java的简单代码，但报错了求解释

Question

为什么代码会报错？

 import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        Fraction a = new Fraction(in.nextInt(), in.nextInt());
        Fraction b = new Fraction(in.nextInt(),in.nextInt());
        a.print();
        b.print();
        a.plus(b).print();
        a.multiply(b).plus(new Fraction(5,6)).print();
        a.print();
        b.print();
        in.close();
    }

    class Fraction{
         int a;
         int b;

         Fraction(int a,int b){
             this.a=a;
             this.b=b;
         }  

         Fraction plus(Fraction r) {
             Fraction obj=new Fraction(0,0);
             obj.a=this.a*r.b+r.a*this.b;
             obj.b=this.b*r.b;
             return obj ;
         }

         Fraction multiply(Fraction r){
             Fraction obj=new Fraction(0,0);
             obj.a=this.a*r.a;
             obj.b=this.b*r.b;           
             return obj;
         }

         void print() {
             int min=(this.a<this.b?this.a:this.b);
             for(int i=min;i>1;i--){
                 if(this.a%i==0&&this.b%i==0){
                     this.a=this.a/i;
                     this.b=this.b/i;
                     break;
                 }
             }
             if(a==1&&b==1)  System.out.println("1");
             else System.out.println(this.a+"/"+this.b);
         }
    }
}

错误：No enclosing instance of type Main is accessible. Must qualify the allocation with an enclosing instance of type Main (e.g. x.new A() where x is an instance of Main).

Answer 1

你把Fraction定义成Main的内部类了，而内部类的实例化必须依赖于外部类的。
试试把它挪到Main类的外面。