初学者Java不需要构造函数上的参数错误 [英] Beginner Java required no arguments error on constructor

问题描述

因此，作为一个类的作业，我试图制作一个简单的圆形类，然后将其用作构造函数。
这是我的课程代码：

So as an assignment for a class i'm trying to make a simple circle class and then use it as an constructor. Here is my class code:

public class Circle
{
  private double radius;
  private double pi;

  public void setRadius(double rad)
  {
    radius = rad;
  }

  public double getRadius()
  {
    return radius;
  }

  public double getArea()
  {
    return pi * radius * radius;
  }

  public double getDiameter()
  {
    return radius * 2;
  }

  public double getCircumference()
  {
    return 2 * pi * radius;
  }
}

这里是使用该类构造构造函数的程序：

Here's the program that uses the class to make a constructor:

import java.util.Scanner; //scanner class for input

  public class CircleDemo
{
  public static void main(String[]args)
  {
    double radiusIn; //gets radius from input

    Scanner keyboard=new Scanner(System.in); //activates scanner class in program

    System.out.print("Enter the radius of a circle: ");
    radiusIn=keyboard.nextDouble();

    Circle circularObject= new Circle(radiusIn);

    System.out.println("The circle's area is" + circularObject.getArea());
    System.out.println("The circle's diameter is" + circularObject.getDiameter());
    System.out.println("The circle's circumference is" + circularObject.getCircumference());
  }
}

，我得到错误：
Error ：类Circle中的构造函数Circle不能应用于给定类型；
必需：无参数
：双
原因：实际参数和正式参数列表的长度不同

and i get the error: Error: constructor Circle in class Circle cannot be applied to given types; required: no arguments found: double reason: actual and formal argument lists differ in length

我什么都没看到我的代码有误，但随后我再次使用了老师给我们的示例。

I don't see anything wrong with my code, but then again i'm using the sample that my teacher gave us.

推荐答案

您正在调用构造函数这里

You are calling the constructor here

Circle circularObject= new Circle(radiusIn);

但是您的 Circle 类，该类与您要传入的参数匹配。返回您的 Circle 类，并定义一个构造函数，该构造函数使用 double 参数：

But there is no constructor defined in your Circle class which matches the argument you are passing in. Return to your Circle class and define the constructor which takes a double argument:

public Circle(double val)
{
    //Implementation
}

请注意，如果没有为类提供显式构造函数，Java会自动为您提供具有空实现，无参数的公共构造函数供您使用。一旦定义了第一个显式构造函数，该默认构造函数就消失了，这意味着您必须手动重新定义它。

Note that if you do not provide an explicit constructor for a class, Java will automatically provide you with an empty-implementation, no argument, public constructor for you to use. Once you define your first explicit constructor, that default constructor is gone, meaning you would have to re-define it manually.

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