Java Learning Notes (1) __java

HashMap        It is a map,which means it stores <key, value> pairs. Same key with different Valueswill is stored once in the "map" and only the "last one" into map would bestored, i.e. W Hen a value into the map, if the key already exists, thevalue'll be replaced by the new one, which means for you wil L LOSE the value.
       It uses hashfunction. Objects for key must override the Hashcode () method inherited the Fromclass object. HashMap Re-hash The returned value of Hashcode () to find Thebucket. The reason of re-hashing is to avoid poor quality hash functions.
       When twodifferent keys have the same hash code, Euqals () method comes to rescue . The
       null key andnull value are allowed in HashMap.
       Note This theimplementation of HASHMAP is not synchronized. ArrayList VS Vector · Vector issynchronized while ArrayList are not.
· Vector is slow as it thread safe. In Comparisonarraylist is fast
· By default, a vector double the size of it array whenneeded, and ArrayList increases its array size by 50% Thread Safety The following description is from Wikipedia.
· Thread Safety ISA computer Programming concept applicable in the context of Multi-threadprograms
· A piece of Codeis thread safe if it manipulatesshared data structures only in a manner that guarantees safe execution Bymu Ltiple threads at the same time.