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5. list visitor examples
Singleton constructor for list algorithm
/**
* Computes the length of the IList host.*/
public class GetLength implements IListAlgo {/**
* Singleton Pattern.*/
public static final GetLength Singleton = new GetLength();private GetLength() { } |
Empty and non-empty cases of algorithm
/*** Returns Integer(0).
* @param nu not used* @return Integer(0)
*/public Object emptyCase(
MTList host, Object... nu) {return 0;
} |
/*** Return the length of the host's rest
* plus 1.* @param nu not used.
* @return Integer>0.
*/public Object nonEmptyCase(
NEList host, Object... nu) {Object restLen
= host.getRest().execute(this);return 1 + (Integer)restLen);
} |
Singleton constructor for list algorithm
package listFW;/**
* Computes a String reprsentation of IList showing a left parenthesis followed* by elements of the IList separated by commas, ending with with a right parenthesis.
*/public class ToStringAlgo implements IListAlgo {
public static final ToStringAlgo Singleton = new ToStringAlgo();private ToStringAlgo() { } |
Empty and non-empty cases of algorithm
/*** Returns "()".
*/public Object emptyCase(
MTList host, Object... inp) {return "()";
} |
/*** Passes "(" + first to the rest of IList* and asks for help to complete the
* computation.*/
public Object nonEmptyCase(NEList host, Object... inp) {
return host.getRest().execute(ToStringHelper.Singleton,
"(" + host.getFirst());}
} //closing parenthesis for the class |
Singleton constructor for helper visitor
/**
* Helps ToStringAlgo compute the String representation of the rest of the list.*/
class ToStringHelper implements IListAlgo {public static final ToStringHelper Singleton = new ToStringHelper();
private ToStringHelper() { } |
Empty and non-empty cases of algorithm
/**
* Returns the accumulated String + ")".* At end of list: done!
*/public Object emptyCase(
MTList host, Object... acc) {return acc[0] + ")";} |
/**
* Continues accumulating the String* representation by appending ", " + first
* to acc and recurse!*/
public Object nonEmptyCase(NEList host, Object... acc) {
return host.getRest().execute(this,
acc[0]+ ", " + host.getFirst());
}} |
We now can use to ToStringAlgo to implement the toString() method of an IList.
package listFW;public class MTList implements IList {
/*** Singleton Pattern
*/public final static MTList Singleton
= new MTList();private MTList() { }
/*** Calls the empty case of the
* algorithm algo, passing to it* itself as the host parameter
* and the given input inp as the* input parameter.
* This method is marked as final* to prevent all subclasses from
* overriding it. Finalizing a* method also allows the compiler
* to generate more efficient* calling code.
*/public final Object execute(
IListAlgo algo,Object... inp) {
return algo.emptyCase(this, inp);
}public String toString() {
return (String)ToStringAlgo.Singleton.emptyCase(this);
}} |
package listFW;public class NEList implements IList {
/*** The first data element.
*/private Object _first;
/*** The rest or "tail" of this NEList.
* Data Invariant: _rest != null;*/
private IList _rest;/**
* Initializes this NEList to a given* first and a given rest.
* @param f This NEList's first element* @param r !=null; rest of this NEList.
*/public NEList(Object f, IList r) {
_first = f;_rest = r;
}/**
* Returns the first data element of* this NEList.
* Marked final for the above reasons.*/
public final Object getFirst() {return _first;
}/**
* Returns the rest of this NEList which* is an IList.
* Marked final for the above reasons.*/
public final IList getRest() {return _rest;
}/**
* Calls the nonEmptyCase method of the* IListAlgo parameter, passing itself
* as the host parameter and the given* input as the input parameter.
* Marked final for the above reasons.*/
public final Object execute(IListAlgo algo, Object... inp) {
return algo.nonEmptyCase(this, inp);}
public String toString() {return (String)ToStringAlgo
.Singleton.nonEmptyCase(this);}
} |
Download the above code
here .
Source:
OpenStax, Design patterns. OpenStax CNX. Jun 04, 2009 Download for free at http://cnx.org/content/col10678/1.2
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