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Java Tip 30: Polymorphism and Java
What to do when switch statements start reappearing in your code
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Anyway, back to the plot!
As Java doesn't have functions or pointers to functions, we can't port the C code to Java. However, Listing 5 demonstrates a mapping from an enumerated type to a function, and Listing 6 shows you how to achieve the same effect in Java.
//everthing else the same as Listing 4. public class Foo implements Traceable { private Hashtable map = new Hashtable(); private class MinimalTrace implements Traceable {/* impl */} private class VerboseTrace implements Traceable {/* impl */} public Foo() { //initialize map map.put( Mode.MIMINAL, new MinimalTrace() ); map.put( Mode.VERBOSE, new VerboseTrace() ); } public void Trace( OutputDevice device, Mode mode) { Traceable t=(Traceable)map.get( mode ); t.Trace( device ); } }
Foo now has innerclasses that implement the different levels of Trace() and uses a Hashtable to map the Mode to the implementation class. To improve the efficiency of the Trace() method, we can introduce a currentMode instance variable with an associated currentImpl to avoid the overhead of the Hashtable.get() method, as shown in Listing 7.
public class Foo implements Traceable
{
private Mode currentMode;
private Traceable currentImpl;
private Hashtable map = new Hashtable();
private class MinimalTrace implements Traceable {/* impl */}
private class VerboseTrace implements Traceable {/* impl */}
public Foo()
{
//init map as Listing 6
//assign currentMode and currentImpl to a default
}
public void Trace( OutputDevice device, Mode mode )
{
if(mode!=currentMode)
{
currentMode=mode;
currentImpl=(Traceable)map.get( mode );
}
currentImpl.Trace( device );
}
}
One final area of improvement would be to defer the instantiation of the implementation classes until the classes are required, a technique known as lazy instantiation. I will discuss lazy instantiation in depth in a future article, including how the implications and benefits for Java differ from those for C++.
I will leave it as an exercise for the reader to modify the design of Foo to incorporate lazy instantiation.
About the author
Philip Bishop is technical director and a consultant at Eveque Systems Ltd in the U.K. Eveque Systems specializes in the design and implementation of distributed object systems using Java, CORBA, C++, ODBMS, and so on.- Feedback
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this is actualy good. By Anonymous on October 13, 2009, 11:15 pmthis is actualy good.
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is it?By Anonymous on October 12, 2009, 9:48 pmwhy?
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well explained concepts..By Anonymous on September 14, 2009, 9:39 amwell explained concepts..
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smartBy Anonymous on July 29, 2009, 12:29 pmsmart way of getting rid of switch statement in c++.You use pointers to a functions, comparing it to java, You can use innerclasses which do operations for You.
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Java BooksBy Anonymous on May 6, 2009, 12:50 amthis is the java book
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