[java] Static nested class in Java, why?

I was looking at the Java code for LinkedList and noticed that it made use of a static nested class, Entry.

public class LinkedList<E> ... {
...

 private static class Entry<E> { ... }

}

What is the reason for using a static nested class, rather than an normal inner class?

The only reason I could think of, was that Entry doesn't have access to instance variables, so from an OOP point of view it has better encapsulation.

But I thought there might be other reasons, maybe performance. What might it be?

Note. I hope I have got my terms correct, I would have called it a static inner class, but I think this is wrong: http://java.sun.com/docs/books/tutorial/java/javaOO/nested.html

This question is related to java class static member

One of the reasons for static vs. normal have to do with classloading. You cannot instantiate an inner class in the constructor of it's parent.

PS: I've always understood 'nested' and 'inner' to be interchangeable. There may be subtle nuances in the terms but most Java developers would understand either.

To my mind, the question ought to be the other way round whenever you see an inner class - does it really need to be an inner class, with the extra complexity and the implicit (rather than explicit and clearer, IMO) reference to an instance of the containing class?

Mind you, I'm biased as a C# fan - C# doesn't have the equivalent of inner classes, although it does have nested types. I can't say I've missed inner classes yet :)

static nested class is just like any other outer class, as it doesn't have access to outer class members.

Just for packaging convenience we can club static nested classes into one outer class for readability purpose. Other than this there is no other use case of static nested class.

Example for such kind of usage, you can find in Android R.java (resources) file. Res folder of android contains layouts (containing screen designs), drawable folder (containing images used for project), values folder (which contains string constants), etc..

Sine all the folders are part of Res folder, android tool generates a R.java (resources) file which internally contains lot of static nested classes for each of their inner folders.

Here is the look and feel of R.java file generated in android: Here they are using only for packaging convenience.

/* AUTO-GENERATED FILE.  DO NOT MODIFY.
 *
 * This class was automatically generated by the
 * aapt tool from the resource data it found.  It
 * should not be modified by hand.
 */

package com.techpalle.b17_testthird;

public final class R {
    public static final class drawable {
        public static final int ic_launcher=0x7f020000;
    }
    public static final class layout {
        public static final int activity_main=0x7f030000;
    }
    public static final class menu {
        public static final int main=0x7f070000;
    }
    public static final class string {
        public static final int action_settings=0x7f050001;
        public static final int app_name=0x7f050000;
        public static final int hello_world=0x7f050002;
    }
}

Simple example :

package test;

public class UpperClass {
public static class StaticInnerClass {}

public class InnerClass {}

public static void main(String[] args) {
    // works
    StaticInnerClass stat = new StaticInnerClass();
    // doesn't compile
    InnerClass inner = new InnerClass();
}
}

If non-static the class cannot be instantiated exept in an instance of the upper class (so not in the example where main is a static function)

I don't know about performance difference, but as you say, static nested class is not a part of an instance of the enclosing class. Seems just simpler to create a static nested class unless you really need it to be an inner class.

It's a bit like why I always make my variables final in Java - if they're not final, I know there's something funny going on with them. If you use an inner class instead of a static nested class, there should be a good reason.

One of the reasons for static vs. normal have to do with classloading. You cannot instantiate an inner class in the constructor of it's parent.

PS: I've always understood 'nested' and 'inner' to be interchangeable. There may be subtle nuances in the terms but most Java developers would understand either.

Well, for one thing, non-static inner classes have an extra, hidden field that points to the instance of the outer class. So if the Entry class weren't static, then besides having access that it doesn't need, it would carry around four pointers instead of three.

As a rule, I would say, if you define a class that's basically there to act as a collection of data members, like a "struct" in C, consider making it static.

JVM knows no nested classes. Nesting is just syntactic sugar.

Below images shows Java file:

Below images show class files representation of the java file :

Notice that 2 class files are generated, one for parent and another for nested class.
Non-static nested class' objects have access to the enclosing scope. That access to the enclosing scope is maintained by holding an implicit reference of the enclosing scope object in the nested object

Nested class is a way to represent the intent that the nested class type represents a component of the parent class.

public class Message {

private MessageType messageType; // component of parent class

public enum MessageType {
    SENT, RECEIVE;
}
}



class Otherclass {

public boolean isSent(Message message) {
    if (message.getMessageType() == MessageType.SENT) { // accessible at other places as well
        return true;
    }
    return false;
}
}

private static nested class represents Point#3 & the fact the nested type can only be the subcomponent to the parent class. It can't be used separately.

public class Message {

 private Content content; // Component of message class

 private static class Content { // can only be a component of message class

  private String body;
  private int sentBy;

  public String getBody() {
     return body;
  }

  public int getSentBy() {
     return sentBy;
  }

}
}

class Message2 {
  private Message.Content content; // Not possible
}

More details here.

There are non-obvious memory retention issues to take into account here. Since a non-static inner class maintains an implicit reference to it's 'outer' class, if an instance of the inner class is strongly referenced, then the outer instance is strongly referenced too. This can lead to some head-scratching when the outer class is not garbage collected, even though it appears that nothing references it.

Adavantage of inner class--

one time use
supports and improves encapsulation
readibility
private field access

Without existing of outer class inner class will not exist.

class car{
    class wheel{

    }
}

There are four types of inner class.

normal inner class
Method Local Inner class
Anonymous inner class
static inner class

point ---

from static inner class ,we can only access static member of outer class.
Inside inner class we cananot declare static member .
inorder to invoke normal inner class in static area of outer class.

Outer 0=new Outer(); Outer.Inner i= O.new Inner();
inorder to invoke normal inner class in instance area of outer class.

Inner i=new Inner();
inorder to invoke normal inner class in outside of outer class.

Outer 0=new Outer(); Outer.Inner i= O.new Inner();
inside Inner class This pointer to inner class.

this.member-current inner class outerclassname.this--outer class
for inner class applicable modifier is -- public,default,

final,abstract,strictfp,+private,protected,static
outer$inner is the name of inner class name.
inner class inside instance method then we can acess static and instance field of outer class.

10.inner class inside static method then we can access only static field of

outer class.

class outer{

    int x=10;
    static int y-20;

    public void m1() {
        int i=30;
        final j=40;

        class inner{

            public void m2() {
                // have accees x,y and j
            }
        }
    }
}

Simple example :

package test;

public class UpperClass {
public static class StaticInnerClass {}

public class InnerClass {}

public static void main(String[] args) {
    // works
    StaticInnerClass stat = new StaticInnerClass();
    // doesn't compile
    InnerClass inner = new InnerClass();
}
}

If non-static the class cannot be instantiated exept in an instance of the upper class (so not in the example where main is a static function)

One of the reasons for static vs. normal have to do with classloading. You cannot instantiate an inner class in the constructor of it's parent.

PS: I've always understood 'nested' and 'inner' to be interchangeable. There may be subtle nuances in the terms but most Java developers would understand either.

Static inner class is used in the builder pattern. Static inner class can instantiate it's outer class which has only private constructor. You can not do the same with the inner class as you need to have object of the outer class created prior to accessing the inner class.

class OuterClass {
    private OuterClass(int x) {
        System.out.println("x: " + x);
    }
    
    static class InnerClass {
        public static void test() {
            OuterClass outer = new OuterClass(1);
        }
    }
}

public class Test {
    public static void main(String[] args) {
        OuterClass.InnerClass.test();
        // OuterClass outer = new OuterClass(1); // It is not possible to create outer instance from outside.
    }
}

This will output x: 1

There are non-obvious memory retention issues to take into account here. Since a non-static inner class maintains an implicit reference to it's 'outer' class, if an instance of the inner class is strongly referenced, then the outer instance is strongly referenced too. This can lead to some head-scratching when the outer class is not garbage collected, even though it appears that nothing references it.

From http://docs.oracle.com/javase/tutorial/java/javaOO/whentouse.html:

Use a non-static nested class (or inner class) if you require access to an enclosing instance's non-public fields and methods. Use a static nested class if you don't require this access.

JVM knows no nested classes. Nesting is just syntactic sugar.

Below images shows Java file:

Below images show class files representation of the java file :

Notice that 2 class files are generated, one for parent and another for nested class.
Non-static nested class' objects have access to the enclosing scope. That access to the enclosing scope is maintained by holding an implicit reference of the enclosing scope object in the nested object

Nested class is a way to represent the intent that the nested class type represents a component of the parent class.

public class Message {

private MessageType messageType; // component of parent class

public enum MessageType {
    SENT, RECEIVE;
}
}



class Otherclass {

public boolean isSent(Message message) {
    if (message.getMessageType() == MessageType.SENT) { // accessible at other places as well
        return true;
    }
    return false;
}
}

private static nested class represents Point#3 & the fact the nested type can only be the subcomponent to the parent class. It can't be used separately.

public class Message {

 private Content content; // Component of message class

 private static class Content { // can only be a component of message class

  private String body;
  private int sentBy;

  public String getBody() {
     return body;
  }

  public int getSentBy() {
     return sentBy;
  }

}
}

class Message2 {
  private Message.Content content; // Not possible
}

More details here.

Static inner class is used in the builder pattern. Static inner class can instantiate it's outer class which has only private constructor. You can not do the same with the inner class as you need to have object of the outer class created prior to accessing the inner class.

class OuterClass {
    private OuterClass(int x) {
        System.out.println("x: " + x);
    }
    
    static class InnerClass {
        public static void test() {
            OuterClass outer = new OuterClass(1);
        }
    }
}

public class Test {
    public static void main(String[] args) {
        OuterClass.InnerClass.test();
        // OuterClass outer = new OuterClass(1); // It is not possible to create outer instance from outside.
    }
}

This will output x: 1

Adavantage of inner class--

one time use
supports and improves encapsulation
readibility
private field access

Without existing of outer class inner class will not exist.

class car{
    class wheel{

    }
}

There are four types of inner class.

normal inner class
Method Local Inner class
Anonymous inner class
static inner class

point ---

from static inner class ,we can only access static member of outer class.
Inside inner class we cananot declare static member .
inorder to invoke normal inner class in static area of outer class.

Outer 0=new Outer(); Outer.Inner i= O.new Inner();
inorder to invoke normal inner class in instance area of outer class.

Inner i=new Inner();
inorder to invoke normal inner class in outside of outer class.

Outer 0=new Outer(); Outer.Inner i= O.new Inner();
inside Inner class This pointer to inner class.

this.member-current inner class outerclassname.this--outer class
for inner class applicable modifier is -- public,default,

final,abstract,strictfp,+private,protected,static
outer$inner is the name of inner class name.
inner class inside instance method then we can acess static and instance field of outer class.

10.inner class inside static method then we can access only static field of

outer class.

class outer{

    int x=10;
    static int y-20;

    public void m1() {
        int i=30;
        final j=40;

        class inner{

            public void m2() {
                // have accees x,y and j
            }
        }
    }
}