Java Multiple class declarations in one file

Question

In Java  you can define multiple top level classes in a single file  providing that at most one of these is public  see JLS   7 6    See below for example    Is there a tidy name for this technique  analogous to inner  nested  anonymous   The JLS says the system may enforce the restriction that these secondary classes can t be referred to by code in other compilation units of the package  e g   they can t be treated as package-private   Is that really something that changes between Java implementations    e g   PublicClass java   package com example multiple   public class PublicClass       PrivateImpl impl   new PrivateImpl       class PrivateImpl       int implementationData

User · Accepted Answer

My suggested name for this technique  including multiple top-level classes in a single source file  would be  mess   Seriously  I don t think it s a good idea - I d use a nested type in this situation instead  Then it s still easy to predict which source file it s in  I don t believe there s an official term for this approach though   As for whether this actually changes between implementations - I highly doubt it  but if you avoid doing it in the first place  you ll never need to care

User · Answer

I believe you simply call PrivateImpl what it is  a non-public top-level class  You can also declare non-public top-level interfaces as well   e g   elsewhere on SO  Non-public top-level class vs static nested class  As for changes in behavior between versions  there was this discussion about something that  worked perfectly  in 1 2 2  but stopped working in 1 4 in sun s forum  Java Compiler - unable to declare a non public top level classes in a file

User · Answer

You can have as many classes as you wish like this  public class Fun       Fun             System out println  Fun constructor              void fun             System out println  Fun mathod              public static void main String   args            Fun fu   new Fun            fu fun            Fen fe   new Fen            fe fen            Fin fi   new Fin            fi fin            Fon fo   new Fon            fo fon            Fan fa   new Fan            fa fan            fa run             class Fen       Fen             System out println  fen construuctor               void fen             System out println  Fen method             class Fin       void fin             System out println  Fin method             class Fon       void fon             System out println  Fon method              class Fan       void fan             System out println  Fan method              public void run             System out println  run

User · Answer

javac doesn t actively prohibit this  but it does have a limitation that pretty much means that you d never want to refer to a top-level class from another file unless it has the same name as the file it s in   Suppose you have two files  Foo java and Bar java   Foo java contains    public class Foo   Bar java contains    public class Bar class Baz   Let s also say that all of the classes are in the same package  and the files are in the same directory    What happens if Foo java refers to Baz but not Bar and we try to compile Foo java  The compilation fails with an error like this   Foo java 2  cannot find symbol symbol    class Baz location  class Foo   private Baz baz              1 error   This makes sense if you think about it  If Foo java refers to Baz  but there is no Baz java  or Baz class   how can javac know what source file to look in   If you instead tell javac to compile Foo java and Bar java at the same time  or even if you had previously compiled Bar java  leaving the Baz class where javac can find it  then this error goes away  This makes your build process feel very unreliable and flaky  however   Because the actual limitation  which is more like  don t refer to a top-level class from another file unless it has the same name as the file it s in or you re also referring to a class that s in that same file that s named the same thing as the file  is kind of hard to follow  people usually go with the much more straightforward  though stricter  convention of just putting one top-level class in each file  This is also better if you ever change your mind about whether a class should be public or not   Sometimes there really is a good reason why everybody does something in a particular way

User · Answer

Yes you can  with public static members on an outer public class  like so   public class Foo        public static class FooChild extends Z           String foo             public static class ZeeChild extends Z               and another file that references the above   public class Bar        public static void main String   args            Foo FooChild f   new Foo FooChild            System out println f              put them in the same folder  Compile with   javac folder   java   and run with    java -cp folder Bar

User · Answer

1 Is there a tidy name for this technique  analogous to inner  nested  anonymous     Multi-class single-file demo      2 The JLS says the system may enforce the restriction that these secondary classes can t be referred to by code in other compilation units of the package  e g   they can t be treated as package-private  Is that really something that changes between Java implementations    I m not aware of any which don t have that restriction - all the file based compilers won t allow you to refer to source code classes in files which are not named the same as the class name    if you compile a multi-class file  and put the classes on the class path  then any compiler will find them

User · Answer

According to Effective Java 2nd edition  Item 13        If a package-private top-level class  or interface  is used by only   one class  consider making the top-level class a private nested class   of the sole class that uses it  Item 22   This reduces its   accessibility from all the classes in its package to the one class   that uses it  But it is far more important to reduce the accessibility   of a gratuitously public class than a package-private top-level class            The nested class may be static or non-static based on whether the member class needs access to the enclosing instance  Item 22

User · Answer

Just FYI  if you are using Java 11   there is an exception to this rule  if you run your java file directly  without compilation   In this mode  there is no restriction on a single public class per file  However  the class with the main method must be the first one in the file

User · Answer

No  You can t  But it is very possible in Scala   class Foo  val bar    a   class Bar  val foo    b

[java] Java: Multiple class declarations in one file

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