问题
How can I have a HashMap with unique keys in Java? Or even does this make any sense to have unique keys in HashMap or the keys are unique by default? I am a newbie. thx
回答1:
The keys are unique in all maps. The difference between the various maps implementations concern the possibility of null keys, the order of iteration and concurrency issues.
回答2:
Hash map key is unique. Add duplicate key, then it will be overwritten.
HashMap hm = new HashMap();
hm.put("1", new Integer(1));
hm.put("2", new Integer(2));
hm.put("3", new Integer(3));
hm.put("4", new Integer(4));
hm.put("1", new Integer(5));// value integer 1 is overwritten by 5
By default Hashmap is not synchronized.
回答3:
HasMap has unique keys. as .keySet() returns Set which has unique members
回答4:
HashMap is a collection to store (key,value) pairs and According to the documentation of HashMap the keys are always unique.
If you add a key which already exists(collision) in the hashmap, the old value will be replaced.
回答5:
A generic hashmap is usually implemented as an associative array, so let's say your array has N elements, from 0 to N-1, when you want to add a new (key, value) pair, what it's done behind the scenes is (just conceptually):
- index = hash(key) mod N
- array[index] = value
So, by construction, a key is mapped to one and one only array entry.
Please note that it's actually a bit more complex than this: I am ignoring on purpose things like collision handling, rehashing, etc, you may have a good general idea here https://en.wikipedia.org/wiki/Hash_table
回答6:
Try to look at the Java API for Map which is interface that HashMap implements. The first sentence is:
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
来源：https://stackoverflow.com/questions/8923273/how-can-i-have-a-hashmap-with-unique-keys-in-java