Sometimes,
we can declare classes inside the class, such type of classes are called inner
classes. Inner class concept was introduced in java 1.1v. It supports AWT and
string components event handling.
Need of an inner class – Inner class
is used for creating an object logically inside another object with clear
separation of properties region. Hence, we can know that a particular object
belongs to which object.
Example –
public class
Test1 {
int empno;
String ename;
class
Test2{
int hno;
String city;
String state;
}
}
Explanation – If the
Test2 properties are needed for more than one outer object, then we must create
Test2 class as outer class, and we should create this class object in that
outer object class with HAS-A relation.
Based
on the purpose and position of declaration, all the inner classes are divided
into following four types –
1. Normal or Regular inner class – If we
declare any named class inside a class directly without the static modifier,
such type of inner classes are called Normal or Regular inner classes.
Example –
public class
Test1 {
int empno;
String ename;
class
Test2{
int hno=10;
String city;
String state;
}
public static void
main(String[] args){
System.out.println("Outer classes");
Test1 t = new
Test1();
Test1.Test2
t1 = t.new
Test2();
System.out.println(t1.hno=10);
}
}
Output –
Outer classes
10
Note – The applicable modifiers
for outer classes are public, default, final, abstract, strictfp, but for inner
classes the applicable modifiers are public, default, final, abstract,
strictfp, private, protected, static.
2. Method local inner classes – Sometimes,
we can declare a class inside a method and such type of inner classes are
called as method local inner classes. The main objective of method inner class
is to define method specific repeatedly requiring functionality. Method local
inner class can fulfill the nested method requirement. We can access the method
local inner class only within the method in which we have declare it (i.e.) we
can’t access from outside of the method. Hence, this method local inner classes
are most rarely used type of inner classes because of their lesser scope.
Example –
public class
Test1 {
public void
m1()
{
class
Test2{
public void
sum(int a,int b)
{
System.out.println(a+b);
}
}
Test2 t1 = new
Test2();
t1.sum(10, 20);
t1.sum(50, 60);
}
public static void
main(String[] args){
Test1 t = new
Test1();
t.m1();
}
}
Output –
30
110
3. Anonymous Inner class – Anonymous
class is one type of inner class which is a nameless class/interface. Like
other class, it is not individual class. The main objective of the anonymous
inner classes is for instant use. There are three types of anonymous inner
classes. They are –
- · Anonymous inner class that extends a class.
- · Anonymous inner class that implements an interface.
- · Anonymous inner class that defines inside method arguments.
Example –
class
Test2{
public void
display()
{
System.out.println("Test2 class display() method");
}
}
public class
Test1 {
public static void
main(String[] args){
Test2 t1 = new
Test2(){
public void
display()
{
System.out.println("Anonymous display() method");
}
};
t1.display();
Test2 t2 = new
Test2();
t2.display();
}
}
Output –
Anonymous display() method
Test2 class display() method
Explanation –
- · Test2 t2 = new Test2(); here, we are just creating an object for Test2.
- · Test2 t1 = new Test2(){ }; here, we create a child class for Test2 and for that child class we create an object with the parent reference.
Difference between General classes and
Anonymous inner classes
|
Anonymous Inner class
|
General Class
|
|
It can
extend only one class at a time.
|
A normal
java class can also extend only one class at a time.
|
|
It can
implement only one interface at a time.
|
It can
implement any number of interfaces simultaneously.
|
|
It can
extend a class or implement an interface but not both simultaneously.
|
It can
extend a class and can implement any number of interfaces simultaneously.
|
4. Static Nested class – Sometimes,
we can declare class with static modifiers. Such types of inner classes are
called as static inner class. In normal classes, inner class object is always
associated with outer class object. If the outer class object does not exist,
there is no chance of inner class object. The static nested classes nested
class object isn’t associated with outer class object, that is without existence
of outer class object also there may be a chance of existence of static nested
class object.
Example –
public class
Test1 {
static class
Test2{
public void
m1()
{
System.out.println("Static nested class method");
}
}
public static void
main(String[] args){
Test1.Test2
t = new
Test1.Test2();
t.m1();
//Static class without outer class
Test2 t1 = new
Test2();
t1.m1();
}
}
Output –
Static nested class method
Static nested class method
Difference between Normal Inner class and
Static Inner class
|
Normal Inner class
|
Static Nested class
|
|
Inner
class object is always associated with outer class object i.e., without
existence of outer class object there is no chance of existence of inner
class object.
|
Static
nested class object isn’t associated with outer class object i.e., without an
existence of outer class object also there may be a chance of existence of
static nested class object.
|
|
Inside
normal inner classes, we can’t declare static members.
|
Inside
static nested classes, we can declare the static members.
|
|
In
normal inner class, we can’t declare main method and hence we can’t invoke
inner class directly.
|
We can
declare main method inside the static nested classes and hence we can invoke
static nested class directly.
|
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