java collections - keyset vs entrySet in map

Question

I put a string array elements is a  map  where elements of string array is key and frequency of word is value  e g    String   args     if   it   is   to   be   it   is   up   me   to   delegate      then the map will have entries like   if 1  it 2         Set lt String gt  keys   m keySet    System out println  keyset of the map     keys     prints all keys   if   it   is   to   be   it   is   up   me   to   delegate   Set lt Map Entry lt String  Integer gt  gt  entrySet   m entrySet    Iterator lt Map Entry lt String  Integer gt  gt  i   entrySet iterator    while i hasNext         Map Entry lt String  Integer gt  element   i next        System out println  Key    element getKey      value    element getValue         prints all key values pairs    Using entry set prints all values   Key  if  value  1 Key  it  value  2 Key  is  value  2 Key  to  value  2 Key  be  value  1 Key  up  value  1 Key  me  value  1 Key  delegate  value  1   But the block of code below should print exactly the same output as above  but it does not   Iterator lt String gt  itr2   keys iterator    while itr2 hasNext           System out println itr1 next               System out println m get itr1 next              System out println  Key    itr2 next      value    m get itr2 next          It prints   Key  if  value  2 Key  is  value  2 Key  be  value  1 Key  me  value  1   But if we uncomment line 1 in the while loop i e       System out println itr1 next           and comment the line       System out println  Key    itr2 next      value    m get itr2 next        Then we get all keys    if   it   is   to   be   it   is   up   me   to   delegate     If we use m get   with itr2 next    then the iterator does not have few keys

User · Answer

Traversal over the large map entrySet   is much better than the keySet    Check this tutorial how they optimise the traversal over the large object with the help of entrySet   and how it helps for performance tuning

User · Answer

To make things simple   please note that every time you do itr2 next   the pointer moves to the next element i e  here if you notice carefully  then the output is perfectly fine according to the logic you have written   This may help you in understanding better   1st Iteration of While loop pointer is before the 1st element   Key  if  value  2   itr2 next   if  m get itr2 next   it   gt 2   2nd Iteration of While loop pointer is before the 3rd element   Key  is  value  2  itr2 next   is  m get itr2 next   to   gt 2   3rd Iteration of While loop pointer is before the 5th element   Key  be  value  1  itr2 next    be   m get itr2 next    up    gt  1    4th Iteration of While loop pointer is before the 7th element   Key  me  value  1  itr2 next    me   m get itr2 next    delegate    gt  1    Key  if  value  1 Key  it  value  2 Key  is  value  2 Key  to  value  2 Key  be  value  1 Key  up  value  1 Key  me  value  1 Key  delegate  value  1  It prints   Key  if  value  2 Key  is  value  2 Key  be  value  1 Key  me  value  1

User · Answer

Every time you call itr2 next   you are getting a distinct value  Not the same value  You should only call this once in the loop   Iterator lt String gt  itr2   keys iterator        while itr2 hasNext             String v   itr2 next            System out println  Key    v    value    m get v

User · Answer

An Iterator moves forward only  if it read it once  it s done  Your   m get itr2 next       is reading the next value of itr2 next     that is why you are missing a few  actually not a few  every other  keys

User · Answer

Every call to the Iterator next   moves the iterator to the next element  If you want to use the current element in more than one statement or expression  you have to store it in a local variable  Or even better  why don t you simply use a for-each loop   for  String key   map keySet          System out println key         map get key        Moreover  loop over the entrySet is faster  because you don t query the map twice for each key  Also Map Entry implementations usually implement the toString   method  so you don t have to print the key-value pair manually   for  Entry lt String  Integer gt  entry   map entrySet          System out println entry

[java] java collections - keyset() vs entrySet() in map

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