问题
I'm using a HashMap: byte[] key and String value. But I realize that even I put the same object (same byte array and same string value) by using
myList.put(TheSameByteArray, TheSameStringValue)
into HashMap, the table still inserts a new object with different HashMapEntry. Then function containsKey() cannot work.
Can someone explains this for me? How can I fix this? Thanks. (Android Java)
@Override public boolean containsKey(Object key) {
if (key == null) {
return entryForNullKey != null;
}
int hash = Collections.secondaryHash(key);
HashMapEntry<K, V>[] tab = table;
for (HashMapEntry<K, V> e = tab[hash & (tab.length - 1)];
e != null; e = e.next) {
K eKey = e.key;
if (eKey == key || (e.hash == hash && key.equals(eKey))) {
return true;
}
}
return false;
}
回答1:
A byte[]
(or any array) can't work properly as a key in a HashMap
, since arrays don't override equals
, so two arrays will be considered equal only if they refer to the same object.
You'll have to wrap your byte[]
in some custom class that overrides hashCode
and equals
, and use that custom class as the key to your HashMap.
回答2:
Adding to Eran's clear answer,Since byte[] or any array doesnt override hashcode and equals(it uses the default methods of Object class ),you can always wrap around a String Object which takes byte[] as constructor argument.Not only does String form good keys in Map,they are immutable too(the operations in a Hash based map are faster)
http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#String(byte[])
回答3:
NOTE: This is an extremely hack-y way of making an array or a string, a key in a HashMap, without overriding either the equals() or the hashCode() methods. I'll include the answer in a generic way, so readers can get the idea and implement as per their requirements.
Say, I have two numbers, n
and r
. I want a key-value pair with [n,r]
as the key, and (n+r)
as the value.
Map<List<Integer>, Integer> map = new HashMap<List<Integer>, Integer>();
List<Integer> key = Arrays.asList(n, r);
if( map.containsKey(key) )
return map.get(key);
What if the map did not contain the key?
map.put(Collections.unmodifiableList(Arrays.asList(n, r)), (n+r));
The unmodifiable
part (without going into any further depth), ensures that the key cannot change the hash code.
Now, map.containsKey(key)
will be true.
Note: This is not a good way to do it. It is just a workaround.
来源:https://stackoverflow.com/questions/31938180/hashmap-with-byte-array-key-and-string-value-containskey-function-doesnt-wo