Inner class standards

Hi This is kind of a trivial question.
I know that in Sun's coding standards doc
see below
http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html
a class name is supposed to be Capitalized,
now what about an inner class in no where it says an inner class should or should not be capitalized...

Given that the Naming Standards you pointed to states nothing about Inner nor Outer classes, but only gives the standard for Classes in general, I would make the inference that both Inner and Outer classes should be capitalized just on this information alone.
However, the ability to differentiate between a member variable and an inner class, as was demonstrated above, is further reason to follow the same convention for Inner as for Outer classes.

Similar Messages

General class and setting properties vs specific inner class

This is a general question for discussion. I am curious to know how people differentiate between instantiating a standard class and making property adjustments within a method versus defining a new inner class with the property adjustments defined within.
For example, when laying out a screen do you:
- instantiate all the objects, set properties, and define the layout all within a method
- create an inner class that defines the details of the layout (may reference other inner classes if complex) that is then just instantiated within a method
- use some combination of the two depending on size and complexity.
- use some other strategy
Obviously, by breaking the work up into smaller classes you are simplifying the structure since each class is taking on less responsibility, as well as hiding the details of the implementaion from higher level classes. On the other hand, if you are just instantiating an object and making some SET calls is creating an inner class overkill.
Is there a general consensus for an approach? I am curious to hear the approach of others.

it's depends on your design..
usually, if the application is simple and is not expected to be maintain (update..etc..) than I just have all the building of the gui within the same class (usually..the main class that extends JFrame).
if the application follows the MVC pattern, than I would have a seperate class that build the GUI for a particular View. I would create another class to handle the ActionEvent, and other event (Controller)
I rarely use inner class...and only use them to implements the Listerner interface (but only for simple application)..

State pattern and inner classes

Hi,
I'm implementing the standard state design pattern. I have an object and that object delegates certain operations to various state classes. My problem is the state classes may need private information of the context class. So I can think of a few designs:
* make the private info of the context class default so the state classes can access them
* make appropriate getters for the private info (so again the state classes can access them
* pass the private info to the state classes
* make the state classes inner classes so the state classes have access to private info with any change
I would think the inner class solution is best. any comments? thanks.

I prefer inner classes for state objects.
But if they become very big, maintenance can be an issue and other approaches can be more appropriate.
JCG

Problems javadoc inner classes...

Hi all,
I have a problems documenting an inner class. I've implemented it this way:
* In file TestClass.java I have an inner class "class MainClass" (this is the "upper" class of the file)
* MainClass instantiates a TestClass (which in the file is declared as an inner class within MainClass: "public class TestClass").
When I try to document it I get the following error message:
"No public or protected classes found to document."
What do I have to do? I'm using "Standard Doclet version 1.4.1" and why I have to use the inner class is a matter of saving a copy of the entire object, and I think this is the only solution to it, so what do I do?? Do I declare the "outer" class in a different way?
Very pleased for all help!!
Best regards,
Anjelica

* In file TestClass.java I have an inner class "class
MainClass" (this is the "upper" class of the file)
* MainClass instantiates a TestClass (which in the
file is declared as an inner class within MainClass:
"public class TestClass").So TestClass is an inner class of MainClass?? Then your file should be called MainClass.java.
Try if that helps.

Creating Private Inner Classes in Separate Files

I sometimes find myself wanting to use private inner classes to do things, but then moving the classes to separate files and giving them package access just because I don't like having single large files.
Is there a way to create private inner classes on a class but just save them in another file?
Thanks,
John

For me, short file sizes usually make design structure
more clear. This can make maintenance easier. It can
also make browsing the code easier, even if you have a
good editor or IDE. It is also less intimadating
psychologically (for me, anyway) to work with a number
of small files, each one with a distinct purpose, than
it is to open up a monster, even if the monster does
represent a coherent design unit in some sense. I
think this psychological impact may be more important
than most people give it credit for.The psychological impact is lessened if you use an IDE like VisualAge (where only one method at a time is generally displayed) or use the "Show Source Of Selected Element Only" option in Eclipse.
It's one thing to say a method should be short and a class should have as few methods as possible. Those forces reduce complexity and ease maintenance. It's another to say a source file should be short. A source file is just a storage artifact; source code could be stored in a database without changing how the programmer interacts with it. The fact that the standard java compiler requires the implementation of nested classes to be stored inside the source file of their containing class is a minor inconvenience. Don't let it discourage you from using inner classes when they make sense. The design should not be driven by source file size considerations.
>
But you have added code only with the sole intent of
making a source file smaller. If Java had amechanism
for storing nested classes in other files youwouldn't
do this. My point below was that you shouldn't let
source file size override the decision to use anested
class.Why shouldn't I let it? There are plenty of
non-trivial benefits (the ones I gave above, for
starters) to working with smaller files.Because all of those benefits can be gained from using a decent IDE. Eclipse is free. It can show only the current method and it can collapse nested classes.
You say "If
Java had a mechanism...." Well, I could answer: It
does have such a mechanism, and that mechanism is
packages.Packages are not a mechanism for creating private inner classes in separate files. Eclipse has a mechanism for making the fact that they reside in the same source file a non-issue.
>>
I am not being cavalier. I have no argument, onlyan
opinion.Again, you are perfectly entitled to your opinion.
But if it is truly an opinion, and nothing more, why
bother telling me about it. You might as well post
your favorite color. It is the reasons for your
opinion that interest me, and you still have not
really given any.I have had lengthy arguments about the issue of method and class size. Like I said before, I prefer very small classes and methods. I also think the number of nested classes should be as small as possible. But I have no problem with large files. Files are just one way to organize source code. The size of the things in the files matters, not the files themselves.

JDev Compiler Error? - Compiling inner classes

This maybe just an interpretation of the language standard, but the compiler behavaviour is different from previous versions of JDeveloper and other Java IDE's, with no explanation found.
Using JDeveloper 9.02.829 the following error is received.
Error(13,9): class test$abc not found in class test.test
when compiling the following class
package test;
public class test
class abc { }
public test()
public void t ()
abc a = new abc();
The solution is to ensure the package name and class name are different (case sensitive) iff the class contains an inner class
Whilst this can be done it can pose a problem porting older code into this release of JDeveloper, whilst no problem exists with other IDE's or older versions of the JDevloper IDE.
The question is this a bug or a feature?

This is a bug that got fixed a while back. The bug is with
the fact that the class and the package have the same name.
The bug is definitely fixed in 9.0.3 Preview Release which
is currently available on OTN. 9.0.3 Production should be
available anyday now.
Michel

A doubt on Inner classes

Hi,
The below program throws a compilation error saying "The method fn() in the type Outer.Inner is not applicable for the arguments (String)". I am expecting it to call the private String fn(String in) method on Outer class. This might be silly, but I am not able to find the reason why?
public class Outer {
private String fn(String in){
return in+"outer";
class Inner{
private String fn(){
return fn("");
Can anyone please shed some light on it?
Thanks in advance,
Jose John

TPD Opitz-Consulting com wrote:
EJP wrote:
The reason is that the inner fn(), not being an override (i.e. not taking the same arguments), is instead an overload (different arguments), and overloads hide what they overload in outer scopes.This is wrong. No inheritance is involved here.I fail to see where EJP actually mentions inheritance.
The point may be that you cannot access a non static method without an object. YOU are wrong. The Eclipse quickfix is simply the standard way of referring to a member of an outer class even when the property/method is 'overlapped' by one of the inner class.

Problem with Outer and Inner Classes....or better way?

Right now I'm trying to create an Inner class.
My .java file compiles ok, and I create my jar file too.
But when I try to instantiate the Inner class, it fails:
java.lang.NoClassDefFoundError: com/myco/vlXML/vlXML$vlDocument.
Here's the class code:
public class vlXML{
    private ArrayList myDocList=new ArrayList(); //holds documents
    public vlXML(){
    private class vlDocument{
        public vlDocument(){
        //stuff goes here
    public vlDocument vlDOC(){
        return new vlDocument();
    public void addDocument(){
        vlXML xxx=new vlXML();
        vlDocument myDoc=xxx.vlDOC();
        myDocList.add(myDoc);
    public int getNumDocs(){
        return myDocList.size();
}Then, from a jsp page, I call:
vlXML junk1=new vlXML();
junk1.addDocument();...and get the error...
Can someone help me figure out why it's failing?
Thanks....

You nailed it - thanks....(duh!)
While I have your attention, if you don't mind, I have another question.
I'm creating a Class (outer) that allows my users to write a specific XML file (according to my own schema).
Within the XML file, they can have multiple instances of certain tags, like "Document". "Document"s can have multiple fields.
Since I don't know how many "Documents" they may want, I was planning on using an Inner Class of "Document", and then simply letting them "add" as many as necessary (thus the original code posted here).
Does that seem like an efficient (logical/reasonable) methodology,
or is there there something better?
Thanks Again...

Problem with final variables and inner classes

variables accessed by inner classes need to be final. Else it gives compilation error. Such clases work finw from prompt. But when I try to run such classes through webstart it gives me error/exception for those final variables being accessed from inner class.
Is there any solution to this?
Exception is:
java.lang.ClassFormatError: com/icorbroker/fx/client/screens/batchorder/BatchOrderFrame$2 (Illegal Variable name " val$l_table")
     at java.lang.ClassLoader.defineClass0(Native Method)
     at java.lang.ClassLoader.defineClass(ClassLoader.java:502)
     at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:123)
     at com.sun.jnlp.JNLPClassLoader.defineClass(Unknown Source)
     at com.sun.jnlp.JNLPClassLoader.access$1(Unknown Source)
     at com.sun.jnlp.JNLPClassLoader$1.run(Unknown Source)
     at java.security.AccessController.doPrivileged(Native Method)
     at com.sun.jnlp.JNLPClassLoader.findClass(Unknown Source)
     at java.lang.ClassLoader.loadClass(ClassLoader.java:299)
     at java.lang.ClassLoader.loadClass(ClassLoader.java:255)
     at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:315)
     at com.icorbroker.fx.client.screens.batchorder.BatchOrderFrame.<init>(BatchOrderFrame.java:217)
     at com.icorbroker.fx.client.screens.batchorder.BatchOrderViewController.createView(BatchOrderViewController.java:150)
     at com.icorbroker.fx.client.screens.RealTimeViewController.initialize(RealTimeViewController.java:23)
     at com.icorbroker.fx.client.screens.batchorder.BatchOrderViewController.<init>(BatchOrderViewController.java:62)
     at com.icorbroker.fx.client.screens.displayelements.DisplayPanel$3.mousePressed(DisplayPanel.java:267)
     at java.awt.Component.processMouseEvent(Component.java:5131)
     at java.awt.Component.processEvent(Component.java:4931)
     at java.awt.Container.processEvent(Container.java:1566)
     at java.awt.Component.dispatchEventImpl(Component.java:3639)
     at java.awt.Container.dispatchEventImpl(Container.java:1623)
     at java.awt.Component.dispatchEvent(Component.java:3480)
     at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:3450)
     at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3162)
     at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3095)
     at java.awt.Container.dispatchEventImpl(Container.java:1609)
     at java.awt.Window.dispatchEventImpl(Window.java:1590)
     at java.awt.Component.dispatchEvent(Component.java:3480)
     at java.awt.EventQueue.dispatchEvent(EventQueue.java:450)
     at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:197)
     at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
     at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:144)
     at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:136)
     at java.awt.EventDispatchThread.run(EventDispatchThread.java:99)

I've also been having the same problem. The only work-around seems to be to slightly change the code, recompile & hope it works. See http://forum.java.sun.com/thread.jsp?forum=38&thread=372291

Problem with final variables and inner classes (JDK1.1.8)

When using JDK1.1.8, I came up with following:
public class Outer
    protected final int i;
    protected Inner inner = null;
    public Outer(int value)
        i = value;
        inner = new Inner();
        inner.foo();
    protected class Inner
        public void foo()
            System.out.println(i);
}causing this:
Outer.java:6: Blank final variable 'i' may not have been initialized. It must be assigned a value in an initializer, or in every constructor.
public Outer(int value)
^
1 error
With JDK 1.3 this works just fine, as it does with 1.1.8 if
1) I don't use inner class, or
2) I assign the value in initializer, or
3) I leave the keyword final away.
and none of these is actually an option for me, neither using a newer JDK, if only there is another way to solve this.
Reasons why I am trying to do this:
1) I can't use a newer JDK
2) I want to be able to assign the variables value in constructor
3) I want to prevent anyone (including myself ;)) from changing the value in other parts of the class (yes, the code above is just to give you the idea, not the whole code)
4) I must be able to use inner classes
So, does anyone have a suggestion how to solve this problem of mine? Or can someone say that this is a JDK 1.1.8 feature, and that I just have to live with it? In that case, sticking to solution 3 is probably the best alternative here, at least for me (and hope that no-one will change the variables value). Or is it crappy planning..?

You cannot use a final field if you do not
initialize it at the time of declaration. So yes,
your design is invalid.Sorry if I am being a bit too stubborn or something. :) I am just honestly a bit puzzled, since... If I cannot use a final field in an aforementioned situation, why does following work? (JDK 1.3.1 on Linux)
public class Outer {
        protected final String str;
        public Outer(String paramStr) {
                str = paramStr;
                Inner in = new Inner();
                in.foo();
        public void foo() {
                System.out.println("Outer.foo(): " + str);
        public static void main( String args[] ) {
                String param = new String("This is test.");
                Outer outer = new Outer(param);
                outer.foo();
        protected class Inner {
                public void foo() {
                        System.out.println("Inner.foo(): " + str);
} producing the following:
[1:39] % javac Outer.java
[1:39] % java Outer
Inner.foo(): This is test.
Outer.foo(): This is test.
Is this then an "undocumented feature", working even though it shouldn't work?
However, I assume you could
get by with eliminating the final field and simply
passing the value directly to the Inner class's
constructor. if not, you'll have to rethink larger
aspects of your design.I guess this is the way it must be done.
Jussi

Problem with constructor of inner class.

Hi everybody!
I have an applet which loads images from a database.
i want to draw the images in a textarea, so i wrote an inner class, which extends textarea and overrides the paint method.
but everytime i try to disply the applet in the browser this happens:
java.lang.NoClassDefFoundError: WohnungSuchenApplet$Malfl�che
at java.lang.Class.getDeclaredConstructors0(Native Method)
at java.lang.Class.privateGetDeclaredConstructors(Class.java:1590)
at java.lang.Class.getConstructor0(Class.java:1762)
at java.lang.Class.newInstance0(Class.java:276)
at java.lang.Class.newInstance(Class.java:259)
at sun.applet.AppletPanel.createApplet(AppletPanel.java:567)
at sun.plugin.AppletViewer.createApplet(AppletViewer.java:1778)
at sun.applet.AppletPanel.runLoader(AppletPanel.java:496)
at sun.applet.AppletPanel.run(AppletPanel.java:293)
at java.lang.Thread.run(Thread.java:536)
so my class has no own constructor, it just has the paint method overwritten.
my class looks like this:
public class Malfl�che extends javax.swing.JTextArea{
public void paint(Graphics g){
Color grey=new Color(220,220,220);
g.drawImage(img,10,10,null);
how should a constructor for this class look like?
sorry i am quite new to this, so i really dont have a clue!
my class does not have any attributes or requires any so it doesnt need a constructor, doesnt it?
thanks a lot
tim

First off, unlike regular classes, inner classes can be declared public, private, protected, and default.
Secondly, why are you using the JTextArea to display an image, why not use a JLabel, which takes an Image object as its constructor.
Thirdly, when you drew your image you did not give it a width and height
g.drawImage(img, 0,0, img.getWidth(null), img.getHeight(null), null);
otherwise it will make your image 1 X 1 pixels. not big enough to see.

Help: Factory Class using Inner Class and Private Constructor?

The situation is as follows:
I want a GamesCollection class that instantiates Game objects by looking up the information needed from a database. I would like to use Game outside of GamesCollection, but only have it instantiated by GamesCollection to ensure the game actually exist. Each Game object is linked to a database record. If a Game object exist, it must also exist in the database. Game objects can never be removed from the database.
I thought about making the Game object an inner class of GamesCollection, but this means that Game class constructor is still visible outside. So what if I made Game constructor private? Well, now I can't create Game objects without a static method inside Game class (static Object factory).
Basically what I need is a constructor for the inner Game class accessible to GamesCollection, but not to the rest of the world (including packages). Is there a way to do this?

leesiulung wrote:
As a second look, I was initially confused about your first implementation, but it now makes more sense.
Let me make sure I understand this:
- the interface is needed to make the class accessible outside the outer classBetter: it is necessary to have a type that is accessible outside of GameCollection -- what else could be the return type of instance?
- the instance() method is the object factory
- the private modifier for the inner class is to prevent outside classes to instantiate this objectRight.
However, is a private inner class accessible in the outer class? Try it and see.
How does this affect private/public modifiers on inner classes?Take about five minutes and write a few tests. That should answer any questions you may have.
How do instantiate a GameImpl object? This basically goes back to the first question.Filling out the initial solution:
public interface Game {
    String method();
public class GameCollection {
    private static class GameImpl implements Game {
        public String method() {
            return "GameImpl";
    public Game instance() {
        return new GameImpl();
    public static void main(String[] args) {
        GameCollection app = new GameCollection();
        Game game = app.instance();
        System.out.println(game.method());
}Even if you were not interested in controlling game creation, defining interfaces for key concepts like Game is always going to be a good idea. Consider how you will write testing code, for example. How will you mock Game?

Threaded inner classes & heap memory exhaustion

(_) how can i maximize my threading without running out of
heap memory?
push it to the limit, but throttle back before an
java.lang.OutOfMemoryError.
(_) within 1 threaded class ThreadClass, i have two threaded inner classes. for each instance of ThreadClass i only
start one instance of each inner class.
and, i start hundreds of ThreadClass, but not until the previously running ThreadClass object exits, so only one should be running at any given time.
so, what about threaded inner classes?
are they good? bad? cause "OutOfMemoryErrors"?
are those inner threads not dying?
what are common causes of:
java.lang.OutOfMemoryError: java heap space?
my program runs for about 5-minutes, then
bails with the memory error.
how can i drill down and see what
is eating-up all my memory?
thanks.

A Thread class is not the same as a thread of
execution. Those inner class based threads of
execution are not dying.maybe. but this is the way i test a thread's life:
public void run() {
System.out.println("thread start");
System.out.println("thread dies and release memory");
}for each inner thread, and the outer thread, this approach for
testing thread life reveals that they die.
Why don't you use a thread pool?ok. i will think about how to do this.
>
If not, you need to ensure those inner threads have
exited and completed.what is a 100% sure check to guarantee a thread exits other than
the one i use above?
note:
the outer thread is running on a remote host, and the inner threads
are running locally. here are the details:
public class BB implements Runnable, FinInterface {
public void run() {
    // do some work on the remote machine
private void startResultsHandler(OisXoos oisX) {
     ResultHandler rh = new ResultHandler(oisX);
     rh.start();
public void startDataProxy(OisXoos oisX, String query) {
     DataProxy dp = new DataProxy(oisX, query);
     dp.start();
        public class ResultsHandler extends Thread {
           // runs locally; waits for results from servers
           public void run() {
               ObjectInputStream ois = new ObjectInputStream(oisX.input);
                Set result = (Set) ois.readObject();
        } // ____ class :: _ ResultsHandler _ :: class ____
       public class DataProxy extends Thread {
           // runs locally; performs db queries on behalf of servers
           public void run() {
               ObjectOutputStream oos = new ObjectOutputStream(oisX.output);
                while(moreData) {
                    .... // sql queries
                    oos.writeObject(data);
             StartResultsHandler(oisX);
        } // _____ class :: _ DataProxy _ :: class _____
}now, the BB class is not started locally.
the inner threads are started locally to both service data requests
by the BB thread as well as wait for its results.
(_) so, maybe the inner threads cannot exit (but they sure look
like they exit) until their parent BB thread exits.
(_) yet, those inner threads have no knowledge that the BB
thread is running.
externalizing those inner thread classes will put 2-weeks of work
in the dust bin. i want to keep them internal.
thanks.
here this piece of code that controls everything:
while(moreData) {
FinObjects finObj = new BB();
String symb = (String) data_ois.readObject();
OisXoos oisX = RSAdmin.getServer();
oisX.xoos.writeObject(finObj);
finObj.startDataProxy(finObj, oisX, symb);
}

Compiler error when useing switch statements in an inner class

I have defined several constants in a class and want to use this constans also in an inner class.
All the constants are defined as private static final int.
All works fine except when useing the switch statement in the inner class. I get the compiler error ""constant expression required". If I change the definition from private static final to protected static final it works, but why?
What's the difference?
Look at an example:
public class Switchtest
   private static final int AA = 0;
   protected static final int BB = 1;
   private static int i = 0;
   public Switchtest()
      i = 0; // <- OK
      switch(i)
         case AA: break; //<- OK, funny no problem
         case BB: break; //<- OK
         default: break;
private class InnerClass
      public InnerClass()
         i = 0; // <- OK: is accessible
         if (AA == i) // <- OK: AA is seen by the inner class; i is also accessible
            i = AA + 1;
         switch(i)
            case AA: break; // <- STRANGE?! Fail: Constant expression required
            case BB: break; // <- OK
            default: break;
}Thank's a lot for an explanation.

Just a though:
Maybe some subclass of Switchtest could decalare its own variable AA that is not final, but it can not declare its own BB because it is visible from the superclass. Therefore the compiler can not know for sure that AA is final.

Passing Inner class name as parameter

Hi,
How i can pass inner class name as parameter which is used to create object of inner class in the receiving method (class.formane(className))
Hope somebody can help me.
Thanks in advance.
Prem

No, because an inner class can never have a constructor that doesn't take any arguments.
Without going through reflection, you always need an instance of the outer class to instantiate the inner class. Internally this instance is passed as a parameter to the inner class's constructor. So to create an instance of an inner class through reflection you need to get the appropriate constructor and call its newInstance method. Here's a complete example:import java.lang.reflect.Constructor;
class Outer {
    class Inner {
    public static void main(String[] args) throws Exception{
        Class c = Class.forName("Outer$Inner");
        Constructor cnstrctr = c.getDeclaredConstructor(new Class[] {Outer.class});
        Outer o = new Outer();
        Inner i = (Inner) cnstrctr.newInstance(new Object[]{o});
        System.out.println(i);
}

Inner class standards

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