CodeGym/Help with Java Tasks/Can you explain what is wrong?

Level 8

Wietzendorf

22.01.2019
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10comments

Can you explain what is wrong?

Question about the task

Creating cats

Java Syntax, Level 5, Lesson 9

Resolved

What is wrong?

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 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. 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). */ public class Cat { //write your code here public String name; public int weight = 3; public int age = 5; public String color; public String address = null; public Cat(String name){ name = "bon"; this.weight = weight; this.age = age; this.color = color; } public Cat(String name, int weight, int age){ this.name = name; this.weight = weight; this.age = age; } public Cat(String name, int age){ name = "bob"; this.age = age; this.weight = weight; this.color = color; } public Cat(int weight, String color){ this.name = null; this.address = null; this.weight = weight; this.color = color; } public Cat(int weight, String color, String address){ this.weight = weight; this.color = color; this.address = address; } public static void main(String[] args) { } }

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Angeget

Level 22 , Potsdam, Germany

22 January 2019, 20:35

solution

1. Use this.name in the first constructor, to let the compiler identifies which name variable you mean. you do not need to use this for weight, age etc.. . because there are no parameters whith the same name as instances variable shadowing these variables. 2. For the requirement: The class must have a constructor that takes a name as an argument, but initializes all variables except the address public Cat(String name,){ this. name = name; weight = 3; age = 5; color = "red"; } intialize rather weight and age in constructor 3. For the requirement: The class must have a constructor that takes a name, weight, and age as arguments, and initializes all variables except the address. public Cat(String name, int weight, int age){ this(name) // to call the constructor Cat(name) weight = 4; age = 2; color = "blue"; } the requirement says, initialize all the variables except the address

Level 8 , Wietzendorf, Germany

23 January 2019, 11:13

Okay thank you very much. That helped me a lot ! Got the task done after your help and the help from Guadalupe and researching !

Guadalupe Gagnon

Level 37 , Tampa, United States

22 January 2019, 19:35

useful

Hi MP, So first off, the 'this' keyword only has two purposes (as i understand). First it helps the compiler understand which variable you refer to when the variables that are passed into the constructor have the same exact name as the class properties. Take this bit of your code as an example:

public Cat(int weight, String color){
   // this.name = null; not needed here as no local variable called 'name'
   // this.address = null; not needed here either, same reason.
   name = null;
   address = null;
   this.weight = weight; // needed here because local weight and Cat.weight have same name
   this.color = color;  // needed here, same reason
}

And second, the this keyword is also used to call a constructor from another constructor, for example if you had a constructor (name) and one with (name, age); you could do this:

public Cat(String name){
   this(name, 8);
}

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

Here the first constructor, the one with name, just passed name (with a default value of 8 to age, which i just came up with) to the next constructor to do its job. Now you don't need to write the same code essentially for both, just in the last constructor in line. You can chain multiple constructors together like this and save a lot of time from writing duplicate code.

Guadalupe Gagnon

Level 37 , Tampa, United States

22 January 2019, 19:36

useful

Let me know when you solve this and i'll share my solve with you

Guadalupe Gagnon

Level 37 , Tampa, United States

22 January 2019, 19:51

useful

I'll give you a correct version of the first constructor, assuming that chaining them together might not make sense yet (my original solve did not chain): So the first constructor has only 'name' passed in, that means that age, weight, color, and address are unknown. The requirements say that only name and address can be null, so all other values need to be some default that you decide:

public Cat(String name){
   this.name = name; // our one known value passed in
   age = 8;
   weight = 10;
   color = "Black";
   address = null;
}

copy this example with all the constructors and you should pass.

Level 8 , Wietzendorf, Germany

23 January 2019, 11:12

Thank you as always, after adjusting it with your tip and researching on google I manged to complete the task. I still don't get the whole class and constructor thing yet, I have to read a little more and try out more that will hopefully close the gap soon. But thank you for your effort ! It really helps me understanding! Couldn't I write it like I did in the Cat (String name) constructor? Wouldn't it still work? Afaik the compiler should take out the default value of age, weight, color from the Cat variables. But the task requirements were probably strict so I had to give those constructors value, right?

Level 8 , Wietzendorf, Germany

23 January 2019, 11:37

But I don't understand the this(name,8) code you wrote. I had to use it in another task I just had (thanks btw :D) but I don't understand why it has to start with this(..).

Angeget

Level 22 , Potsdam, Germany

23 January 2019, 11:49

As you did it before, it should have worked also, but according to the requirement, you should have initialize age, weight, and color in the constructor. Each declared instance variable has a default value. For int the def. value is 0 The constructor is there to initialize the data of your class

Angeget

Level 22 , Potsdam, Germany

23 January 2019, 11:50

I wrote this(name) and it just mean call the constructor Cat(name)

Angeget

Level 22 , Potsdam, Germany

23 January 2019, 11:59

I recommand you to reread this useful links: https://codegym.cc/groups/posts/33-getting-started-with-classes-writing-your-own-classes-constructors with constructors. I think this will be a great help