Comparing two maps

Question

I have two maps declared as Map lt String  Object gt   The Object here could be another Map lt String  Object gt   and so on   I want to check if two maps are exactly the same without knowing their depth  Instead of using recursion can I compare the output of the toString   called on each map  Or is there a simpler way to compare the maps

User · Answer

As long as you override equals   on each key and value contained in the map  then m1 equals m2  should be reliable to check for maps equality   The same result can be obtained also by comparing toString   of each map as you suggested  but using equals   is a more intuitive approach   May not be your specific situation  but if you store arrays in the map  may be a little tricky  because they must be compared value by value  or using Arrays equals    More details about this see here

User · Answer

Quick Answer  You should use the equals method since this is implemented to perform the comparison you want  toString   itself uses an iterator just like equals but it is a more inefficient approach  Additionally   as  Teepeemm pointed out  toString is affected by order of elements  basically iterator return order  hence is not guaranteed to provide the same output for 2 different maps  especially if we compare two different maps     Note Warning  Your question and my answer assume that classes implementing the map interface respect expected toString and equals behavior  The default java classes do so  but a custom map class needs to be examined to verify expected behavior    See  http   docs oracle com javase 7 docs api java util Map html  boolean equals Object o       Compares the specified object with this map for equality  Returns true   if the given object is also a map and the two maps represent the same   mappings  More formally  two maps m1 and m2 represent the same   mappings if m1 entrySet   equals m2 entrySet     This ensures that the   equals method works properly across different implementations of the   Map interface    Implementation in Java Source  java util AbstractMap   Additionally  java itself takes care of iterating through all elements and making the comparison so you don t have to  Have a look at the implementation of AbstractMap which is used by classes such as HashMap       Comparison and hashing                 Compares the specified object with this map for equality   Returns         lt tt gt true lt  tt gt  if the given object is also a map and the two maps        represent the same mappings   More formally  two maps  lt tt gt m1 lt  tt gt  and         lt tt gt m2 lt  tt gt  represent the same mappings if         lt tt gt m1 entrySet   equals m2 entrySet    lt  tt gt    This ensures that the         lt tt gt equals lt  tt gt  method works properly across different implementations        of the  lt tt gt Map lt  tt gt  interface                 lt p gt This implementation first checks if the specified object is this map         if so it returns  lt tt gt true lt  tt gt    Then  it checks if the specified        object is a map whose size is identical to the size of this map  if        not  it returns  lt tt gt false lt  tt gt    If so  it iterates over this map s         lt tt gt entrySet lt  tt gt  collection  and checks that the specified map        contains each mapping that this map contains   If the specified map        fails to contain such a mapping   lt tt gt false lt  tt gt  is returned   If the        iteration completes   lt tt gt true lt  tt gt  is returned                 param o object to be compared for equality with this map         return  lt tt gt true lt  tt gt  if the specified object is equal to this map             public boolean equals Object o            if  o    this              return true           if    o instanceof Map               return false          Map lt K V gt  m    Map lt K V gt   o          if  m size      size                return false           try               Iterator lt Entry lt K V gt  gt  i   entrySet   iterator                while  i hasNext                      Entry lt K V gt  e   i next                    K key   e getKey                    V value   e getValue                    if  value    null                        if    m get key   null  amp  amp  m containsKey key                            return false                    else                       if   value equals m get key                            return false                                            catch  ClassCastException unused                return false            catch  NullPointerException unused                return false                     return true          Comparing two different types of Maps  toString fails miserably when comparing a TreeMap and HashMap though equals does compare contents correctly    Code   public static void main String args      HashMap lt String  Object gt  map   new HashMap lt String  Object gt     map put  2    whatever2    map put  1    whatever1    TreeMap lt String  Object gt  map2   new TreeMap lt String  Object gt     map2 put  2    whatever2    map2 put  1    whatever1     System out println  Are maps equal  using equals      map equals map2    System out println  Are maps equal  using toString   equals                map toString   equals map2 toString       System out println  Map1   map toString     System out println  Map2   map2 toString         Output   Are maps equal  using equals  true Are maps equal  using toString   equals    false Map1  2 whatever2  1 whatever1  Map2  1 whatever1  2 whatever2

[java] Comparing two maps

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