我在尋找哪些構造函數和訪問修飾符的組合？

Question

我有一個用戶定義的類Fraction其中包含幾個構造函數和成員方法。我想創建一個獨立的「獨立函數」類，它使用Fraction的兩個實例作爲參數來創建第三個Fraction或者修改並返回它傳遞的實例之一。

class MyFractionProject{ 
public static void main(String[] args){ 
    class Fraction{ 
     private int numerator; 
     private int denominator; 
     public Fraction(){ 
      numerator = 0; 
      denominator = 1; 
     }//default constructor 

     public Fraction (int num, int denom){ 
      numerator = num; 
      denominator = denom; 
     }//sample constructor 

     //other constructors 


     //accessors and mutators (setters and getters) are here 

     public Fraction multiply(Fraction otherFraction){ 
      Fraction result = new Fraction(//multiply the Fractions); 
      return result; 
     }//sample member method, calls a constructor and accessor 
     //a bunch of other member methods are here 

    }//end Fraction 

    //New standalone utility class goes here: 
    class FractionUtility{ 
     //suite of ~5 functions which take two Fraction objects as arguments 
     //and return a Fraction object, either one of the arguments or a new 
     //instance 
     public static FractionUtility UtilityMultiply(Fraction fr1, Fraction fr2){ 
      //lots of stuff 
     }//Does this HAVE to return a FractionUtility, or can it return a Fraction? 
     //Can I declare variables outside the static FractionUtility methods if they 
     //will be needed every time the method is called? Will they be accessible 
     //inside the static methods? Do I need to instead declare them inside every 
     //static method so that they're visible each time I call the method? 
    }//end FractionUtility 

    //bunch of other stuff in main(), successfully uses Fraction but not 
    //FractionUtility 

}//end main() 
} 

Utility類別需要與Fraction正文分開定義。我需要有幾個不同的分數實例，但從不需要實例化FractionUtility。這使得它看起來像是一個靜態類會起作用，但是當我這樣做時會拋出錯誤 - 通常不能從靜態上下文訪問非靜態分數變量。

我可以看到在main（）之外定義兩個類然後導入它們是有意義的，但我不知道如何去做，或者如果我這樣做了什麼規則。

Answer 1

好吧，你似乎只想在同一個文件中聲明幾個不相關的類。

這就是靜態內部類的用途。

例如，你可以這樣做：

public class Something { 

static class MyClass { 
    private int data = 0; 

    public MyClass() { 
    } 
    public MyClass(int data) { 
     this.data = data; 
    } 

    public MyClass makeNew(MyClass otherinstance) { 
     MyClass result = new MyClass(this.data + otherinstance.data); 
     return result; 
    } 
} 


static class MyUtilityClass { 
} 

public static void main(String[] args) { 
    MyClass myClass = new MyClass(); 
    MyClass copy = myClass.makeNew(new MyClass()); 
    MyUtilityClass utilityClass = new MyUtilityClass(); 
} 
}