Inner/Nested Classes

Could someone tell me what the rules of accessibility are for static and non-static inner classes being able to access elements of the enclosing class

A non-static or inner class can access all of the enclosing class's members, including private members. This is because each instance of an inner class is associated with an instance of the outer class, and inner classes are not allowed to have static members except for constants.
A static nested class can access only the static members of the enclosing class. This is because instances of the nested class are not associated with instances of the outer class.

Similar Messages

Extending inner [nested] classes

OK I've got a class (A) with a nested class (B).
B accesses variables etc. in A and does its own thing, but what I'd like to do is make B abstract and have a number of classes extend it - which is used depends on program arguments.
The problem I'm having is that the new class (C) that extends B does not seem to have access to any of the variables in A.
I originally tried:
class C extends B
and
class C extends A$B
I also tried this, which I found on this site:
class C extends A.B
(thats A-dot-B) but it still doesnt seem to be working.
Is it possible to extend [abstract] inner classes like this? If so, how? Any help appreciated. cheers.

Here I found a solution. However, it is not so elegant. In fact, I am also having trouble in extending a nested class in an elegant way.
Here, A is the original top-level class and A.B is the nested class of A. A2 extends A. B2 and B3 extends A.B. Main is the class where the main() method can be found.
// A.java
public class A {
private B b;
public void setB (B b) {
    this.b = b;
public B getB () {
    return b;
public abstract class B {
    public abstract String getName();
// A2.java
public class A2 extends A {
public class B2 extends A.B {
    public String getName () {
      return "Peter";
public class B3 extends A.B {
    public String getName () {
      return "John";
public class Main {
public static void main (String[] args) {
    A a = new A2 ();
    A.B b = ((A2)a).new B2 ();
    a.setB (b);
    System.out.println (a.getB().getName()); // John
    b = ((A2)a).new B3 ();
    a.setB (b);
    System.out.println (a.getB().getName()); // Peter
}You may see here that I used "A" as the type of "a". Te reason is I want to use "a" as an "A", not "A2". If you don't want to do casting, you may simply use "A2" as the type. The class A2 can be blank. It is only a "container" of the classes B2 and B3.

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

Nested Classes and Static Methods

I was perusing the Java Tutorials on Nested Classes and I came across this...
[http://java.sun.com/docs/books/tutorial/java/javaOO/nested.html|http://java.sun.com/docs/books/tutorial/java/javaOO/nested.html]
I was reading the documentation and I read a slightly confusing statement. I was hoping some further discussion could clarify the matter. The documentation on "Nested Classes" says (I highlighted the two statements, in bold, I am having trouble piecing together.)
Static Nested Classes
As with class methods and variables, a static nested class is associated with its outer class. And like static class methods, a static nested class cannot refer directly to instance variables or methods defined in its enclosing class ? it can use them only through an object reference.
Note: A static nested class interacts with the instance members of its outer class (and other classes) just like any other top-level class. In effect, a static nested class is behaviorally a top-level class that has been nested in another top-level class for packaging convenience.
So this means a nested static inner class CANNOT refer to its own instanced variables, like a normal static method, but CAN refer to its outer class instanced variables?

So this means a nested static inner class CANNOT refer to its own instanced variables, like a normal static method, but CAN refer to its outer class instanced variables?No, it means that a static nested class cannot refer to instance variables of its enclosing class. Example:public class Foo {
    int i;
    static class Bar {
        int j = i; // WRONG! Bar class is static context
}~

Nested classes and separate files in Javadoc...

Hello. I am writing an application, and it has several .java files, and some of the classes contained therein contain nested classes, for example
ClassOne.java
class ClassOne
// stuff
   class innerClassOne
      //more stuff
}ClassTwo.java
class ClassTwo
// stuff
   class innerClassTwo
      //more stuff
}What I want to do is generate documentation like the online java documentation for my app, that is, documentation that will contain ALL the classes and nested classes involved, and link everything that comes in java already to the sun online java docs.
I've figured out the -link command for javadoc, and I can make that work, but I've run into some other snags.
1.) Javadoc isn't finding or documenting my nested classes, despite them being marked up. Google does not wish to help.
2.) Some classes reference other classes of mine that aren't conatined in the same .java file, but do exist in their own .java files in the dame local directory. Javadoc does not like this, and gives errors.
3.) Running javadoc on one file at a time overwrites the old file's documentation, thus I cannot seem to get an index.html javadoc file that links to all my classes, nor can I get the classes to link to each other.
What do I need to do to fix these?

So this means a nested static inner class CANNOT refer to its own instanced variables, like a normal static method, but CAN refer to its outer class instanced variables?No, it means that a static nested class cannot refer to instance variables of its enclosing class. Example:public class Foo {
    int i;
    static class Bar {
        int j = i; // WRONG! Bar class is static context
}~

Questions concerning Nested Classes

Hi!
Just read some articles about nested classes (include the ones from the Java Tutorial and the Effective Java chapter), and while most of it is perfectly clear, three questions remain:
1.) When I declare a member class, how should I declare the access specifiers (private, protected etc.) for the member class's attributes and methods? If I declare a member class as private, anything but private for the attributes and methods wouldn't make sense, would it?
2.) The Java Tutorial says:
Also, because an inner class is associated with an instance, it cannot define any static members itself.While this is true for static methods, it seems to be possible to declare static variables inside inner classes. This is confusing me... does it actually make sense to declare static variables inside inner classes (or member classes in general)? Or should the be placed in the declaring class?
3.) Another confusing quote from the Java Tutorial:
Static nested classes do not have access to other members of the enclosing class.This is true for instance variables and methods but not for static variables, which are also members of the enclosing class, aren't they?
Thanks in advance,
OIiver

Trollhorn wrote:
Hi!
Just read some articles about nested classes (include the ones from the Java Tutorial and the Effective Java chapter), and while most of it is perfectly clear, three questions remain:
1.) When I declare a member class, how should I declare the access specifiers (private, protected etc.) for the member class's attributes and methods? If I declare a member class as private, anything but private for the attributes and methods wouldn't make sense, would it?
private static class MyInner implements Runnable
@Override public void run() // Must be public!
2.) The Java Tutorial says:
Also, because an inner class is associated with an instance, it cannot define any static members itself.While this is true for static methods, it seems to be possible to declare static variables inside inner classes. This is confusing me... does it actually make sense to declare static variables inside inner classes (or member classes in general)? Or should the be placed in the declaring class?Wrong. It can define static final member variables and I see no reason to move them to the outer class, if they are used only by the inner class.
3.) Another confusing quote from the Java Tutorial:
Static nested classes do not have access to other members of the enclosing class.This is true for instance variables and methods but not for static variables, which are also members of the enclosing class, aren't they?I agree with you here.
Edited by: baftos on Jun 6, 2009 9:18 AM

Breakpoints in nested classes don't work

I've noticed that when I set a breakpoint in a nested class, it doesn't work to stop the code, unless I set the breakpoint after I've created an instance of the class.
In the code example below, if I set breakpoints A, B, C and D, only breakpoints A and C will be hit when MainClass.main runs.
However, if I set breakpoint D once breakpoint C is hit, then breakpoint D works.
I've noticed this behavior in both JBuilder 4 and Eclipse, which are both using the Sun 1.3.1 JRE/JDK.
Does anyone know if there is a way to get breakpoints in nested classes to work as expected (they stop the thread whenever that point in the code is executed, and setting them before the class is loaded/instantiated doesn't change that behavior).
BTW, in my research on this topic, I found a mention of "class preparation", a step that the JVM goes through when loading a class. I wonder if the class has to go through this step before the breakpoint can be enabled.
--James
public class MainClass {
    public static void main(String[] args) {
        Object o = OtherClass.Test(3); // Breakpoint A
        System.out.println(o);
class OtherClass {
    static Object Test(int v) {
        Object o = new NestedClass(v);
        return o;                      // Breakpoint C
    static class NestedClass {
        int x;
        NestedClass(int v) {
            x = v;                     // Breakpoint B
        public String toString() {
            return "NestedClass(x=" + x + ")"; // Breakpoint D
}

You wrote:
-> I've noticed this behavior in both JBuilder 4 and Eclipse, which
-> are both using the Sun 1.3.1 JRE/JDK.
Take a look at Bug-ID 4299394
Synopsis: TTY: Deferred line breakpoints can't be set on inner classes
http://developer.java.sun.com/developer/bugParade/bugs/4299394.html
-> BTW, in my research on this topic, I found a mention of "class preparation",
-> a step that the JVM goes through when loading a class. I wonder if the class
-> has to go through this step before the breakpoint can be enabled.
A breakpoint cannot be set in a class before the class prepare event for
that class. Also: the fully qualified name for the inner or nested class
must be constructed properly.
Any breakpoint requested before the target class is loaded must be 'deferred',
which means the debugger keeps track of the breakpoint request and sets
it when the class load eventually occurs.
-> However, if I set breakpoint D once breakpoint C is hit, then
-> breakpoint D works.
By the time you get to the breakpoint at C, the inner class containing D
(OtherClass$NestedClass.class) was located, loaded, and prepared as required
by the "Object o = new NestedClass(v);" statement.

Referencing an inner swingworker class in a JPanel class...?

Hi,
I've got an inner swingworker class defined inside an extended JPanel (we'll call it middleJ) class. Basically the swingworker class runs off a filtered file chooser. I have a method in the JPanel class that instantiates the inner class and kicks off the swing worker (we'll call it kickOffSwing). Now if I instantiate middleJ in an outer class and call the kickOffSwing() method, that should run the filtered filechooser right? Is this the right way to go about running a lengthy background process inside a nested class structure, or is there a more organised way of doing things?
Cheers!

Never mind. Got it working fine! I often find the act of putting together a question helps focus on the task at hnad.

Are static nested classes thread-safe?

There doesn't seem to be any definitive answer to this. Given the following code, is it thread-safe?
public class SomeMultiThreadedWebController {
public HttpServletResponse someMethodToExecuteViaWebRequest(HttpServletRequest request) {
    simpleQueryBuilder("SELECT...").addParameter("asdf","asdf").createQuery(EMF.getEntityManager()).executeUpdate();
protected static class SimpleQueryBuilder {
         private String queryString;
         private Map<String, Object> params = new HashMap<String, Object>();
         public SimpleQueryBuilder(String queryString) {
              this.queryString = queryString;
         public SimpleQueryBuilder addParameter(String name, Object value) {
              params.put(name, value);
              return this;
         public Query createQuery(EntityManager em) {
              Query query = em.createQuery(queryString);
              for (Entry<String, Object> entry : params.entrySet()) {
                   query.setParameter(entry.getKey(), entry.getValue());
              return query;
    public static SimpleQueryBuilder simpleQueryBuilder(String queryString) {
         return new SimpleQueryBuilder(queryString);
}Forget whether or not someone would do this, as this is just an example. I'm really trying to get at whether or not the instance variables inside the static nested class are thread-safe. Thanks for any responses.

Hello,
I believe you understand what you're talking about, but you state it in a way that is very confusing for others.
Let me correct this (essentially, incorrect uses of the terminology):
I agree that thread-safe or not is for an operation, for a member, it has some sort of contextual confusion.
Member has a much broader meaning in the [Java Language Specification|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.4] . Even "class member" applies to both an attribute, a method, or an inner class or interface.
I think you mean "member variable" of a class (aka "attribute" or "field"). By the way, static or not is irrelevant to the rest of the discussion.
For an operation or a member, if there's only one thread could access it atomically in one moment, we could call it thread-safe.Mmm. I was tempted to say yes (I'm reluctant to commit myself). With an emphasis on "_The encapsulating class_ makes this member's usage thread-safe".
Still, just synchronizing each operation on a member is not enough to make all usages "thread-safe":
Consider a java.util.Vector: each add/get is synchronized, so it is atomic, fine.
However if one thread adds several values, let's say 3, one by one, to a vector that initially contains 0 values, and another thread reads the vector's size() (another properly synchronized method), the reader thread may witness a size anywhere among 0, 1, 2, 3, which, depending on the business logic, may be a severely inconsistent state.
The client code would have to make extra work (e.g. synchronizing on the vector's reference before the 3 adds) to guarantee that the usage is thread-safe.
Thus any synchronized method(With the limit stated above)
or immutable member (like primitive type) are thread-safe.
Additionally for a member, if it's immutable, then it's thread-safe. You mean, immutable primitive type, or immutable object. As stated previously, an immutable reference to a mutable object isn't thread-safe.
a static final HashMap still have thread-safe issue in practice because it's not a primitive.The underlined part is incorrect. A primitive may have thread-safety issues (unless it's immutable), and an object may not have such issues, depending on a number of factors.
The put, get methods, which will be invoked probably, are not thread-safe although the reference to map is.Yes. And even if the put/get methods were synchronized, the client code could see consistency issues in a concurrent scenario, as demonstrated above.
Additional considerations:
1) read/write of primitive types are not necessarily atomic: section [ §17.7 of the JLS|http://java.sun.com/docs/books/jls/third_edition/html/memory.html#17.7] explicitly states that writing a long or double value (2 32-bits words) may not be atomic, and may be subject to consistency issues in a concurrent scenario.
2) The Java Memory Model explicitly allows non-synchronized operations on non-volatile fields to be implemented in a "thread-unsafe" way by the JVM. Leading way to a lot of unintuitive problems such as the "Double-Checked Locking idiom is broken". Don't make clever guess on code execution path unless you properly synchronize access to variables across threads.
Edited by: jduprez on Mar 4, 2010 9:53 AM

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!

Cloning nested classes

Does anyone know where I can get information on how
clone() behaves with nested classes? thanks ...

I'm actually talking about nested classes (such
as inner classes), not objects as iv's. Can I assume that an inner class's
native iv's get a shallow copy as well? I can think of a number of ambiguous
situations and am wondering if these issues are discussed anywhere, if there
are any general cautions, etc? Maybe there is nothing to this but on the
face of it it seems complex.

Nameless Nested Classes passed in Function Parameter?

I am trying to get my head around the following code example:
javax.swing.SwingUtilities.invokeLater( new Runnable() { public void run() { createAndShowGUI(); }});What is actually getting passed to the SwingUtilities.invokeLater() function?
Can anyone explain all of the parts and pieces of :
new Runnable() { public void run() { createAndShowGUI(); }} ?
Here is my best guess:
1) { public void run() { createAndShowGUI(); }} is an unnamed nested class that is instantiated using the Runnable() interface.
2) this class has one method called run() that executes createAndShowGUI();
Is my best guess even close to what is really going on?

richard.broersma wrote:
1) { public void run() { createAndShowGUI(); }} is an unnamed nested class that is instantiated using the Runnable() interface.Yes, that's essentially correct. It's called an anonymous inner class.
This is similar to doing
// a non-anonymous inner class
private class MyClass implements Runnable
public void run()
createAndShowGUI();
MyClass myclass = new MyClass();
javax.swing.SwingUtilities.invokeLater(myclass);
2) this class has one method called run() that executes createAndShowGUI();Yes. It must have a parameterless public void run method since it implements the Runnable interface.
Is my best guess even close to what is really going on?Yes, you're catching on.

Instantiating a nested class from JNI

Im able to instantiate my public member class "Inner" as long as the constructor does not take any arguments. If it requires arguments in the constructor, I get a crash report. I need to be able to instantiate the object with supplied initial arguments. Is this a bug in the JVM? Any suggestions?
/dan
Crash report follows after code:
class HelloWorld {
   public class Inner {
      private int myValue = 50;
      public Inner() {}
      public Inner(int myValue) {
         this.myValue = myValue;
      public Inner(int myValue, int there) {
         this.myValue = myValue;
}C++ library:
   jclass inr_clz = env->FindClass("LHelloWorld$Inner;");
   if (inr_clz == NULL) {
      return -25;               // failed to find the class
//   mid = env->GetMethodID(inr_clz, "<init>", "(LHelloWorld;)V");              // works
   mid = env->GetMethodID(inr_clz, "<init>", "(LHelloWorld;II)V");            // also works
   if (mid == NULL) {
      return -30;                    // failed to get method
   jint value = 400;
//   inner = env->NewObject(inr_clz, mid);                           // works
   inner = env->NewObject(inr_clz, mid, value, value);             // crash here
   if (inner == NULL) {
      return -40;                     // out of memory error
   ...Crash message:
# An unexpected error has been detected by HotSpot Virtual Machine:
# Internal Error (53484152454432554E54494D450E43505001A3), pid=2008, tid=2628
# Java VM: Java HotSpot(TM) Client VM (1.5.0_06-b05 mixed mode)
# An error report file with more information is saved as hs_err_pid2008.log
# If you would like to submit a bug report, please visit:
# http://java.sun.com/webapps/bugreport/crash.jsp
#

Im able to instantiate my public member class "Inner"
as long as the constructor does not take any
arguments. If it requires arguments in the
constructor, I get a crash report. I need to be able
to instantiate the object with supplied initial
arguments. Is this a bug in the JVM? Any
suggestions?It's your code.
Here's a working example:
==================
public class NativeInnerTest {
     public static void main(String[] args) {
          new Outer().doNative();
public class Outer {
     static {
          System.loadLibrary("outer");
     public class Inner {
          private int val;
          Inner (int val) {
               this.val = val;
          public void foo() {
               System.out.println("Value is: " + val);
     public native void doNative();
}=====Native Code======================
#include "Outer.h"
JNIEXPORT void JNICALL Java_Outer_doNative (JNIEnv *env, jobject obj) {
     jclass clz = env->FindClass("Outer$Inner");
     if (clz == NULL) {
          printf("Failed to find class Outer$Inner");
          return;
     jmethodID mid = env->GetMethodID(clz,"<init>", "(LOuter;I)V");
     if (mid == NULL) {
          printf ("Failed to find Inner constructor\n");
          return;
     jint val = 42;
     jobject inner = env->NewObject(clz,mid,obj,val);
     if (inner == NULL) {
          printf ("Failed to construct Inner object\n");
          return;
     jmethodID fooID = env->GetMethodID(clz, "foo", "()V");
     if (fooID == NULL) {
          printf ("Failed to find method id for foo()\n");
          return;
     env->CallVoidMethod(inner, fooID);
}================================
The problem in your code is twofold. First, your type signature in the call to GetMethodID is incorrect. An inner class constructor taking a single integer argument should have a type signature like this:
(LHelloWorld;I)V
You can get the proper type signatures using the 'javap' tool. For my example, the output of javap looks like this:
[jim@daisy tmp]$ /usr/java/jdk1.6.0/bin/javap -s -private Outer.Inner
Compiled from "Outer.java"
public class Outer$Inner extends java.lang.Object{
private int val;
Signature: I
final Outer this$0;
Signature: LOuter;
Outer$Inner(Outer, int);
Signature: (LOuter;I)V
public void foo();
Signature: ()V
}Second, your call to NewObject does not have the proper parameter types. Here's your code:
inner = env->NewObject(inr_clz, mid, value, value);The corresponding correct line from my example is:
jobject inner = env->NewObject(clz,mid,obj,val);The constructor expects a 'HelloWorld' object and an integer as denoted in the correct
type signature.
You are passing in the integer 'val' twice when you should be doing something like this:
inner = env->NewObject(inr_clz, mid, obj, value);where 'obj' is the jobject passed to you by the VM along with the JNIEnv pointer. This jobject represents the instance of the enclosing class which every non static nested class must have.
Jim S.

Instanciation of nested class using Class.newInstance();

I am having a problem to instanciate a nested class using Class
new instance method. The problem is as follows:
class A {
public static class B {
String xxx;
class Instance {
public static void main ( String args []) {
String name = "A"
Class name_class = Class.forName(name);
Object name_object = name_class.newInstance();
Class [] in_classes = name_class.getClasses();
for (i =0; i < in_classes.length; i ++) {
Class inner = in_classes ;
System.out.println("class =" + inner.getName());
Here: Object inner_object = inner.newInstance();
An exception is thown at <Here> line where the newInstance is called
on nested class.
Anybody run into this problem, please help.
Thanks

Well, it seems to work for me :D
public class Test {
     public static void main ( String args []) {
          try {
               String name = "A";
               Class name_class = Class.forName(name);
               Object name_object = name_class.newInstance();
               Class [] in_classes = name_class.getClasses();
               for (int q =0; q < in_classes.length; ++q) {
                    Class inner = in_classes [q];
                    System.out.println("class =" + inner.getName());
                    A.B inner_object = (A.B) inner.newInstance();
                    System.out.println (inner_object.xxx);
          } catch (final Exception e) {
               e.printStackTrace ();
               System.out.println (e.getMessage ());
class A {
     public static class B {
          public String xxx = "Hi";
}

Why a nested class can instantiate an abstract class?

Hi people!
I'm studying for a SCJP 6, and i encountered this question that i can figure it out but i don't find any official information of my guess.
I have the following code:
public class W7TESTOQ2 {
    public static void main(String[] args) {
       // dodo dodo1 = new dodo();
        dodo dodo2 =new dodo(){public String get(){return "filan";}};
        dodo.brain b = dodo2.new brain(){public String get(){return "stored ";}};
        System.out.print(dodo2.get()+" ");
        System.out.println(b.get());
abstract class dodo
    public String get()
        return "poll";
    abstract class brain
        public abstract String get();
}I know that abstract classes cannot be instantiated but i see that in this example, with a nested anonymous class it does (dodo and brain classes). My guess is that declaring the nested class it makes a subclass of the abstract class and doing so it can be instantiated. But i can't find any documentation about this.
Does anybody know?
Really thanks in advance.
Regards,
Christian Vielma

cvielma wrote:
Another question about this. Why can't i declare a constructor in the nested class? (it gives me return type required)You cannot declare a constructor, because in one line you're declaring a new class (the anonymous inner class) as well as constructing an instance of it.
What you can do, however, is if the abstract class (or the superclass, in any case, it doesn't need to be abstract) defines several constructors, you can call them.
public abstract MyClass {
    public MyClass() {
        // Default do nothing constructor
    public MyClass(String s) {
        // Another constructor
// Elsewhere
MyClass myclass = new MyClass("Calling the string constructor") {
};But you can't define your own constructors in the anonymous inner class.
Also, since the class is anonymous, what would you name the constructor? :)
Edited by: Kayaman on 26.6.2010 22:37

Inner/Nested Classes

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