In Java, besides having fields and methods, a class can also have other classes as members. Just like fields and methods, a class member of can be static or non-static. A non-static class member is called an inner class. Inner class is a programming construct based on the powerful concept of closure prevalent in the functional programming paradigm. It allows on-the-fly creation of objects, which can communicate transparently with the enclosing object inside of which they come into existence. We illustrate the rationale and use of inner classes via a progression from top level helper classes to named static nested classes, to non-static named classes, and to anonymous inner classes with dynamic on-the-fly instantiation.
1. helpers are the variants
Consider again the familiar problem of computing a
String representation of an
IList that displays a comma-separated list of its elements delimited by a pair of matching parentheses. We have seen at least one way to compute such a
String representation using a visitor called
ToStringAlgo . Below are three different algorithms,
ToString1 ,
ToString2 and
ToString3 , that compute the same
String representation.
Main Visitor
ToString1 |
Tail-recursive helper
ToString1Help |
package listFW.visitor;
import listFW.*;public class ToString1 implements IListAlgo
{public static final ToString1
Singleton = new ToString1();private ToString1() {}
public Object emptyCase(IMTList host, Object... nu) {
return "()";}public Object nonEmptyCase(
INEList host, Object... nu) {return host.getRest().execute(
ToString1Help.Singleton,"(" + host.getFirst());
}} |
/**
* Helps ToString1 compute the String* representation of the rest of the list.
* It takes as input the accumulated* string representation of the preceding
* list. This accumulated string contains* the left most parenthesis and all the
* elements of the preceding list, each* separated by a comma.
*/class ToString1Help implements IListAlgo {
public static final ToString1HelpSingleton = new ToString1Help();
private ToString1Help() {}public Object emptyCase(
IMTList host, Object... acc) {return acc[0] + ")";}public Object nonEmptyCase(
INEList host, Object... acc) {return host.getRest().execute(this,
acc[0]+ ", "
+ host.getFirst());}
} |
|
Main Visitor
ToString2 |
Tail-recursive helper
ToString2Help |
package listFW.visitor;
import listFW.*;public class ToString2 implements IListAlgo
{public static final ToString2
Singleton = new ToString2();private ToString2() {}public Object emptyCase(IMTList host,
Object... nu) {return "()";
}public Object nonEmptyCase(INEList host, Object... nu) {return host.getRest().execute(
ToString2Help.Singleton,host.getFirst().toString());
}} |
/**
* Helps ToString2 compute the Stringrepresentation of the rest of the list.
*/class ToString2Help implements IListAlgo {
public static final ToString2HelpSingleton = new ToString2Help();
private ToString2Help() {}public Object emptyCase(
IMTList host, Object... acc) {return "(" + acc[0]+ ")";
}public Object nonEmptyCase(
INEList host, Object... acc) {return host.getRest().execute(this,
acc[0]+ ", "
+ host.getFirst());}
} |
|
