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https://issues.apache.org/jira/browse/COLLECTIONS-747?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17034241#comment-17034241
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Walter Laan edited comment on COLLECTIONS-747 at 2/11/20 8:34 AM:
------------------------------------------------------------------
No, you will need the actual array Class to make a copy. See
java.util.ArrayList#toArray() which return Object[] and toArray(T[]) which
creates a copy. Or you could use the class from a key value, but that won't
work if all of them are null, which is not forbidden as far as I checked.
For example, if want to keep the minimum of two keys, you have to do:
{code:java}
@SafeVarargs
public MultiKey(K key1, K key2, K... keys) {
int length = keys.length + 2;
@SuppressWarnings("unchecked") // cast is correct because
K[].getClass() is passed
K[] tmpKeys = (K[]) Arrays.copyOf(keys, keys.length + 2,
keys.getClass());
for(int i = length - 1; i > 1; i--) {
tmpKeys[i] = tmpKeys[i - 2];
}
tmpKeys[0] = key1;
tmpKeys[1] = key2;
this.keys = tmpKeys;
}
// which then allows null keys, though need to specify type instead of just
diamond (<>)
MultiKeyClassCastException[] keys = new MultiKey<>(key1, key2).getKeys();
MultiKeyClassCastException[] keys2 = new MultiKey<>(key1, key2, key3).getKeys();
MultiKeyClassCastException[] keys3 = new
MultiKey<MultiKeyClassCastException>(null, null).getKeys();
{code}
[https://stackoverflow.com/questions/529085/how-to-create-a-generic-array-in-java]
[https://softwareengineering.stackexchange.com/a/331109]
was (Author: wlaan):
No, you will need the actual array Class to make a copy. See
java.util.ArrayList#toArray() which return Object[] and toArray(T[]) which
creates a copy. Or you could use the class from a key value, but that won't
work if all of them are null, which is not forbidden as far as I checked.
For example, if want to keep the minimum of two keys, you have to do:
{code:java}
@SafeVarargs
public MultiKey(K key1, K key2, K... keys) {
int length = keys.length + 2;
@SuppressWarnings("unchecked") // cast is correct because
K[].getClass() is passed
K[] tmpKeys = (K[]) Arrays.copyOf(keys, keys.length + 2,
keys.getClass());
for(int i = length - 1; i > 1; i--) {
tmpKeys[i] = tmpKeys[i - 2];
}
tmpKeys[0] = key1;
tmpKeys[1] = key2;
this.keys = tmpKeys;
}
// which then allows null keys, though need to specify type instead of just
diamond (<>)
MultiKeyClassCastException[] keys = new MultiKey<>(key1,
key2).getKeys();
MultiKeyClassCastException[] keys2 = new MultiKey<>(key1, key2,
key3).getKeys();
MultiKeyClassCastException[] keys3 = new
MultiKey<MultiKeyClassCastException>(null, null).getKeys();
{code}
[https://stackoverflow.com/questions/529085/how-to-create-a-generic-array-in-java]
[https://softwareengineering.stackexchange.com/a/331109]
> MultiKey.getKeys class cast exception
> -------------------------------------
>
> Key: COLLECTIONS-747
> URL: https://issues.apache.org/jira/browse/COLLECTIONS-747
> Project: Commons Collections
> Issue Type: Bug
> Components: KeyValue
> Affects Versions: 4.4
> Reporter: Walter Laan
> Priority: Major
>
> When using an non-array constructor of MultiKey, an Object[] is created,
> which cannot be cast to a K[]
> {code}
> import org.apache.commons.collections4.keyvalue.MultiKey;
> public class MultiKeyClassCastException {
> public static void main(String[] args) {
> MultiKeyClassCastException key1 = new MultiKeyClassCastException();
> MultiKeyClassCastException key2 = new MultiKeyClassCastException();
> MultiKeyClassCastException[] keys = new MultiKey<>(key1,
> key2).getKeys();
> }
> }
> // running gives (same error if in module):
> Exception in thread "main" java.lang.ClassCastException: class
> [Ljava.lang.Object; cannot be cast to class [LMultiKeyClassCastException;
> ([Ljava.lang.Object; is in module java.base of loader 'bootstrap';
> [LMultiKeyClassCastException; is in unnamed module of loader 'app')
> at MultiKeyClassCastException.main(MultiKeyClassCastException.java:8)
> {code}
> AFAIK, the only way to fix this (with source compatibility), is to only have
> a varargs constructors. but I think this breaks binary compatibility though
> and cannot be used with the existing constructors.
> {code}
> @SafeVarargs
> public MultiKey(K... keys) {
> this.keys = keys;
> }
> {code}
> Workaround is to use array constructor with correct typed array or
> {code}Object[] keys = multiKey.getKeys();{code} explicitly.
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