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Question about the task

Creating cats

Java Syntax, Level 5, Lesson 9

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it says that this constructor already exists or something like that...

Create the Cat class with five constructors:
- Name,
- Name, weight, age
- Name, age (standard weight)
- Weight, color (name, address, and age are unknown; the cat is homeless)
- Weight, color, address (someone else's pet)

The constructor's job is to make the object valid.
For example, if the weight is unknown, then you need to specify some average weight.
A cat can't weigh nothing.
The same applies to age and color. But there may or may not be a name (i.e. name might be null).
The same applies to the address (it might be null).

Requirements:

The Cat class must have a String variable name.
The Cat class must have an int variable age.
The Cat class must have an int variable weight.
The Cat class must have a String variable address.
The Cat class must have a String variable color.
The class must have a constructor that takes a name as an argument, but initializes all variables except the address.
The class must have a constructor that takes a name, weight, and age as arguments, and initializes all variables except the address.
The class must have a constructor that takes a name and age as arguments, and initializes all variables except the address.
The class must have a constructor that takes a weight and color as arguments, and initializes all variables except the name and address.
The class must have a constructor that takes a weight, color, and address as arguments, and initializes all variables except the name.

package com.codegym.task.task05.task0517; /* Creating cats */ public class Cat { public String name = null; public String address = null; public String color; public int age, weight; public Cat(String name) { this.name = name; this.age = 1; this.weight = 2; this.color = "black"; } public Cat(String name, int weight, int age) { this.name = name; this.age = age; this.weight = weight; this.color = "black"; } public Cat(String name, int age) { this.name = name; this.age = age; this.weight = 2; this.color = "black"; } public Cat(String color, int weight) { this.age = 1; this.weight = weight; this.color = color; } public Cat(String color, int weight, String address) { this.age = 1; this.weight = weight; this.color = color; this.address = address; } public static void main(String[] args) { } }

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Karolis Garšva

Level 16 , Gravesend, United Kingdom

14 June 2021, 21:05

Oh, so the sequence I write arguments also maters... I get it. Thank you

Guadalupe Gagnon

Level 37 , Tampa, United States

14 June 2021, 20:21

public Cat(String name, int age) {
    this.name = name;
    this.age = age;
    this.weight = 2;
    this.color = "black";
}

public Cat(String color, int weight) {
    this.age = 1;
    this.weight = weight;
    this.color = color;
}

These are duplicate constructors. Each of those constructors has String and int as the parameter types. But even if the computer knew the difference between String name - String color and int age - int weight; it still could be very confusing. Just take, for example, if you created a new cat like this:

Cat test = new Cat("Ruby", 9);

What constructor would that use??? The (color, weight) one or the (name, age) one?

Yusuf el Turki

Level 5 , Rhein, Germany

15 June 2021, 03:29

Actually i did pass this task. But after that i saw this question i am confused. How can a constructor choose which parameter should choose for example is "Ruby" name or color? Can you please explain 😀

Guadalupe Gagnon

Level 37 , Tampa, United States

15 June 2021, 14:00

What you name the parameters matters not to the computer. All it looks at is the type. So, using all the constructors in the code above and abstracting out the code, this is what the computer sees:

public Cat(String)
public Cat(String, int, int)
public Cat(String, int)
public Cat(String, int)
public Cat(String, int, String)

You can see that line 3 and 4 are exactly the same. That is the way the computer sees it. It doesn't matter what you name the variables:

public Cat(String anyString, int anythingBut5)
public Cat(String anyString, int mustBe5)

The computer can't read English (or French, German, Polish, Chinese, Spanish, ect). It wont know that if you create a cat with the number 5 as the second argument that it should use the constructor with the name "mustBe5". It will only call the constructor where the argument types you use match the parameter types of the constructor (this works the same for methods). If two have the same types it won't know which to correctly call. The rule is that no two constructors can have parameter types exactly matching another constructor. This rule applies for methods two when you have at least two methods with the same name. So, some abstract examples:

/* this is illegal code as the parameter types are the same*/
public Cat(String, int)
public Cat(String, int)

/*method example of illegal code for same reason*/
public void myMethod(String, int)
public void myMethod(String, int)

/* this is legal code as the parameter types now are not an exact match*/
public Cat(String, int)
public Cat(int, String)

/*method example of legal code for same reason*/
public void myMethod(String, int)
public void myMethod(int, String)