    return valor * qtdPros;
  }
}