Inner classes in Java, the mystery within.

Inner classes, also called Nested Classes, are nothing but classes that are defined within other classes. The nesting is a relationship between classes, not objects. Inner classes have clearly two benefits, name control & access control. In Java, this benefit is not as important because Java packages give the name control. Java inner classes have feature that makes them richer and more useful. An object of an inner class has an implicit reference to the outer class object that instantiated it. Through this pointer, it gains access to any variable of the outer object. Only static inner classes don’t have this pointer. It is actually invisible when we write the code, but compiler takes care of it. Inner classes are actually a phenomenon of the compiler and not the JVM. Inner classes may be defined with following access modifiers : public, protected, private, or with default package access. The syntax for inner class is as follows:

[modifiers] class OuterClassName {
    code...
    [modifiers] class InnerClassName {
        code....
    }
}
Code language: Java (java)

Inner Classes:

Following properties can be noted about Inner classes:

• The outer class (the class containing the inner class) can instantiate as many number of inner class objects as it wishes, inside it’s code.
• If the inner class is public & the containing class as well, then code in some other unrelated class can as well create an instance of the inner class.
• In above case the inner class can be created as follows:

<OuterClassName> outerObj = new <OuterClassName>(arguments);
	outerObj.<InnerClassName> innerObj = outerObj.new <InnerClassName>(arguments);
Code language: Java (java)

• No inner class objects are automatically instantiated with an outer class object.
• If the inner class is static, then static inner class can be instantiated without an outer class instance, otherwise, the inner class object must be associated with an instance of the outer class.
• Inner class code has free access to all elements of the outer class object that contains it, by name (no matter what the access level of the elements is), if the inner class has a varible with same name then the outer class’s variable can be accesse like this:

<OuterClassName>.this.<variableName>
Code language: Java (java)

• The outer class can call even the private methods of the inner class.

Static Inner Classes:

Syntax for static inner class is as follows:

<access-specifier> class OuterClassName {
    public static class <StaticInnerClassName> {
        . . .
    }
    . . .
}
Code language: Java (java)

for static inner classes following additional properties hold:

• Static members of the outer class are visible to the static inner class, what ever their access level be.
• Non-static members of the outer class are not available, because there is not instance of the outer class.
• An inner class may not have static members unless the inner class is itself marked as static.
• Sometimes static nested class are not refered to as inner class at all, as they don’t require outer classes instance.
• A static inner class is just like any other inner class, but it dose not have the reference to its outer class object that generated it.

There are two more types of inner classes, i.e local inner classes & anonymous inner classes. The local inner class are defined within a method. Anonymous inner classes are also defined with in a method but have no name.

Local Inner Classes:

Syntax of the local inner class is as follows:

<access-specifier> class <OuterClassName> {
	code...
	<access-specifier> <return-type> <MethodName>(<arguments>){
		class <LocalInnerClassName>{
			code...
		}
		code...
	}
	code...
}
Code language: Java (java)

• Local classes are never declared with an access specifier (that is, public or private). Their scope is always restricted to the block in which they are declared.
• Local classes have a great advantage: they are completely hidden from the outside world.
• They can not only access the instance variables but local variables of the method (in which they are defined) as well, but the local varible has to be declared final.

Anonymous Inner Classes

When using local inner classes, you can often go a step further. If you want to make only a single object of this class, you don’t even need to give the class a name. Such a class is called an anonymous inner class. Usually the inner class extend some interface or extend other class. This syntax for anonymous classes is very cryptic.

new SuperType(construction parameters) {
	inner class methods and data
}
Code language: Java (java)

• Here, SuperType can be an interface, such as ActionListener; then, the inner class implements that interface. Or SuperType can be a class; then, the inner class extends that class.

• An anonymous inner class cannot have constructors because the name of a constructor must be the same as the name of a class, and the class has no name. Instead, the construction parameters are given to the superclass constructor. In particular, whenever an inner class implements an interface, it cannot have any construction parameters. Nevertheless, you must supply a set of parentheses as in

new InterfaceType () { methods and data }
Code language: Java (java)

• It is recommended to refrain from using them as many programmers find too many anonymous classes hard to read.

The following table shows the types of nested classes:

Types of Nested Classes

Type	Scope	Inner
static nested class	member	no
inner [non-static] class	member	yes
local class	local	yes
anonymous class	only the point where it is defined	yes

Class Files Generation for Inner Classes

As we mentioned earlier each class can have more than one inner classes. Once the main class is compiled which has several inner classes, the compiler generates separate class files for each of the inner class. Have a look at below example.

// Main class
public class Main {

	// Inner class Test1
	class Test1 {
	}

	// Inner class Test2
	class Test2 {
	}


	public static void main(String [] args) {
	
		// Anonymous inner class 1
		new Object() {
		};

		// Anonymous inner class 2
		new Object() {
		};

		System.out.println("Hello World");
	}
}
Code language: Java (java)

Here we have a Main class which has four inner classes. Test1, Test2, Anonymous inner class 1 and Anonymous inner class 2. Once we compile this class using javac command, the compiler will generates following class files.

Main.class
Main$Test1.class
Main$Test2.class
Main$1.class
Main$2.class
Code language: Java (java)