When to use Nested Class/Inner Classes ?

I am not very clear, when to use nested/inner classes..
The scenario is :-
class ABC
//ABC need to store multiple instance of class XYZ
class XYZ
int member1;
int member2;
One approach is
class ABC
class XYZ
//vector of class XYZ instances is stored in class-ABC
or another approach is Class XYZ can be in separate JAVA file.
Query:-
1) Is there any difference between nested or Inner class...or are they same?
2) When should they be used....Is it good to use in above scenario.
3) What are the disadvtanges/advantages of using the Nested
class.

Query:-
1) Is there any difference between nested or Inner
class...or are they same?I really don't get it. Yes there is a difference between having an inner class, and a class in a separate file, but a nested class is an inner class.
2) When should they be used....Is it good to use in
above scenario.To write an inner class is a design decision. Do other classes need to know about the class XYZ or not? Do the XYZ class need to know about the inner working of the ABC class? How complex is the XYZ class etc.
>
3) What are the disadvtanges/advantages of using the
Nested
class.See above.
/Kaj

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Building Tree hierarchy Using nested loops and class cl_gui_column_tree

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Edited by: Yayati6260 on Jul 14, 2011 7:25 AM

Hello all,
Thanks the issue is solved. The node key was not getting a unique identification as nodekey. I resolved the issue by generating unique identification for each level. Thanks all,
Regards
Yayati Ekbote

Nested and inner classes

What's the difference between them? As far as i can tell, a nested class is usually public and a inner class is almost always private. Can some one please provide an example?
Thanks a lot guys.

inner classes == nested classes - static classes
In other words, inner class should belongs to the instance of the enclosing class.

How to use a static inner class?

what's the implication of identifier "static" before an inner class.Thank you.

not sure what implication is, due to my poor vocabulary. But here's an uuh... "example" of a static inner class:
public class Example {
static class AnotherClass {
}There you have class "Example" with the static inner class "AnotherClass." If you wanted to use "AnotherClass" you would do something like:
public class Whatever {
Example.AnotherClass ac = new Example.AnotherClass();
}I don't think I'm mistaken there, but if I am someone please correct me.

Using static interface inner class.. how?

Can anyone tell me how I'm able to use an inner class that is a static interface?
Specifically I'm getting DocumentEvents through a DocumentListener..
Then within DocumentEvents there is an inner class which is a static interface called:
DocumentEvent.ElementChange, I'm trying to use the methods contained within the interface ElementChange.
Can anyone throw a helping hand as to how I go about this?
Thanks :)

public class A ... {
public static interface B {
}Just creates a public interface called A.B
How are you supposed to use this with your DocumentEvent? I can't say

Why are protected fields not-accessible from sub-classed inner class?

I ran across a situation at work where we have to sub-class an inner class from a third-party package. So, it looks something like this...
package SomePackage;
public class Outer {
   protected int x;
   public class Inner {
package OtherPackage;
public class NewOuter extends Outer {
   public class NewInner extends Outer.Inner {
}Now the NewInner class needs to access protected variables in the Outer class, just like the Inner class does. But because NewOut/NewInner are in a different package I get the following error message.
Variable x in class SomePackage.Outer not accessible from inner class NewOuter. NewInner.You can still access those protected variables from the NewOuter class though. So, I can write accessor methods in NewOuter for NewInner to use, but I am wondering why this is. I know that if NewOuter/NewInner are in the same package as Outer/Inner then everything works fine, but does not when they are in different packages.
I have to use JDK1.1.8 for the project so I don't know if there would be a difference in JDK1.2+, but I would think that nothing has changed. Anyway, if you know why Java disallows access as I have detailed please let me know.

Although I don't have the 1.1.8 JDK installed on my system, I was able to compile the following code with the 1.3.1_01 JDK and run it within a Java 1.1.4 environment (the JVM in the MSIE 5.5 browser). As long as you don't access any of the APIs that were introduced with 1.2+ or later, the classes generated by the JDK 1.2+ javac are compatible with the 1.1.4+ JVM.
//// D:\testing\SomePackage\Outer.java ////package SomePackage ;
public class Outer {
    protected int x ;
    public Outer(int xx) {
        x = xx ;
    public class Inner {
}//// D:\testing\OtherPackage\NewOuter.java ////package OtherPackage;
import SomePackage.* ;
public class NewOuter extends Outer {
    public NewOuter(int xx) {
        super(xx) ;
    public class NewInner extends Outer.Inner {
        public int getIt() {
            return x ;
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import java.awt.* ;
import java.applet.* ;
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        initComponents ();
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}//// d:\testing\build.cmd ////set classpath=.;D:\testing
cd \testing\SomePackage
javac Outer.java
cd ..\OtherPackage
javac NewOuter.java
cd ..
javac Test.java//// d:\testing\Test.html ////<HTML><HEAD></HEAD><BODY>
<APPLET CODE="Test.class" CODEBASE="." WIDTH=200 HEIGHT=100></APPLET>
</BODY></HTML>

Inner classes - a general question

i'm having some difficulties understanding when to use a static inner class
and when to use a non static inner class
lets say i'm implementing a LinkedList , and i want my nodes and my iterator
implemented in inner classes .
when and why should i declare them static or non static.
thank you very much in advance.

Inner classes are never static. Nested classes can be either static or non-static; a non-static nested class is called an inner class.
Anyway, an inner class implicitly gets a reference to an object of the outer class, while a static nested class doesn't, so the decision should depend on whether or not it needs that.
An iterator would need a reference to its collection so it would be a non-static nested class. A node wouldn't so it would be a static nested class.

Private inner class and static private inner

Hi,
I understand the concept and usage of inner classes in general.
When should we go for a private inner class and when for a static private inner class? I tried searching but it wasn't of much help.
Basically I need to design a caching solution in which I need to timestamp the data object. After timestamping, data will be stored in a HashMap or some other collection. I'm planning to use a wrapper class (which is inner and private) which holds the data object and timestamp. I can make the program work by using either normal inner class or static inner class, however would like to know which is better in such case. Also If I can get some general guidelines as to when to use a staic inner class and when to use a normal inner class, it would help me.
Thanks in advance.

user1995721 wrote:
When should we go for a private inner class and when for a static private inner class?
I can make the program work by using either normal inner class or static inner class, however would like to know which is better
If I can get some general guidelines as to when to use a static inner class and when to use a normal inner class, it would help me.Making the inner class static is helpful in that it limits visibility.
If the inner class needs to access non-static fields or methods from the containing class instance
the inner class has to be non-static.

When we going to use static inner class

Hi
when we r going use static inner class
inner classes use for to create adaptorclasses that implement an interface.
what about Static inner class
if possible give some examples
Thanks in adv

static inner classes are used when the inner class does not require to access the encompassing class's variables/methods. By default non-static inner classes obtain a reference to the outer class instance through which they access the outer class variables and methods
ram.

Why Use Nested Classes

Hi All,
Why do we use Nested Classes ? Explain it with example ....
If we say - It is a way of logically grouping classes that are only used in one place. Then we can say we use it through "Package" so why inner class ??
Thanks in advance ....

A real life example to jschell's explaination may be this.
The pistons in your automobile engine are encapsulated by the engine block -- or the engine block encapsulates the pistons inside. This is the desired effect because as soon as we take the pistons out of the engine block, and put them say, in the backseat of the car instead, they no longer perform the function they were intended. There is another side effect to this analogy. When the pistons are properly installed in the engine block, there is only one movement the pistons can move -- up and down movement, that's it... they cannot rotate, spin, twist around or anything inside the engin block. Furthermore, the up and down movement is the only useful thing a piston can do. As soon as we throw the pistons in the backseat of the car, they can roll around, spin, and generally rotate to any configuration or orientation -- which does not produce any useful power.
Inner classes are generally the same way. So we can see this example with a little code -- which we like.
public class Engine
Piston[] pistons = new Piston[8];
class Piston
// the usual methods here..
}Of course, if your program is extremely sohisticated, this may not be a good solution -- for example, if you are making a catalog of automotive parts, then you would probably want your <tt>Piston</tt> class to be stand-alone.
Edited by: pierrot_2. Also notice that the class <tt>Piston</tt> is not an INNER class of Backseat!

Cann't extend a inner class where as can Implement a nested Interface

i cann't extend a inner class in some other outer class . Where as i can implement the nested Interface in some other class. Why????
for example:-
class ABC
class Inner
/* class body */
interface TempInterface
/* interfacebody */
class OuterClass
class InnerTwo extends ABC.inner //(line 1)Will give error
class InnerTwo implements ABC.TempInterface //(line 2)Will run fine
the line 1 is going to give compilation error i.e not in the scope but the line 2 will run fine .
Both of the things are in the same class but giving 2 other results.
I am not getting sufficient reasons for it.
Can any one help me????
Reagrds
Arunabh

As far as the language is concerned, the classonly
exists in the context of an instance of theenclosing
class.This still exhibits a class/object confusion to me.
It should be 'instance only exists ...' or 'instance
can only exist'. The class only exists in the
scope of the enclosing class, but this is another
issue.I'm not following what you're saying. The second sentence sounds like you're disagreeing with me. The last sentence sounds like you're agreeing with me.
A non-static nested class is an instance member, just like an instance method or instance variable. As such, as far as the language is concerned, that class only exists in the context of an instance of the enlcosing class.It's not just instances of the nested class--its the class definition itself that only exists within the context of an instance of the enclosing class. That's why you have to do anEclosingIntstance.NestedClass and can't do EnclosingClass.NestedClass.

Java Inner classes within the Threading paradigm

Hi all.
Im familiar with static nested classes, method local classes, inner classes and anonymous classes.
But what I am a little perplexed by, is when exactly to use inner classes or static nested classes? (forgetting anonymous classes and method local classes).
I read this article (http://www.javaworld.com/javaworld/javaqa/2000-03/02-qa-innerclass.html) and the first point makes for a good argument, but why nest the class? Why not define it as an external class?
Also you typically find nested classes within the Threading paradigm, but why?
I typically would create a top level class (non nested) that would implement the Runnable interface, override run() and then
use this class when constructing a new Thread.
Any clarification would be greatly received.
Thanks and Happy Friday.

Boeing-737 wrote:
I read this article (http://www.javaworld.com/javaworld/javaqa/2000-03/02-qa-innerclass.html) and the first point makes for a good argument, but why nest the class? Why not define it as an external class?
I was going to provide some reasons, but when I read the article I found they were already there. Following the DRY principle I'm going to let the article speak for itself.
Also you typically find nested classes within the Threading paradigm, but why?No, I don't. If that's where you typically find them then you haven't had a varied experience.
I typically would create a top level class (non nested) that would implement the Runnable interface, override run() and then
use this class when constructing a new Thread.So carry on doing that, if it works for you.

Inner Classes doubts

Hi All,
I am trying to learn Inner classes in Java. I am referring to the book Core Java by Horstmann and Cornell.
I know that there are various types of inner classes namely:
- Nested Inner classes
- Local Inner classes
- Annonymous Inner classes
- static inner classes
First I am on with Nested Inner classes :
Following is the code which I am executing :
package com.example.innerclass;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Date;
import javax.swing.JOptionPane;
import javax.swing.Timer;
public class InnerClassTest {
      * @param args
     public static void main(String[] args) {
          // TODO Auto-generated method stub
          TalkingClock clock = new TalkingClock(1000,true);
          clock.start();
//          JOptionPane.showMessageDialog(null,"Quit Program");
//          System.exit(0);
class TalkingClock
     private int interval;
     private boolean beep;
     public TalkingClock(int interval, boolean beep){
          this.interval = interval;
          this.beep = beep;
     public void start(){
          ActionListener listener = new TimePrinter();
          Timer t = new Timer(interval,listener);
          t.start();
     private class TimePrinter implements ActionListener{
          public void actionPerformed(ActionEvent event){
               Date now = new Date();
               System.out.println("At the tone time is : "+now);
               if(beep)
                    Toolkit.getDefaultToolkit().beep();
}Following are my doubts :
1. Why do we need to give the line
JOptionPane.showMessageDialog(null,"Quit Program");
System.exit(0);without this line the program doesn't show any output.
2. I didn't understand this syntax.
You can write inner object constructor more explicitly using the syntax. :
outerObject.new InnerClass(construction parameters)
For e.g.
ActionListener listener = this.new TimePrinter();
Here the outer class reference of the newly constructed TimePrinter object is set to this reference of the method that creates the inner class object. the this. qualifier is redundant. However, it is also possible to set the outer class reference to another object by explicilty naming it. For e.g if TimePrinter were a public inner class, you could construct a TimePrinter for any talking clock.
TalkingClock jabberer = new TalkingClock(1000,true);
TalkingClock.TimePrinter listener = jabberer.new TimePrinter();
Please do help me understand this concept.
Thanks
Siddharth

I have understood that this explanation :
i) assuming that TimePrinter is an inner class of TalkingClock, that you'd need an instance of the later in order to create an instance of the former.Yes.
Being a non-static inner class, it can not be instantiated out of context ... which context is the outer class.No. See my reply 11. The "context" is an instance of the outer class - it bears repeating.
ii) jabberer is the outer instance that you are providing.Yes (more accurately it's a reference to an instance of the outer class, but that would be nit-picking).
The left side is identifying the class, the right side is identifying the instanceNo.
I'm not sure what you're calling left side and right side.
If you're talking about both sides of the equals sign, then no, it's completely wrong.
If you're talking about both sides of the "point" sign, then it's wrong too, just a bit less wrong.
Let's revise this step by step (good thought process).
1. in first line we are getting an outer class reference with this code
TalkingClock jabberer = new TalkingClock(1000,true);
this line is very natural and easily understood. Yes. The correct wording would be merely "we are getting a reference to an instance of the outer class". Sorry to insist densely.
2. Now when we come to the second line, i.e. where we try to instantiate an inner class with this line of code
TalkingClock.TimePrinter listener = jabberer.new TimePrinter();
- I do understand the concept that we need an instance of outer class in order to create an instance of inner class as inner class is visible only to outer class.No. We need an instance of the outer class as the inner class is non-static, and by definition needs an instance of the outer class. That has nothing to do with visibility (public vs private vs...). Again, some words have special meanings in the Java world.
- I also do understand that it cant be instantiated out of context. I see you like this expression, but it is too vague and misleads you. Please forget about it for a moment (no offense to otherwise helpful and knowledgeable abillconsl).
- I also do understand that left side is identifying the class and right side is identifying the instance. ANDAgain I'm afraid of which "sides" you're talking about.
- that in this line TalkingClock.TimePrinter listener = new TalkingClock().new TimePrinter();
the outer class is anonymous (new TalkingClock()) as we don't require its name here Poor choice of words again. Anonymous classes do exist in Java, but are a totally different concept, that is not related to this line.
- Also in this line TalkingClock.TimePrinter listener = jabberer.new TimePrinter();
I understood the left side part i.e. TalkingClock.TimePrinter listener =
We are attaching the outer class reference with the inner class that's absolutely understandable. Not at all!
This just declares a variable listener, whose type is TalkingClock.TimePrinter (or more accurately com.example.innerclass.TalkingClock.TimePrinter).
Then follows an assignment:
WHAT I don't understand is the right hand side, i.e., the statement jabberer.new TimePrinter();
1. I am unable to digest the fact that we can do something like anobject.new
new is an operator that is used to instantiate an instance of an object. I am unable to digest that we can do x.new?See my previous reply. This is short-hand syntax Sun chose to pass a reference to an instance of the outer class (here, jabberer) to the constructor of the inner class. They could have chosen something else. Again, bear with it.
I only know that we can do is new SomeClass(); AND NOT instance.new SomeClass();
Now you know better:
The second form is valid - only if SomeClass is a non-static inner class defined in a class of which instance is an instance.
2. Is there something to this conceptually OR this is only a syntax and that I should learn it.
I want to understand and grasp if there is some concept behind it rather than just learn and mug up. See my previous reply. Each instance of a non-static inner class stores a reference to an instance of the outer class. There must be a way (a syntax) to specify which instance (of the outer class) should be stored in the instance (of the inner class).
This particular syntax is just a syntax, the "concept" is that the instance of the inner class stores a (unmodifiable) reference to the instance of the outer class that was specified when the instance of the inner class was created.
I don't know if that deserves to be called a concept, but that's an interesting thing to keep in mind (there are some not-so-obvious implications in terms of, e.g. garbage collection).
Best regards.
J.

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.

Inner classes question

Hi guys,
I have a question to ask - when i'm writing an inner class and i want to create a new instance from it, the Eclipse forces me to use an instance of the containing class to call new.
Is it how it's done or am i doing something wrong?
public class A
    // Some code here
    public class B
         //Some code here
}Trying to create an instance:
A a = new A();
A.B b = a.new B();

What you have defined is "non-static" inner class.A "non-static" can only be instantiated only if we create an object of outer class.True.
>
There are two ways to create "inner" class object,
1)The way you have written the codeTrue.
2)
    public class A
{    // Some code here
public void createInner()
classB b=new ClassB();
public classB
//Some code here
That code won't compile. Even if you corrected it so that it did compile, it doesn't really help in the OP's situation. Did you read the advice of Saish and Jos about why class 'B' probably shouldn't be public, anyway?

When to use Nested Class/Inner Classes ?

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