Using Enumerated Types as Array Indexes
Ada, the programming language mandated for the military for a time,
was a wonderful language that didn't deserve to die. But it did, and
now Java is alive and well and nobody seems to be doing anything at
all with Ada.
Java is a pretty handy language in its own right. But in Ada one
could define arrays to be indexed by enumerated types. Can Java do
that? If not, why not?
I wrote a piece of code that implements an array of <String>s, indexed
by objects of class <Coord>, an enumerated type, that I'm including
below. Obviously the functionality of this class could be copied to
make it possible to create _virtual_ arrays of _any_ element class,
indexed by _any_ enumerated type. Isn't this precisely the thing
generics were designed for? But how does one write a generic class to
implement an array indexed by enumerated types?
I've also included below three attempts to create a generic type that
implements an array indexed by enumerated types, but none of them
compile. Can anyone give me some pointers on this?
Kevin Simonson
Script started on Tue Aug 16 06:47:31 2011
sh-4.1$ ls
ArrayEnum.class Coord.class EnumArray2.java Offsetable.java
ArrayEnum.java EnumArray1.java EnumArray3.java
sh-4.1$ : First the Java file that _does_ work.
sh-4.1$ cat ArrayEnum.java
enum Coord { X_LFT, Y_LFT, X_RHT, Y_RHT }
public class ArrayEnum
{
String[] arrEnm;
public ArrayEnum ()
{
arrEnm = new String[ Coord.values().length];
}
public void set ( Coord index
, String element)
{
arrEnm[ index.ordinal()] = element;
}
public String get ( Coord index)
{
return arrEnm[ index.ordinal()];
}
public static void main ( String[] arguments)
{
if (0 < arguments.length && arguments.length % 2 == 0)
{ ArrayEnum demo = new ArrayEnum();
Coord[] allCoords = Coord.values();
Coord chosen;
int index;
for (Coord enm : Coord.values())
{ demo.set( enm, enm + "_orig_value");
}
System.out.println( "Before setting values:");
for (Coord enm : Coord.values())
{ System.out.println
( "demo.get( " + enm + ") == \"" + demo.get( enm) + "\".");
}
for (int arg = 0; arg < arguments.length; arg += 2)
{ index = -1;
for (;;)
{ if (++index == allCoords.length)
{ chosen = null;
break;
}
if (arguments[ arg].toUpperCase().equals( "" +
allCoords[ index]))
{ chosen = allCoords[ index];
break;
}
}
if (chosen != null)
{ System.out.println
( "demo.set( " + chosen + ", \"" + arguments[ arg + 1] +
");");
demo.set( chosen, arguments[ arg + 1]);
}
else
{ System.out.println
( "Couldn't match argument \"" + arguments[ arg]
+ "\" with a <Coord>
value!");
}
}
System.out.println( "After setting values:");
for (Coord enm : Coord.values())
{ System.out.println
( "demo.get( " + enm + ") == \"" + demo.get( enm) + "\".");
}
}
else
{ System.out.println( "Usage is");
System.out.println( " java ArrayEnum (<coord> <accompanying-
string>)+");
}
}
}
sh-4.1$ : I compile it.
sh-4.1$ javac ArrayEnum.java
sh-4.1$ : It compiles without error messages, so I run it with some
values.
sh-4.1$ java ArrayEnum x_rht Kevin x_lft Sandy y_rht Joshua
Before setting values:
demo.get( X_LFT) == "X_LFT_orig_value".
demo.get( Y_LFT) == "Y_LFT_orig_value".
demo.get( X_RHT) == "X_RHT_orig_value".
demo.get( Y_RHT) == "Y_RHT_orig_value".
demo.set( X_RHT, "Kevin);
demo.set( X_LFT, "Sandy);
demo.set( Y_RHT, "Joshua);
After setting values:
demo.get( X_LFT) == "Sandy".
demo.get( Y_LFT) == "Y_LFT_orig_value".
demo.get( X_RHT) == "Kevin".
demo.get( Y_RHT) == "Joshua".
sh-4.1$ : Then the three attempts at generic implementations, with the
sh-4.1$ : compilation error messages that accompanied them.
sh-4.1$ cat EnumArray1.java
import java.util.Iterator;
public class EnumArray1< Ty, En extends Enum< En>> implements Iterator
{
Ty[] enumArray;
En nextToRead;
public EnumArray1 ()
{
En[] vlues = En.values();
enumArray = new Ty[ vlues.length];
nextToRead = 0 < vlues.length ? vlues[ 0] : null;
}
public Ty get ( En index)
{
return enumArray[ index.ordinal()];
}
public void set ( En index
, Ty vlue)
{
enumArray[ index.ordinal()] = vlue;
}
public int size ()
{
return enumArray.length;
}
public boolean last ( En enm)
{
return enm.ordinal() + 1 == enumArray.length;
}
public boolean first ( En enm)
{
return enm.ordinal() == 0;
}
public En succ ( En enm)
{
int index = enm.ordinal() + 1;
return index < enumArray.length ? En.values()[ index] : null;
}
public En pred ( En enm)
{
int index = enm.ordinal() - 1;
return 0 <= index ? En.values()[ index] : null;
}
public boolean hasNext()
{
return nextToRead != null;
}
public Object next ()
{
Object nxtEnm = (Object) nextToRead;
nextToRead = next( nextToRead);
return nxtEnm;
}
public void remove ()
{
}
}
sh-4.1$ javac EnumArray1.java
EnumArray1.java:10: cannot find symbol
symbol : method values()
location: class java.lang.Enum<En>
En[] vlues = En.values();
^
EnumArray1.java:11: generic array creation
enumArray = new Ty[ vlues.length];
^
EnumArray1.java:44: cannot find symbol
symbol : method values()
location: class java.lang.Enum<En>
return index < enumArray.length ? En.values()[ index] : null;
^
EnumArray1.java:50: cannot find symbol
symbol : method values()
location: class java.lang.Enum<En>
return 0 <= index ? En.values()[ index] : null;
^
EnumArray1.java:61: next() in EnumArray1<Ty,En> cannot be applied to
(En)
nextToRead = next( nextToRead);
^
5 errors
sh-4.1$ cat EnumArray2.java
import java.util.Iterator;
public class EnumArray2< Ty, En extends Enum< En>> implements Iterator
{
Object[] enumArray;
En nextToRead;
public EnumArray2 ()
{
En[] vlues = En.values();
enumArray = new Object[ vlues.length];
nextToRead = 0 < vlues.length ? vlues[ 0] : null;
}
public Ty get ( En index)
{
return (Ty) enumArray[ index.ordinal()];
}
public void set ( En index
, Ty vlue)
{
enumArray[ index.ordinal()] = (Object) vlue;
}
public int size ()
{
return enumArray.length;
}
public boolean last ( En enm)
{
return enm.ordinal() + 1 == enumArray.length;
}
public boolean first ( En enm)
{
return enm.ordinal() == 0;
}
public En succ ( En enm)
{
int index = enm.ordinal() + 1;
return index < enumArray.length ? En.values()[ index] : null;
}
public En pred ( En enm)
{
int index = enm.ordinal() - 1;
return 0 <= index ? En.values()[ index] : null;
}
public boolean hasNext()
{
return nextToRead != null;
}
public Object next ()
{
Object nxtEnm = (Object) nextToRead;
nextToRead = next( nextToRead);
return nxtEnm;
}
public void remove ()
{
}
}
sh-4.1$ javac EnumArray2.java
EnumArray2.java:10: cannot find symbol
symbol : method values()
location: class java.lang.Enum<En>
En[] vlues = En.values();
^
EnumArray2.java:44: cannot find symbol
symbol : method values()
location: class java.lang.Enum<En>
return index < enumArray.length ? En.values()[ index] : null;
^
EnumArray2.java:50: cannot find symbol
symbol : method values()
location: class java.lang.Enum<En>
return 0 <= index ? En.values()[ index] : null;
^
EnumArray2.java:61: next() in EnumArray2<Ty,En> cannot be applied to
(En)
nextToRead = next( nextToRead);
^
Note: EnumArray2.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
4 errors
sh-4.1$ cat EnumArray3.java
import java.util.Iterator;
public class EnumArray3< Ty, En extends Offsetable, Enum> implements
Iterator
{
private static class Associated
{
Offsetable value;
Object element;
Associated ( Offsetable enm)
{ value = enm;
}
}
final En INIT_VALUE;
Associated[] enumArray;
En nextToRead;
private void fillValues ( Offsetable enm
, int index)
{
try
{ fillValues( enm.offsetBy( 1), index + 1);
}
catch (ArrayIndexOutOfBoundsException excptn)
{ enumArray = new Associated[ index + 1];
}
enumArray[ index] = new Associated( enm);
}
public EnumArray3 ( En initialValue)
{
INIT_VALUE = initialValue;
nextToRead = initialValue;
fillValues( (Offsetable) initialValue, 0);
}
public Ty get ( En index)
{
return (Ty) enumArray[ index.ordinal()].element;
}
public void set ( En index
, Ty vlue)
{
enumArray[ index.ordinal()].element = (Object) vlue;
}
public int size ()
{
return enumArray.length;
}
public boolean last ( En enm)
{
return enm.ordinal() + 1 == enumArray.length;
}
public boolean first ( En enm)
{
return enm.ordinal() == 0;
}
public boolean hasNext()
{
return nextToRead != null;
}
public Object next ()
{
Object nxtEnm = (Object) nextToRead;
nextToRead = nextToRead.offsetBy( 1);
return nxtEnm;
}
public void remove ()
{
}
}
sh-4.1$ javac EnumArray3.java
EnumArray3.java:40: cannot find symbol
symbol : method ordinal()
location: interface Offsetable
return (Ty) enumArray[ index.ordinal()].element;
^
EnumArray3.java:46: cannot find symbol
symbol : method ordinal()
location: interface Offsetable
enumArray[ index.ordinal()].element = (Object) vlue;
^
EnumArray3.java:56: cannot find symbol
symbol : method ordinal()
location: interface Offsetable
return enm.ordinal() + 1 == enumArray.length;
^
EnumArray3.java:56: operator + cannot be applied to
Offsetable.ordinal,int
return enm.ordinal() + 1 == enumArray.length;
^
EnumArray3.java:56: incomparable types: <nulltype> and int
return enm.ordinal() + 1 == enumArray.length;
^
EnumArray3.java:61: cannot find symbol
symbol : method ordinal()
location: interface Offsetable
return enm.ordinal() == 0;
^
EnumArray3.java:72: incompatible types
found : Offsetable
required: En
nextToRead = nextToRead.offsetBy( 1);
^
Note: EnumArray3.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
7 errors
sh-4.1$ exit
exit
Script done on Tue Aug 16 06:50:57 2011