java内部类私有构造函数，public成员

Question

I'd like to know if this is the preferred way to design a class; if not what's wrong with it and what's a better option?

class Calculator {

   public Calculator(Input input) {...};
   CalculatorOutput output;

   class CalculatorOutput {
   private CalculatorOutput() {} //private constructor - so no one other than Calculator can instantiate this class
   Double someCalcStuff;
   List<Double> someOtherCalcStuff;
}

    public void calculate() {
    CalculatorOutput op = new CalculatorOutput();
    //..after this function populates the someCalcStuff & someOtherCalcStuff; it just returns an instance of CalculatorOutput
}

...
return op;

}

//usage

Calculator.CalculatorOutput result = new Calculator(input).calculate();
for (Double d: result.someOtherCalcStuff) {...}

The reason I have CalculatorOutput as an inner class of Calculator is b/c it's got no other purpose or context than returning the calculator's output. I have CalculatorOutput's members as public b/c I don't really see a point in making them private , and then accessible via a getter/setter...Since I don't expect this inner class to be inherited, or do any other functionality mentioned in this post: Why use getters and setters?

The user cannot instantiate the inner class (private constructor), so I don't see a real drawback in creating public accessible members of the inner class (CalculatorOutput)

Specific questions:

• Is this approach bad? why?
• What's a better approach to achieve this?

Answer 1

To make the code cleaner, what I would do is create a Calculator package and add the following classes into that package:

1. Calculator
2. CalculatorOutput

And I would make the following changes: (make members private and provide getters and setters where needed)

class Calculator {
    private CalculatorOutput output;

    public Calculator(Input input) {/* logic here */} 

     // getters and setters
}

class CalculatorOutput {

    private Double someCalcStuff;

    private List<Double> someOtherCalcStuff;

    // getters and setters

    CalculatorOutput() {} 
}

public class Test{

    public static void main(String [] args)
    {
        Calculator calc = new Calculator();

        CalculatorOutput output = new CalculatorOutput();

        // use methods as needed
    }
}