Is it Ok to put many classes in one file ?

Hi all,
This is another beginner question. I was wondering, is it good practice to put many classes in one single file? Sometimes I notice there are many classes inside one big file and sometimes each class is in its own separate file. Is there an advantage in separating each class in each different file? For e.g. one big file, called A.java:
class A {
public static void main(String[] args) { }
class B {}
class C {}
class D {}Is it better to have class B in a B.java file, and class C in C.java, etc? In the real-world workplace, what is the usual practice?
Thank you for your replies.
P/S : I love Java, it's so cool. :-)

I have also seen situations, in large software systems, where a compile that fails partway through will leave the top level classes that do not match the file name unbuilt, and some compilers will then not be able to build, because they cannot map the class name to the source file.
? {?

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Is it Ok to put many classes in one file ?

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