How to putAll on Java hashMap contents of one to another but not replace existing keys and values

Question

I need to copy all keys and values from one A HashMap onto another one B  but not to replace existing keys and values   Whats the best way to do that   I was thinking instead iterating the keySet and checkig if it exist or not  I would  Map temp   new HashMap       generic later temp putAll Amap   A clear    A putAll Bmap   A putAll temp

User · Accepted Answer

It looks like you are willing to create a temporary Map, so I'd do it like this:

Map tmp = new HashMap(patch);
tmp.keySet().removeAll(target.keySet());
target.putAll(tmp);

Here, patch is the map that you are adding to the target map.

Thanks to Louis Wasserman, here's a version that takes advantage of the new methods in Java 8:

patch.forEach(target::putIfAbsent);

User · Answer

With Java 8 there is this API method to accomplish your requirement   map putIfAbsent key  value    If the specified key is not already associated with a value  or is mapped to null  associates it with the given value and returns null  else returns the current value

User · Answer

Just iterate and add   for Map Entry e   a entrySet      if  b containsKey e getKey        b put e getKey    e getValue       Edit to add   If you can make changes to a  you can also do   a putAll b    and a will have exactly what you need   all the entries in b and all the entries in a that aren t in b

User · Answer

As others have said  you can use putIfAbsent  Iterate over each entry in the map that you want to insert  and invoke this method on the original map  mapToInsert forEach originalMap  putIfAbsent

User · Answer

Java 8 solution using Map merge As of java-8 you can use Map merge K key  V value  BiFunction remappingFunction  which merges a value into the Map using remappingFunction in case the key is already found in the Map you want to put the pair into     using lambda newMap forEach  key  value  - gt  map merge key  value   oldValue  newValue  - gt  oldValue        using for-loop for  Map Entry lt Integer  String gt  entry  newMap entrySet          map merge entry getKey    entry getValue     oldValue  newValue  - gt  oldValue      The code iterates the newMap entries  key and value  and each one is merged into map through the method merge  The remappingFunction is triggered in case of duplicated key and in that case it says that the former  original  oldValue value will be used and not rewritten  With this solution  you don t need a temporary Map   Let s have an example of merging newMap entries into map and keeping the original values in case of the duplicated antry  Map lt Integer  String gt  newMap   new HashMap lt  gt     newMap put 2   quot EVIL VALUE quot                               this will NOT be merged into newMap put 4   quot four quot                                     this WILL be merged into newMap put 5   quot five quot                                     this WILL be merged into  Map lt Integer  String gt  map   new HashMap lt  gt     map put 1   quot one quot    map put 2   quot two quot    map put 3   quot three quot     newMap forEach  k  v  - gt  map merge k  v   oldValue  newValue  - gt  oldValue     map forEach  k  v  - gt  System out println k    quot   quot    v      1 one 2 two 3 three 4 four 5 five

User · Answer

You can make it in just 1 line if you change maps order in  erickson s solution   mapWithNotSoImportantValues putAll  mapWithImportantValues      In this case you replace values in mapWithNotSoImportantValues with value from mapWithImportantValues with the same keys

User · Answer

public class MyMap    public static void main String   args         Map lt String  String gt  map1   new HashMap lt String  String gt         map1 put  key1    value1        map1 put  key2    value2        map1 put  key3    value3        map1 put null  null        Map lt String  String gt  map2   new HashMap lt String  String gt         map2 put  key4    value4        map2 put  key5    value5        map2 put  key6    value6        map2 put  key3    replaced-value-of-key3-in-map2           used only if map1 can be changes updates with the same keys present in map2      map1 putAll map2           use below if you are not supposed to modify the map1      for  Map Entry e   map2 entrySet            if   map1 containsKey e getKey                 map1 put e getKey   toString    e getValue   toString         System out println map1

User · Answer

Using Guava s Maps class  utility methods to compute the difference of 2 maps you can do it in a single line  with a method signature which makes it more clear what you are trying to accomplish   public static void main final String   args           Create some maps     final Map lt Integer  String gt  map1   new HashMap lt Integer  String gt         map1 put 1   Hello        map1 put 2   There        final Map lt Integer  String gt  map2   new HashMap lt Integer  String gt         map2 put 2   There        map2 put 3   is        map2 put 4   a        map2 put 5   bird            Add everything in map1 not in map2 to map2     map2 putAll Maps difference map1  map2  entriesOnlyOnLeft

[java] How to putAll on Java hashMap contents of one to another, but not replace existing keys and values?

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