I am getting an error that constructor "cannot be applied to given types"

Question

Given this code:

Main.java

class Main {

public static void main(String[] args) {

SpecificThing mine = new SpecificThing();

System.out.println(mine.double_my_number());

}

}

Anything.java

public class Anything {

private int my_number;

public Anything(int a) {

my_number = a;

}

public int get_my_number() {

return my_number;

}

}

SpecificThing.java

public class SpecificThing extends Anything {

public int double_my_number() {

return get_my_number() * 2;

}

}

The editor highlights SpecificThing with the error:

Implicit constructor Anything() is undefined for default constructor. Must define explicit constructor

and the compiler reports the following error:

SpecificThing.java:1: error: constructor Anything in class Anything cannot be applied to given types;

public class SpecificThing extends Anything {

^

required: int

found: no arguments

reason: actual and formal argument lists differ in length

What does that all mean and how do I fix my code?

Answer 1

In Java, when no constructor is defined in a class, the default no-argument constructor is generated. When a class is a subclass, the generated default constructor involves calling the no-argument constructor of the superclass. In this case, however, the superclass (Anything) does not have a no-argument constructor (a.k.a. "Implicit constructor Anything()") because it only defines a constructor that takes an argument. Therefore, Java cannot generate the default no-argument constructor for the SpecificThing class.

One way to fix the code is to define a no-argument constructor in either the Anything class or the SpecificThing class. For instance, in the Anything class, add the following code:

public Anything() {

my_number = 17;

}

In the SpecificThing class, the constructor would need to call the superclass constructor that takes an argument, e.g.:

public SpecificThing() {

super(17);

}

Another way to fix the code is to define a constructor in the SpecificThing class that takes an argument and calls the superclass constructor, and then change the code in main to use that constructor instead, as shown below.

In the SpecificThing class, add the following code:

public SpecificThing(int s) {

super(s);

}

In the main method in Main.java, change the first line to be:

SpecificThing mine = new SpecificThing(17);

Answer 2

default constructor SpecificThing() is looking for default constructor Anything() in class Anything, but Anything() is missing, so it failed on new SpecificThing();

To fix this, just need add default constructor Anything() to class Anything, without parameter list.