Inner class and nested static class in Java both are classes declared
inside another class, known as top
level class in Java. In Java terminology, If you declare a nested class static,
it will called nested static class in Java while non static nested class are
simply referred as Inner Class. Inner classes are also a popular topic in Java
interviews. One of the popular question is difference between inner class and
nested static class , some time also refereed as difference between static and non static nested class in Java. One of the most
important question related to nested classes are Where should you use
nested class in Java? This is one of the tricky Java question, If you have read Effective Java from Joshua Bloch than in
one chapter he has suggested that if a class can only be useful to one
particular class, it make sense to keep that inside the class itself otherwise declare it as top level class.
Nested class improves Encapsulation
and help in maintenance. I actually look JDK itself for examples and if you
look HashMap class, you will find that Map.Entry is a good
example of nested class in Java. Another popular use of nested classes are
implementing Comparator
in Java e.g. AgeCompartor for Person class.
Since some time Comparator is also tied with a particular
class, it make sense to declare them as nested static class in Java. In this Java tutorial we will see what is Inner
class in Java, different types of Inner classes, what is static nested class
and finally difference between static nested class and inner class in Java.
What is Inner Class in Java
Any class which is not a top level or declared inside another class is known
as nested class and out of those nested classes, class which are declared non
static are known as Inner class in Java. there are three kinds of Inner
class in Java:
- Local inner class
- Anonymous inner class
- Member inner class
Local inner class is declared inside a code block
or method. Anonymous inner class is a class which
doesn't have name to reference and initialized at same place where it gets
created. Member inner class is declared as non
static member of outer class. Now with Inner class first question comes in mind
is when to use Inner class in Java, simple answer is any class which is only be
used by its outer class, should be a good candidate of making inner. One of the
common example of Inner classes are Anonymous class which is used to implement Thread or
EventListeners like ActionListerner in Swing, where they only
implement key methods like run()
method of Thread class or actionPerformed(ActionEvent ae).
Here are some important properties of Inner classes in Java:
1) In order to create instance of Inner class, an instance of outer class
is required. Every instance of inner class is bounded to one instance of Outer
class.
2) Member inner class can be make private,
protected or public. its just like any other member of class.
3) Local inner class can not be private, protected or public because
they exist only inside of local block or method. You can only use final
modifier with local inner class.
4) Anonymous Inner class are common to implement Runnable
or CommandListener where you just need to implement one method. They
are created and initialized at same line.
5) You can access current instance of Outer class, inside inner class as Outer.this
variable.
Inner class Example in Java
Here is an example of member inner class, local inner class and anonymous
inner class. For simplicity we have combined all examples of different inner
class in one.
public class
InnerClassTest {
public static void
main(String args[]) {
//creating local
inner class inside method
class Local
{
public
void name()
{
System.out.println("Example of Local
class in Java");
}
}
//creating instance
of local inner class
Local local = new
Local();
local.name(); //calling method from local inner class
//Creating anonymous
inner class in java for implementing thread
Thread anonymous = new Thread(){
@Override
public
void run(){
System.out.println("Anonymous class
example in java");
}
};
anonymous.start();
//example of creating
instance of inner class
InnerClassTest test = new
InnerClassTest();
InnerClassTest.Inner
inner = test.new Inner();
inner.name(); //calling method of inner class
}
/*
* Creating Inner class in Java
*/
private class Inner{
public void name(){
System.out.println("Inner class
example in java");
}
}
}
Output:
Example of Local class in Java
Inner class example in java
Anonymous class example in java
What is nested static class in Java
Nested static class is another class which is declared inside a class as
member and made static. Nested static class is also declared as member of outer
class and can be make private,
public or protected like any other member. One of the
main benefit of nested static class over inner class is that instance of nested
static class is not attached to any enclosing instance of Outer class. You also
don't need any instance of Outer class to create instance of nested static
class in Java. This makes nested static class very convenient to use and
access.
Nested
Static Class Example in Java
Here is an example of nested static class in Java. It look exactly
similar to member inner classes but has quite a few significant difference with
them, e.g. you can access them inside main
method because they are static. In order to create instance of nested
static class, you don’t need instance of enclosing class. You can refer them
with class name and you can also import them using static import
feature of Java 5.
public class
NestedStaticExample {
public static void
main(String args[]){
StaticNested nested = new
StaticNested();
nested.name();
}
//static nested class in java
private static class
StaticNested{
public void name(){
System.out.println("static nested
class example in java");
}
}
}
Though this is very trivial example of nested static class, it
demonstrate some properties of nested static class. Better example of nested
static class can be implementing a custom Comparator e.g. OrderByAmount in How to sort Object in Java using Comparator.
Difference between Inner class and nested
static class in Java.
Both static and non
static nested class or Inner needs to declare inside enclosing class in
Java and that’s why they are collectively known as nested classes but they have couple of differences as shown
below:
1) First and most important difference
between Inner class and nested static class is that Inner class require instance
of outer class for initialization and they are always associated with instance
of enclosing class. On the other hand nested static class is not associated
with any instance of enclosing class.
2) Another difference between Inner class and nested static class is that
later uses static keyword in there class declaration, which means they are
static member of class and can be accessed like any other static member of
class.
3) Nested static class can be imported using static
import in Java.
4) One last difference between Inner class and nested static class is
that later is more convenient and should be preferred over Inner class while
declaring member classes.
That's all on What is Inner class and nested static class in Java.
We have seen local, anonymous and member inner classes and difference between
Inner class and nested static class in Java. Worth noting is where to use
nested static class or Inner class ? Joshua Bloch has suggested to prefer nested static class over Inner
classes in his book Effective Java. Some time Interviewer ask you to write
code to create instance of inner class which can be tricky if you haven't used
them recently. Just remember that every inner class instance is associated with
one outer class instance.
Source:http://javarevisited.blogspot.com/2012/12/inner-class-and-nested-static-class-in-java-difference.html
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