Why there are 2 classes in this code?

If you add two classes like this public class Solution { ......................... } public class Cat { ......................... } in one file (i.e solutions.java), it won't work.

i see, ya i hope too.. what's the difference then between what you've showed and what they made in this challenge?

Not defined in one file: //Not Nested public class Solution { ......................... } public class Cat { ......................... } Defined in one file: //Nested public class Solution { ......................... public class Cat { ......................... } }

You can add multiple, un-nested, classes in the same file. I don't know all the rules for this, but only the top class may be public (top meaning the name of the file, not the position). Take this for example:

public class Solution {}

class Cat {} // good

class Dog{} // good

class Human{} // good

public class CantDoThis{} // this one will be 'red squiggly' because the
// top class, Solution, can be the only public class. In this case
// you would have to remove 'public' or put it in it's own file

It's generally bad practice, however, to put multiple classes in the same file like this unless there is a specific reason to do so.

Ohh! I didn't know that....Nice

From Stackoverflow: "Because one .java file can contain only one public class." If we do this, can we create a Human object in Solution class or any other class in the same package?

If you used the code that I linked above, any of the classes can create objects of each other within that same 'Solution' file. So the Human class can make new Cat/Dog/Solution objects (and the same applies to all the other classes). If you make a new file, then the code in that file will only be able to create new Solution objects, but not Cat, Human, Dog. You can create code in the Solution class like this:

public static Human getHuman(){
   return new Human();
}

And now code in your seperate class file can declare a Human object and get a new object by calling that code (i called my seperate file 'test'):

public class test {
    public static void main(String[] args) {
        Human human = Solution.getHuman();
    }
}

so in the last example, the human object can access the human,cat,dog classes even from another file? also what's the purpose of having multiple nested classes in 1 file instead of on separated files ?

No, not unless there was code in the Human class for that (just like I added code in the Solution class to get a Human). The reason to use nested classes is a little more of a complex subject. One reason, off the top of my head, is that classes can only inherit from only one other class. This can be restricting when you want to create your own class that has the functionality of more than one class, like making a custom Thread and Comparator class hybrid. However you can inherit one, then nest class that inherits the other, then use an object of that nested class to provide the desired functionality. If that made no sense to you, I did say that it was more of an advanced topic. You will be introduced to it in the multithreading quest line.