Beginner Java required no arguments error on const

问题:

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;
  }
}

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());
  }
}

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

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

回答1:

You are calling the constructor here

Circle circularObject= new Circle(radiusIn);

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
}

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.

回答2:

You don't have a constructor in your circle class

public Circle (something here) {}

回答3:

Java provides a default constructor for every class you write. You cannot see it, but it is there and it will be called unless you write your own with any desired parameters.

回答4:

One of the most used implementations of a parametered constructor is like this:

double radius;
public Circle(double radius){
    this.radius = radius;
}

you do not need to have the same name:

double radius;
public Circle(double i){
    radius = i;
}

but the first version is considered best practice and should be the one used. This is of course valid for any form of paramatered constructors:

String name;
int age;
public Person(String name, int age){
    this.name = name;
    this.age = age;
}

回答5:

Please add the constructor for Circle. The modified code will be:

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

  Circle(double x) 
  {
    this.radius = x;
  }

  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;
  }
}