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How to retrieve common key value pairs from two hashmaps

I have two hashmaps:
Map<String, String> mapA = new HashMap<String, String>();
Map<String, String> mapB = new HashMap<String, String>();
TreeSet<String> uniquekeys = new TreeSet<String>();
mapA.put("1","value1");
mapA.put("2","value2");
mapA.put("3","value3");
mapA.put("4","value4");
mapA.put("5","value5");
mapA.put("6","value6");
mapA.put("7","value7");
mapA.put("8","value8");
mapA.put("9","value9");
mapB.put("1","value1");
mapB.put("2","value2");
mapB.put("3","value3");
mapB.put("4","value4");
mapB.put("5","value5");
To get the common key value pairs from the two hashmaps, I have written the below logic:
uniquekeys.addAll(mapA.keySet());
uniquekeys.addAll(mapB.keySet());
and then use the keys from the treeset: uniquekeys to retrieve unique key value pairs from mapA & mapB.
But this is not giving me the details of all the keys from mapA. I understand this way is flawed but I couldn't come up with a proper logic.
Could anyone let me know how can I retrieve key value pairs that are common in mapA and mapB into a new HashMap ?

Try below logic :
Map<String, String> common = new HashMap<String, String>();
for(String key : mapA.keySet()) {
if(mapB.get(key) !=null ) {
if(mapA.get(key).equals(mapB.get(key))) {
common.put(key, mapA.get(key));
}
}
}

You can do it with Java 8 Streams in the following way:
Map<String, String> commonMap = mapA.entrySet().stream()
.filter(x -> mapB.containsKey(x.getKey()))
.collect(Collectors.toMap(x -> x.getKey(), x -> x.getValue()));

Instead of adding all keys to a TreeSet, you can fill the TreeSet with common values:
uniquekeys.addAll(mapA.keySet());
uniquekeys.retainAll(mapB.keySet());
This way, the keys contained in A but not B will be removed. Know you've got your TreeSet, you can do what you want.
However, you can also create your HashMap without TreeSet, as #Ramesh and #NiVeR suggest

Use Guava Util Sets
Set<String> intersectionSet = Sets.intersection(firstSet, secondSet);

How to add a value to a list of values for a single key in a hashmap (Java)

I have written this:
HashMap<String, String> map1 = new HashMap<String, String>();
Map<String, ArrayList<String>> map2 = new HashMap<String, ArrayList<String>>();
i am trying to allow more then 1 value for each key in a hashmap. so if the first key is '1', i want to allow '1' to be paired with values '2' and '3'.
so it be like:
1 --> 2
|--> 3
but when I do:
map2.put(key, value);
it gives error that says "incompatible types" and it can not be converted to ArrayList and it says the error is at the value part of the line.

If you are using Java 8, you can do this quite easily:
String key = "someKey";
String value1 = "someValue1";
String value2 = "someValue2";
Map<String, List<String>> map2 = new HashMap<>();
map2.computeIfAbsent(key, k -> new ArrayList<>()).add(value1);
map2.computeIfAbsent(key, k -> new ArrayList<>()).add(value2);
System.out.println(map2);
The documentation for Map.computeIfAbsent(...) has pretty much this example.

In map2 you need to add ArrayList (you declared it as Map<String, ArrayList<String>> - the second one is the value type) only, that's why it gives you incompatible types.
You would need to do initialize the key with an ArrayList and add objects to it later:
if (!map2.containsKey(key)) {
map2.put(key, new ArrayList<String>());
}
map2.get(key).add(value);
Or you could use Multimap from guava, then you can just map2.put and it won't overwrite your values there but add to a list.

You are little bit away from what you are trying to do.
Map<String, ArrayList<String>> map2 = new HashMap<String, ArrayList<String>>();
this will allow only String as key and an ArrayList as value. So you have to try something like:
ArrayList<String> value=new ArrayList<String>();
value.add("2");
value.add("3");
map2.put("1", value);
When retrieving you also have to follow ans opposite procedure.
ArrayList<String> valueTemp=map2.get("1");
then you can iterate over this ArrayList to get those values ("2" and "3");

Try like this. //use list or set.. but set avoids duplicates
Map<String, Set<String>> map = new HashMap<>();
Set<String> list = new HashSet<>();
// add value to the map
Boolean b = map.containsKey(key);
if (b) {
map.get(key).addAll(list);
} else
map.put(key, list);
}

You can not add different values in same key in Map. Map is override the value in that key. You can do like this way.
Map<String, ArrayList<String>> map = new HashMap<String, ArrayList<String>>();
ArrayList<String> list=new ArrayList<String>();
list.add("2");
list.add("3");
map.put("1", list);
first add value in array list then put into map.

It is all because standard Map implementations in java stores only single pairs (oneKey, oneValue). The only way to store multiple values for a particular key in a java standard Map is to store "collection" as value, then you need to access this collection (from Map) by key, and then use this collection "value" as regular collection, in your example as ArrayList. So you do not put something directly by map.put (except from creating the empty collection), instead you take the whole collection by key and use this collection.
You need something like Multimap, for example:
public class Multimap<T,S> {
Map<T, ArrayList<S>> map2 = new HashMap<T, ArrayList<S>>();
public void add(T key, S value) {
ArrayList<T> currentValuesForGivenKey = get(key);
if (currentValuesForGivenKey == null) {
currentValuesForGivenKey = new ArrayList<T>();
map2.get(key, currentValuesForGivenKey);
}
currentValuesForGivenKey.add(value);
}
public ArrayList<S> get(T key) {
ArrayList<String> currentValuesForGivenKey = map2.get(key);
if (currentValuesForGivenKey == null) {
currentValuesForGivenKey = new ArrayList<S>();
map2.get(key, currentValuesForGivenKey);
}
return currentValuesForGivenKey;
}
}
then you can use it like this:
Multimap<String,String> map2 = new Multimap<String,String>();
map2.add("1","2");
map2.add("1","3");
map2.add("1","4");
for (String value: map2.get("1")) {
System.out.println(value);
}
will print:
2
3
4

it gives error that says "incompatible types" and it can not be converted to ArrayList and it says the error is at the value part of the line.
because, it won't automatically convert to ArrayList.
You should add both the values to list and then put that list in map.

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

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);

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);

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<Integer, String> map1 = new HashMap<Integer, String>();
map1.put(1, "Hello");
map1.put(2, "There");
final Map<Integer, String> map2 = new HashMap<Integer, String>();
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());
}

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)

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.

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) -> map.merge(key, value, (oldValue, newValue) -> oldValue));
// using for-loop
for (Map.Entry<Integer, String> entry: newMap.entrySet()) {
map.merge(entry.getKey(), entry.getValue(), (oldValue, newValue) -> 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<Integer, String> newMap = new HashMap<>();
newMap.put(2, "EVIL VALUE"); // this will NOT be merged into
newMap.put(4, "four"); // this WILL be merged into
newMap.put(5, "five"); // this WILL be merged into
Map<Integer, String> map = new HashMap<>();
map.put(1, "one");
map.put(2, "two");
map.put(3, "three");
newMap.forEach((k, v) -> map.merge(k, v, (oldValue, newValue) -> oldValue));
map.forEach((k, v) -> System.out.println(k + " " + v));
1 one
2 two
3 three
4 four
5 five

public class MyMap {
public static void main(String[] args) {
Map<String, String> map1 = new HashMap<String, String>();
map1.put("key1", "value1");
map1.put("key2", "value2");
map1.put("key3", "value3");
map1.put(null, null);
Map<String, String> map2 = new HashMap<String, String>();
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);
}}

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.

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);

How sort an ArrayList of HashMaps holding several key-value pairs each?

I need to call an external API with an ArrayList of HashMaps holding several predefined key-value pairs each. An example:
ArrayList<HashMap<String, String>> arrayListHashMap = new ArrayList<HashMap<String, String>>();
{
HashMap hashMap = new HashMap<String, String>();
hashMap.put("key", "A key");
hashMap.put("value", "B value");
arrayListHashMap.add(hashMap);
}
{
HashMap hashMap = new HashMap<String, String>();
hashMap.put("key", "B key");
hashMap.put("value", "A value");
arrayListHashMap.add(hashMap);
}
Now I need to sort this construct on the contents of the "value" key. This sort would result in the "key=B key/value=A value" entry as the first one in the arrayListHashMap.
Any help is highly appreciated.
HJW

You need to implement a Comparator<HashMap<String, String>> or more generally Comparator<Map<String, String>> which just extracts the value assocated with the value key, then use Collections.sort. Sample code (with generalization for whatever key you want to sort on):
class MapComparator implements Comparator<Map<String, String>>
{
private final String key;
public MapComparator(String key)
{
this.key = key;
}
public int compare(Map<String, String> first,
Map<String, String> second)
{
// TODO: Null checking, both for maps and values
String firstValue = first.get(key);
String secondValue = second.get(key);
return firstValue.compareTo(secondValue);
}
}
...
Collections.sort(arrayListHashMap, new MapComparator("value"));

You can use the below solution to achieve it:
arrayListHashMap.sort(Comparator.comparing(m -> m.get("value"), Comparator.nullsLast(Comparator.naturalOrder())));

(This is not an answer to the asked question - Jon did this already -, but the comment field is too small for this.)
Your data structure looks like you misunderstood the key-value structure of maps (and Hash maps in your example).
A Map can contain any number of keys, and for each key also a value. A pair of key and value is given by a Map.Entry (which can be obtained by the entrySet() method of the map). If you then want to sort by key, simply use a SortedMap (like TreeMap) instead of the usual HashMap.
You are emulating the individual entries by a HashMap each, then putting them all in a ArrayList ... :-/
Here what I would have done in your example:
Map<String, String> map = new TreeMap<String, String>();
map.put("B key", "B value");
map.put("A key", "B value");
System.out.println(map); // already sorted

How do I convert a Map to List in Java?

How do I convert a Map<key,value> to a List<value>? Should I iterate over all map values and insert them into a list?

List<Value> list = new ArrayList<Value>(map.values());
assuming:
Map<Key,Value> map;

The issue here is that Map has two values (a key and value), while a List only has one value (an element).
Therefore, the best that can be done is to either get a List of the keys or the values. (Unless we make a wrapper to hold on to the key/value pair).
Say we have a Map:
Map<String, String> m = new HashMap<String, String>();
m.put("Hello", "World");
m.put("Apple", "3.14");
m.put("Another", "Element");
The keys as a List can be obtained by creating a new ArrayList from a Set returned by the Map.keySet method:
List<String> list = new ArrayList<String>(m.keySet());
While the values as a List can be obtained creating a new ArrayList from a Collection returned by the Map.values method:
List<String> list = new ArrayList<String>(m.values());
The result of getting the List of keys:
Apple
Another
Hello
The result of getting the List of values:
3.14
Element
World

Using the Java 8 Streams API.
List<Value> values = map.values().stream().collect(Collectors.toList());

map.entrySet() gives you a collection of Map.Entry objects containing both key and value. you can then transform this into any collection object you like, such as new ArrayList(map.entrySet());

a list of what ?
Assuming map is your instance of Map
map.values() will return a Collection containing all of the map's values.
map.keySet() will return a Set containing all of the map's keys.

I guess you want to convert the values contained in the Map to a list? Easiest is to call the values() method of the Map interface. This will return the Collection of value objects contained in the Map.
Note that this Collection is backed by the Map object and any changes to the Map object will reflect here. So if you want a separate copy not bound to your Map object, simply create a new List object like an ArrayList passing the value Collection as below.
ArrayList<String> list = new ArrayList<String>(map.values());

Map<String, Integer> map = new HashMap<String, Integer>();
map.put("java", 20);
map.put("C++", 45);
Set <Entry<String, Integer>> set = map.entrySet();
List<Entry<String, Integer>> list = new ArrayList<Entry<String, Integer>>(set);
we can have both key and value pair in list.Also can get key and value using Map.Entry by iterating over list.

If you want to ensure the values in the resultant List<Value> are in the key-ordering of the input Map<Key, Value>, you need to "go via" SortedMap somehow.
Either start with a concrete SortedMap implementation (Such as TreeMap) or insert your input Map into a SortedMap before converting that to List. e.g.:
Map<Key,Value> map;
List<Value> list = new ArrayList<Value>( new TreeMap<Key Value>( map ));
Otherwise you'll get whatever native ordering the Map implementation provides, which can often be something other than the natural key ordering (Try Hashtable or ConcurrentHashMap, for variety).

// you can use this
List<Value> list = new ArrayList<Value>(map.values());
// or you may use
List<Value> list = new ArrayList<Value>();
for (Map.Entry<String, String> entry : map.entrySet())
{
list.add(entry.getValue());
}

Map<String, String > map = new HapshMap<String, String>;
map.add("one","java");
map.add("two", "spring");
Set<Entry<String, String>> set = map.entrySet();
List<Entry<String, String>> list = new ArrayList<Entry<String, String>> (set);
for(Entry<String, String> entry : list) {
System.out.println(entry.getKey());
System.out.println(entry.getValue());
}

Here's the generic method to get values from map.
public static <T> List<T> ValueListFromMap(HashMap<String, T> map) {
List<T> thingList = new ArrayList<>();
for (Map.Entry<String, T> entry : map.entrySet()) {
thingList.add(entry.getValue());
}
return thingList;
}

public List<Object> convertMapToList(Map<Object, Object> map){
return new ArrayList<>(map.values());
}

If you want an immutable copy of the values:
List<Value> list = List.copyOf(map.values())