Passing enum and EnumMap to a method

[~kurt]

Generics are really new to me, and catching on has been a bit confusing.
The Sun tutorial is like a man page - you gotta read it a few times and
I've been too impatient to do so. I've actually been reading through
the java.util.EnumMap code to get a better grip on things. Since all
my googling didn't turn up an example of how to pass enums (and EnumMaps),
I figured I'd post it here for others if they have the same question.
Some of the recent threads on this topic were very helpful to me, but
I still needed to put it all together.

If anyone sees anything stupid (or any suggestions for improvement,
please let me know).


import java.util.*;

public class TestEnumMap {

enum ThreeD { X, Y, Z }

public static void main(String[] args) {

double dval;

EnumMap<ThreeD, Double> xyz =
new EnumMap<ThreeD, Double>(ThreeD.class);

xyz.put(ThreeD.X, 5.0);
xyz.put(ThreeD.Y, 10.0);
xyz.put(ThreeD.Z, 15.0);
System.out.println("Not Passed");
for (ThreeD i : ThreeD.values()) {
dval = xyz.get(i);
System.out.println("" + i + ": " + dval);
}

printVals(xyz);
printVals(ThreeD.class, xyz);
}
public static void printVals (EnumMap<ThreeD, Double> em) {
System.out.println("Passed, but with just the EnumMap:");
double dval;
for (ThreeD i : ThreeD.values()) {
dval = em.get(i);
System.out.println("" + i + ": " + dval);
}
}

public static <K extends Enum<K>> void printVals(Class<K> keyType,
EnumMap<ThreeD, Double> em) {
double dval;
K[] keyUniverse = keyType.getEnumConstants();
System.out.println("Everything Passed");
for (int i=0; i<keyUniverse.length; i++) {
dval = em.get(keyUniverse);
System.out.println("" + keyUniverse + ": " + dval);
}
}
}


- Kurt

 

[Tom Hawtin]

public static <K extends Enum<K>> void printVals(Class<K> keyType,
EnumMap<ThreeD, Double> em) {
double dval;
K[] keyUniverse = keyType.getEnumConstants();

The question didn't jump out at me, but I'm guessing you are asking how
to get all the elements of an enum given an EnumMap with the enum as the
key type.

AFAIK, the only guaranteed way to obtain the 'keyClass' that I can see
is through serialisation - not pleasant.

All you really can do is pass the information through separately.
However, your method declaration can be tidied up a little:

private static <K extends Enum<K>> void printVals(
Class<K> keyType, Map<K, ?> map
) {

Tom Hawtin

 

[~kurt]

The question didn't jump out at me, but I'm guessing you are asking how
to get all the elements of an enum given an EnumMap with the enum as the
key type.

Yep - just experimenting - seeing how I might use an EnumMap in the future.

private static <K extends Enum<K>> void printVals(
Class<K> keyType, Map<K, ?> map
) {

Thanks,

- Kurt

 

[Tom Hawtin]

The question didn't jump out at me, but I'm guessing you are asking how
to get all the elements of an enum given an EnumMap with the enum as the
key type.

AFAIK, the only guaranteed way to obtain the 'keyClass' that I can see
is through serialisation - not pleasant.

Ah, it seems that you can extract the full set of elements for the enum,
so long as you have a non-empty key-set. Of course if you have an
example element you can getClass, find the enum class and then all the
elements yourself.

import java.util.*;

class EnumMapToAllKeys {
public static void main(String [] args) {
Map<Thread.State, String> map =
new EnumMap<Thread.State, String>(Thread.State.class);
map.put(Thread.State.BLOCKED, "block");
map.put(Thread.State.TIMED_WAITING, "timed");
print(map);
}
/**
* @throws IllegalArgumentException if map is empty(!)
*/
private static <K extends Enum<K>> void print(Map<K, ?> map) {
EnumSet<K> all = EnumSet.copyOf(map.keySet());
all.addAll(EnumSet.complementOf(all));
for (K key : all) {
System.out.println(key+" => "+map.get(key));
}
}
}

It would work properly if EnumSet.copyOf could extract the class
information from EnumMap.keySet. Perhaps an RFE (and patch) is in order
to add that sort of functionality across collections.

Tom Hawtin

 

[Daniel Pitts]

The question didn't jump out at me, but I'm guessing you are asking how
to get all the elements of an enum given an EnumMap with the enum as the
key type.

AFAIK, the only guaranteed way to obtain the 'keyClass' that I can see
is through serialisation - not pleasant.

Ah, it seems that you can extract the full set of elements for the enum,
so long as you have a non-empty key-set. Of course if you have an
example element you can getClass, find the enum class and then all the
elements yourself.

import java.util.*;

class EnumMapToAllKeys {
public static void main(String [] args) {
Map<Thread.State, String> map =
new EnumMap<Thread.State, String>(Thread.State.class);
map.put(Thread.State.BLOCKED, "block");
map.put(Thread.State.TIMED_WAITING, "timed");
print(map);
}
/**
* @throws IllegalArgumentException if map is empty(!)
*/
private static <K extends Enum<K>> void print(Map<K, ?> map) {
EnumSet<K> all = EnumSet.copyOf(map.keySet());
all.addAll(EnumSet.complementOf(all));
for (K key : all) {
System.out.println(key+" => "+map.get(key));
}
}

}

It would work properly if EnumSet.copyOf could extract the class
information from EnumMap.keySet. Perhaps an RFE (and patch) is in order
to add that sort of functionality across collections.

Tom Hawtin

Actually, it would be nice if EnumMap added the method Class<K>
getKeyType() to the interface.