1. What is java nested class
The Java programming language allows you to define a class within another class. Such a class is called nested class. A java nested class is a class within a class.
The relationship between outer class and inner class is not IS-A relationship and it is Has-A relationship. Interfaces also can be nested. Following are possible inner classes.
1.1. Example 1 a class with in class
//outer class
class OuterClass {
//inner class
class InnerClass {
}
}
1.2. Example 2 an interface with in interface
//outer interface
interface OuterInterface {
//inner interface
interface InnerInterface {
}
}
1.3. Example 3 an interface within a class
//outer class
class OuterClass {
//inner interface
interface InnerInterface {
}
}
1.4. Example 4 a class with in an interface
//outer interface
interface OuterInteerface {
//inner class
class InnerClass {
}
}
1.5. Example 5 a static class or interface with in a class
//outer class
public class OuterClass {
static interface IInner{
}
static class Inner implements IInner{
}
}
1.6. Example 6 a class with in method or block
public class MethodLocalOuterClass {
// Inner class with in instance block
{
class InstanceBlockInner{
}
}
// Inner class with in static block
static {
class StaticBlockInner{
}
}
// Inner class with in method
public void test() {
class MethodLocalInner{
}
}
}
1.7. Example 7 a name less class as argument to a method
public class AnonymousLocalExample {
public static void main(String[] args) {
String[] sarray = {"Peter", "Mike", "Zaheer", "John"};
// anonymous class as method argument
Arrays.sort(sarray, new Comparator<String>() {
@Override
public int compare(String o1, String o2) {
return o2.compareTo(o1);
}
});
System.out.println(Arrays.asList(sarray));
}
}
Output :
[Zaheer, Peter, Mike, John]
2. Types of Nested classes
Based on the purpose and position of declaration all Java nested classes are divided into 4 types.
1. Normal or Regular inner classes (Non static)
2. Local Classes
3. Anonymous inner classes
4. Static nested classes
Nested classes are divided into two categories: static and non-static. Nested classes that are declared static are called static nested classes. Non-static nested classes are called inner classes. Following image illustrates various types of nested classes in java.
3. Why do we use java nested class?
- It is a way of logically grouping classes that are only used in one place.
- It increases encapsulation.
- It can lead to more readable and maintainable code.
- Without existing one type of object if there is no chance of existing another type of object then we can go for inner classes.
4. Conclusion
In this article we have covered high level overview of Java nested class, what are the kinds of nested classes based on purpose and position of declaration with basic examples.
