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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
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A HashMap contains values based on the key. It contains only unique elements.
Key notes on internal working of HashMap Key object's hashCode() is required to calculate the index location of Entry object. Key object's equals() method is used to maintain uniqueness of keys in map.
HashMap stores key, value pairs and it does not allow duplicate keys. If the key is duplicate then the old key is replaced with the new value.
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.
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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.
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