HashMap allows duplicates

Question

I have a doubt regarding HashMap  as we all know HashMap allows one null key and value pair  My question here is   If I wrote like this   m put null null   m put null a     Will it throw a  error or exception  or will it override the value or what will be the value of returing

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Hashmap type Overwrite that key if hashmap key is same key  map put  1   1111    map put  1   2222      output  key value 1 2222

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Doesn t allow duplicates in the sense  It allow to add you but it does nt care about this key already have a value or not  So at present for one key  there will be only one value   It silently overrides the value for null key  No exception   When you try  to get  the last inserted value with null will be return   That is not only with null and for any key   Have a quick example      Map m   new HashMap lt String  String gt        m put  1    a       m put  1    b       no exception    System out println m get  1       b

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Each key in a HashMap must be unique  When  quot adding a duplicate key quot  the old value  for the same key  as keys must be unique  is simply replaced  see HashMap put   Associates the specified value with the specified key in this map  If the map previously contained a mapping for the key  the old value is replaced  Returns the previous value associated with key  or null if there was no mapping for key   As far as nulls  a single null key is allowed  as keys must be unique  but the HashMap can have any number of null values  and a null key need not have a null value  Per the documentation       HashMap  permits null values and  a  null key   However  the documentation says nothing about null null needing to be a specific key value pair or null  quot a quot  being invalid

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m put null null      here key null  value null m put null a         here also key null  and value a   Duplicate keys are not allowed in hashmap  However value can be duplicated

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HashMap don t allow duplicate keys but since it s not thread safe it might occur duplicate keys  eg    while  true                final HashMap lt Object  Object gt  map   new HashMap lt Object  Object gt  2               map put  runTimeType   1               map put  title   2               map put  params   3               final AtomicInteger invokeCounter   new AtomicInteger                 for  int i   0  i  lt  100  i                      new Thread new Runnable                          Override                     public void run                             map put  formType   invokeCounter incrementAndGet                                              start                              while  invokeCounter intValue      100                    Thread sleep 10                             if  map size    gt  4       this means you insert two or more formType key to the map                System out println  JSONObject fromObject map

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Code example   HashMap lt Integer String gt  h   new HashMap lt Integer String gt       h put null null   h put null   a     System out println h     Output    null a    It overrides the value at key null

[java] HashMap allows duplicates?

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