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How to use forEach after computeIfPresent

I have this snippet of code, and I want to use forEach after computeIfpresent function. Mainly, if the key is found, then we should loop over the values (list), and fetch each entry and add it to another list. Any idea how I can do that?
List<Long> myArrayList = new ArrayList();
Map<Long, Set<Long>> myMap = new HashMap();
Set<Long> mySet = MyMap().get(id);
if (mySet != null)
{
for (Long ex : mySet)
{
myArrayList.add(ex);
}
}
-->??
myMap.computeIfPresent(id, (key, value) -> value.forEach(ex -> myArrayList.add(ex)));

computeIfPresent is for changing the value inside the HashMap under the given key (id in your case). It means that such operation is not allowed because by running forEach on the key:value pair you are not providing any new mapping value for a map
What you could do would be
myMap.computeIfPresent("a", (k, v) -> {
((Set<Long>)v).forEach(e -> myArrayList.add(e));
return v;
});
but it looks bad and is violating the purpose of computeIfPresent method
You should just use rather traditional approach
if (myMap.containsKey(id)) {
myArrayList.addAll(myMap.get(id));
}
or use an Optional
Optional.ofNullable(map.get(id)).ifPresent(set ->
myArrayList.addAll((Set<Long>)set)
);
...but is this more readable? :)

If I get it right, you are looking for Map.getOrDefault instead of Map.computeIfPresent chained with a foreach. Using Map.getOrDefault the task could be rewritten to:
List<Long> myArrayList = new ArrayList<>();
Map<Long, Set<Long>> myMap = new HashMap<>();
long id = ...;
myArrayList.addAll(myMap.getOrDefault(id, Collections.emptySet()));

Compute map - add element to existing List or create new List and add to it

I have this piece of code:
private static void computeMapAddition(Map<String, List<XXX>> objectMap,
XXX objectToAdd, String key) {
if (objectMap.containsKey(key)) {
List<XXX> objectList = objectMap
.get(key);
objectList.add(objectToAdd);
} else {
List<XXX> objectList = new ArrayList<>();
objectList.add(objectToAdd);
objectMap.put(key, objectList);
}
}
What this code does:
1) if map contains key then retrieve value - which is List - and add element to that list (it can have lots of elements already)
2) if map doesn't contain a key then create new list, add element to the newly created list and put that (key, value) to the map
Is there any way to make it less verbose using Java 8?

java 8 added computeIfAbsent to Map interface. It does exactly what you want:
// return the list if already present or make a new one, insert into the map
// and return the newly created one
List<XXX> objectList = objectMap.computeIfAbsent(key, k -> new ArrayList<>());
// add new object to list
objectList.add(objectToAdd);
Or you can combine it together as
objectMap.computeIfAbsent(key, k -> new ArrayList<>()).add(objectToAdd);

Some addition to #misha answer.
We may use two options:
Map<Integer, List<String>> map = new HashMap<>();
computeIfAbsent (preferred)
map.computeIfAbsent(1, ArrayList::new).add("b");
compute
map.compute(1, (k, v) -> {
v = v != null ? v : new ArrayList<>();
v.add("b");
return v;
});

This work for me:
map.compute(key, (k, v) -> {
v = v != null ? new ArrayList<>(v) : new ArrayList<>();
v.add("text");
return v;
});

Overwriting values in a HashMap that are in an ArrayList<String>

Let's say I have a HashMap with String keys and Integer values:
map = {cat=1, kid=3, girl=3, adult=2, human=5, dog=2, boy=2}
I want to switch the keys and values by putting this information into another HashMap. I know that a HashMap cannot have duplicate keys, so I tried to put the information into a HashMap with the Integer for the keys that would map to a String ArrayList so that I could potentially have one Integer mapping to multiple Strings:
swap = {1=[cat], 2=[adult, dog, boy], 3=[kid, girl], 5=[human]}
I tried the following code:
HashMap<Integer, ArrayList<String>> swap = new HashMap<Integer, ArrayList<String>>();
for (String x : map.keySet()) {
for (int i = 0; i <= 5; i++) {
ArrayList<String> list = new ArrayList<String>();
if (i == map.get(x)) {
list.add(x);
swap.put(i, list);
}
}
}
The only difference in my code is that I didn't hard code the number 5 into my index; I have a method that finds the highest integer value in the original HashMap and used that. I know it works correctly because I get the same output even if I hard code the 5 in there, I just didn't include it to save space.
My goal here is to be able to do this 'reversal' with any set of data, otherwise I could just hard code the value. The output I get from the above code is this:
swap = {1=[cat], 2=[boy], 3=[girl], 5=[human]}
As you can see, my problem is that the value ArrayList is only keeping the last String that was put into it, instead of collecting all of them. How can I make the ArrayList store each String, rather than just the last String?

With Java 8, you can do the following:
Map<String, Integer> map = new HashMap<>();
map.put("cat", 1);
map.put("kid", 3);
map.put("girl", 3);
map.put("adult", 2);
map.put("human", 5);
map.put("dog", 2);
map.put("boy", 2);
Map<Integer, List<String>> newMap = map.keySet()
.stream()
.collect(Collectors.groupingBy(map::get));
System.out.println(newMap);
The output will be:
{1=[cat], 2=[adult, dog, boy], 3=[kid, girl], 5=[human]}

you are recreating the arrayList for every iteration and i can't figure out a way to do it with that logic, here is a good way though and without the need to check for the max integer:
for (Map.Entry<String, Integer> entry : map.entrySet()) {
String key = entry.getKey();
Integer value = entry.getValue();
List<String> get = swap.get(value);
if (get == null) {
get = new ArrayList<>();
swap.put(value, get);
}
get.add(key);
}

Best way is to iterate over the key set of the original map.
Also you have to asure that the List is present for any key in the target map:
for (Map.Entry<String,Integer> inputEntry : map.entrySet())
swap.computeIfAbsent(inputEntry.getValue(),()->new ArrayList<>()).add(inputEntry.getKey());

This is obviously not the best solution, but approaches the problem the same way you did by interchanging inner and outer loops as shown below.
Map<String, Integer> map = new HashMap<String, Integer>();
map.put("cat", 1);
map.put("kid", 3);
map.put("girl", 3);
map.put("adult", 2);
map.put("human", 5);
map.put("dog", 2);
map.put("boy", 2);
HashMap<Integer, ArrayList<String>> swap = new HashMap<Integer, ArrayList<String>>();
for (Integer value = 0; value <= 5; value++) {
ArrayList<String> list = new ArrayList<String>();
for (String key : map.keySet()) {
if (map.get(key) == value) {
list.add(key);
}
}
if (map.containsValue(value)) {
swap.put(value, list);
}
}
Output
{1=[cat], 2=[adult, dog, boy], 3=[kid, girl], 5=[human]}

Best way I can think of is using Map.forEach method on existing map and Map.computeIfAbsent method on new map:
Map<Integer, List<String>> swap = new HashMap<>();
map.forEach((k, v) -> swap.computeIfAbsent(v, k -> new ArrayList<>()).add(k));
As a side note, you can use the diamond operator <> to create your new map (there's no need to repeat the type of the key and value when invoking the map's constructor, as the compiler will infer them).
As a second side note, it's good practice to use interface types instead of concrete types, both for generic parameter types and for actual types. This is why I've used List and Map instead of ArrayList and HashMap, respectively.

Using groupingBy like in Jacob's answer but with Map.entrySet for better performance, as suggested by Boris:
// import static java.util.stream.Collectors.*
Map<Integer, List<String>> swap = map.entrySet()
.stream()
.collect(groupingBy(Entry::getValue, mapping(Entry::getKey, toList())));
This uses two more methods of Collectors: mapping and toList.
If it wasn't for these two helper functions, the solution could look like this:
Map<Integer, List<String>> swap = map.entrySet()
.stream()
.collect(
groupingBy(
Entry::getValue,
Collector.of(
ArrayList::new,
(list, e) -> {
list.add(e.getKey());
},
(left, right) -> { // only needed for parallel streams
left.addAll(right);
return left;
}
)
)
);
Or, using toMap instead of groupingBy:
Map<Integer, List<String>> swap = map.entrySet()
.stream()
.collect(
toMap(
Entry::getValue,
(e) -> new ArrayList<>(Arrays.asList(e.getKey())),
(left, right) -> {
left.addAll(right);
return left;
}
)
);

It seams you override the values instrad of adding them to the already creared arraylist. Try this:
HashMap<Integer, ArrayList<String>> swapedMap = new HashMap<Integer, ArrayList<String>>();
for (String key : map.keySet()) {
Integer swappedKey = map.get(key);
ArrayList<String> a = swapedMap.get(swappedKey);
if (a == null) {
a = new ArrayList<String>();
swapedMap.put(swappedKey, a)
}
a.add(key);
}
I didn't have time to run it (sorry, don't have Java compiler now), but should be almost ok :)

You could use the new merge method in java-8 from Map:
Map<Integer, List<String>> newMap = new HashMap<>();
map.forEach((key, value) -> {
List<String> values = new ArrayList<>();
values.add(key);
newMap.merge(value, values, (left, right) -> {
left.addAll(right);
return left;
});
});

Merging two Maps

I have two maps whose keys are Strings and whose values are Set<MyObject>. Given two Maps, what is the easiest way to merge them such that if two keys are identical, the value is a union of the two sets. You can assume values are never null and if it is useful, we can make these Maps SortedMaps.

You can do this with a stream fairly easily:
Map<T, Set<U>> merged = Stream.of(first, second)
.map(Map::entrySet)
.flatMap(Set::stream)
.collect(Collectors.toMap(Entry::getKey, Entry::getValue, (a, b) -> {
HashSet<U> both = new HashSet<>(a);
both.addAll(b);
return both;
}));
This splits the maps into their Entrys and then joins them with a Collector which resolves duplicates by adding both values to a new HashSet.
This also works for any number of maps.
Some variations which produce the same result:
Stream.of(first, second).flatMap(m -> m.entrySet().stream())
.collect(...);
Stream.concat(first.entrySet().stream(), second.entrySet().stream())
.collect(...); //from comment by Aleksandr Dubinsky
The third parameter for Collectors.toMap is not necessary if there are no duplicate keys.
There is another Collectors.toMap with a fourth parameter that lets you decide the type of the Map collected into.

Are we talking about HashMap instances. In that case lookup is O(1), so you can just take one map, iterate over the entries of that map, see whether the other map contains that key. If not, just add the set. If it contains the key, take the union of the two sets (by adding all elements of one set to another)
To illustrate with some code, where I used a Set to have autocompletion in my IDE
Map<String, Set<Double>> firstMap = new HashMap<String, Set<Double>>( );
Map<String, Set<Double>> secondMap = new HashMap<String, Set<Double>>( );
Set<Map.Entry<String, Set<Double>>> entries = firstMap.entrySet();
for ( Map.Entry<String, Set<Double>> entry : entries ) {
Set<Double> secondMapValue = secondMap.get( entry.getKey() );
if ( secondMapValue == null ) {
secondMap.put( entry.getKey(), entry.getValue() );
}
else {
secondMapValue.addAll( entry.getValue() );
}
}

static void mergeSet(Map<String, Set<String>> map1, Map<String, Set<String>> map2) {
map1.forEach((key1, value1) -> {
map2.merge(key1, value1, (key2, value2) -> key2).addAll(value1);
});
}

How about this (untested):
Map<String,Set<Whatever>> m1 = // input map
Map<String,Set<Whatever>> m2 = // input map
Map<String,Set<Whatever>> ret = // new empty map
ret.putAll(m1);
for(String key : m2.keySet()) {
if(ret.containsKey(key)) {
ret.get(key).addAll(m2.get(key));
} else {
ret.put(key,m2.get(key));
}
}
This solution doesn't modify the input maps, and because it is short and relies on API methods only, I find it quite readable.
Note that putAll() and addAll() are both optional methods in Map and Set. Consequently (and in order to get O(1) lookup), I'd recommend using HashMap and HashSet.
Note that because neither HashSet or HashMap are synchronised you will need to look for some other solution if you want thread-safe code.

The following should merge a map1 into map2 (untested):
for (Entry<String, Set<???>> entry : map1.entrySet( ))
{
Set<???> otherSet = map2.get(entry.getKey( ));
if (otherSet == null)
map2.put(entry.getKey( ), entry.getValue ( ));
else
otherSet.addAll(entry.getValue( ));
}
I don't know what you've parameterized your Sets on, hence the <???>: replace as appropriate.

Something like this (untested):
// Assume all maps are of the same generic type.
public static Map<String, Set<MyObject>> mergeAll(Map m1, Map m2) {
Map<String, Set<MyObject>> merged = new HashMap();
// Merge commom entries into the new map.
for (Map.Entry<String, Set<MyObject>> entry : m1.entrySet()) {
String key = entry.getKey();
Set<MyObject> s1 = new HashSet(entry.getValue());
Set<MyObject> s2 = m2.get(key);
if (s2 != null) s1.addAll(s2);
merged.put(key, s1);
}
// Add entries unique to m2 to the new map.
for (String key : m2.keys()) {
if (!s1.containsKey(key)) merged.put(key, new HashSet(m2.get(key)));
}
return merged;
}
Note that this solution does not mutate either of its arguments.

Map<Integer,String> m1=new HashMap<Integer,String>();
Map<Integer,String> m2=new HashMap<Integer,String>();
m1.put(1,"one");
m1.put(2,"two");
m2.put(3,"three");
m2.put(2,"two");
Set<Integer> s=m2.keySet();
for(int i:s){
if(m1.get(i)==null){
m1.put(i,m2.get(i));
}
}
System.out.println(m1);

Note that all other answers will eventually augment the original sets which you might not want for all use cases, if you don't want that just use a third map as output and create a new set for each key
public static void merge2Maps(Map<String, Set<Double>> a, Map<String, Set<Double>> b, Map<String, Set<Double>> c){
for (Map.Entry<String, Set<Double>> entry : a.entrySet()) {
Set<Double> set = new HashSet<Double>();
c.put(entry.getKey(), set);
set.addAll(entry.getValue());
}
for (Map.Entry<String, Set<Double>> entry : b.entrySet()) {
String key = entry.getKey();
Set<Double> set = c.get(key);
if (set == null) {
set = new HashSet<Double>();
c.put(entry.getKey(), set);
}
set.addAll(entry.getValue());
}
}

If you want to end up with immutable data structures to prevent manipulation of your merged map and map's Set instances then you can take this approach. This solution uses Google's Guava library.
public <K,T> Map<K, Set<T>> mergeToImmutable (
final Map<K, Set<T>> left,
final Map<K, Set<T>> right)
{
return Maps.toMap(
Sets.union(
checkNotNull(left).keySet(),
checkNotNull(right).keySet()
),
new Function<K, Set<T>> () {
#Override
public Set<T> apply (K input) {
return ImmutableSet.<T>builder()
.addAll(MoreObjects.firstNonNull(left.get(input), Collections.<T>emptySet()))
.addAll(MoreObjects.firstNonNull(right.get(input), Collections.<T>emptySet()))
.build();
}
}
);
}

If you define a method to unite non-null Sets as:
static <T> Set<T> union(Set<T>... sets) {
return Stream.of(sets)
.filter(s -> s != null)
.flatMap(Set::stream)
.collect(Collectors.toSet());
}
then merging two maps m1 and m2 having Set<V> values can be performed as follows:
Map<String, V> merged
= union(m1.keySet(), m2.keySet())
.stream()
.collect(Collectors.toMap(k -> k, k -> union(m1.get(k), m2.get(k))));
Or even simpler:
Map<String, V> merged = new HashMap<>();
for (String k : union(m1.keySet(), m2.keySet())
merged.put(k, union(m1.get(k), m2.get(k)));

<K, V> Map<K, List<V>> mergeMapOfLists(Stream<Map<K, List<V>>> stream) {
return stream
.map(Map::entrySet) // convert each map to set of map's entries
.flatMap(Collection::stream) // convert each map entry to stream and flat them to one stream
.collect(toMap(Map.Entry::getKey, Map.Entry::getValue,
(list1, list2) -> {
list1.addAll(list2);
return list1;
})); // convert stream to map; if key is duplicated execute merge fuction (append exisitng list with elements from new list)
}

How to convert List to Map?

Recently I have conversation with a colleague about what would be the optimal way to convert List to Map in Java and if there any specific benefits of doing so.
I want to know optimal conversion approach and would really appreciate if any one can guide me.
Is this good approach:
List<Object[]> results;
Map<Integer, String> resultsMap = new HashMap<Integer, String>();
for (Object[] o : results) {
resultsMap.put((Integer) o[0], (String) o[1]);
}

With java-8, you'll be able to do this in one line using streams, and the Collectors class.
Map<String, Item> map =
list.stream().collect(Collectors.toMap(Item::getKey, item -> item));
Short demo:
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class Test{
public static void main (String [] args){
List<Item> list = IntStream.rangeClosed(1, 4)
.mapToObj(Item::new)
.collect(Collectors.toList()); //[Item [i=1], Item [i=2], Item [i=3], Item [i=4]]
Map<String, Item> map =
list.stream().collect(Collectors.toMap(Item::getKey, item -> item));
map.forEach((k, v) -> System.out.println(k + " => " + v));
}
}
class Item {
private final int i;
public Item(int i){
this.i = i;
}
public String getKey(){
return "Key-"+i;
}
#Override
public String toString() {
return "Item [i=" + i + "]";
}
}
Output:
Key-1 => Item [i=1]
Key-2 => Item [i=2]
Key-3 => Item [i=3]
Key-4 => Item [i=4]
As noted in comments, you can use Function.identity() instead of item -> item, although I find i -> i rather explicit.
And to be complete note that you can use a binary operator if your function is not bijective. For example let's consider this List and the mapping function that for an int value, compute the result of it modulo 3:
List<Integer> intList = Arrays.asList(1, 2, 3, 4, 5, 6);
Map<String, Integer> map =
intList.stream().collect(toMap(i -> String.valueOf(i % 3), i -> i));
When running this code, you'll get an error saying java.lang.IllegalStateException: Duplicate key 1. This is because 1 % 3 is the same as 4 % 3 and hence have the same key value given the key mapping function. In this case you can provide a merge operator.
Here's one that sum the values; (i1, i2) -> i1 + i2; that can be replaced with the method reference Integer::sum.
Map<String, Integer> map =
intList.stream().collect(toMap(i -> String.valueOf(i % 3),
i -> i,
Integer::sum));
which now outputs:
0 => 9 (i.e 3 + 6)
1 => 5 (i.e 1 + 4)
2 => 7 (i.e 2 + 5)

List<Item> list;
Map<Key,Item> map = new HashMap<Key,Item>();
for (Item i : list) map.put(i.getKey(),i);
Assuming of course that each Item has a getKey() method that returns a key of the proper type.

Just in case this question isn't closed as a duplicate, the right answer is to use Google Collections:
Map<String,Role> mappedRoles = Maps.uniqueIndex(yourList, new Function<Role,String>() {
public String apply(Role from) {
return from.getName(); // or something else
}});

Short and sweet.
Using Java 8 you can do following :
Map<Key, Value> result= results
.stream()
.collect(Collectors.toMap(Value::getName,Function.identity()));
Value can be any object you use.

Alexis has already posted an answer in Java 8 using method toMap(keyMapper, valueMapper). As per doc for this method implementation:
There are no guarantees on the type, mutability, serializability, or
thread-safety of the Map returned.
So in case we are interested in a specific implementation of Map interface e.g. HashMap then we can use the overloaded form as:
Map<String, Item> map2 =
itemList.stream().collect(Collectors.toMap(Item::getKey, //key for map
Function.identity(), // value for map
(o,n) -> o, // merge function in case of conflict with keys
HashMap::new)); // map factory - we want HashMap and not any Map implementation
Though using either Function.identity() or i->i is fine but it seems Function.identity() instead of i -> i might save some memory as per this related answer.

Since Java 8, the answer by #ZouZou using the Collectors.toMap collector is certainly the idiomatic way to solve this problem.
And as this is such a common task, we can make it into a static utility.
That way the solution truly becomes a one-liner.
/**
* Returns a map where each entry is an item of {#code list} mapped by the
* key produced by applying {#code mapper} to the item.
*
* #param list the list to map
* #param mapper the function to produce the key from a list item
* #return the resulting map
* #throws IllegalStateException on duplicate key
*/
public static <K, T> Map<K, T> toMapBy(List<T> list,
Function<? super T, ? extends K> mapper) {
return list.stream().collect(Collectors.toMap(mapper, Function.identity()));
}
And here's how you would use it on a List<Student>:
Map<Long, Student> studentsById = toMapBy(students, Student::getId);

A List and Map are conceptually different. A List is an ordered collection of items. The items can contain duplicates, and an item might not have any concept of a unique identifier (key). A Map has values mapped to keys. Each key can only point to one value.
Therefore, depending on your List's items, it may or may not be possible to convert it to a Map. Does your List's items have no duplicates? Does each item have a unique key? If so then it's possible to put them in a Map.

There is also a simple way of doing this using Maps.uniqueIndex(...) from Google guava libraries

Universal method
public static <K, V> Map<K, V> listAsMap(Collection<V> sourceList, ListToMapConverter<K, V> converter) {
Map<K, V> newMap = new HashMap<K, V>();
for (V item : sourceList) {
newMap.put( converter.getKey(item), item );
}
return newMap;
}
public static interface ListToMapConverter<K, V> {
public K getKey(V item);
}

Using java-8 streams
Map<Integer, String> map = results.stream().collect(Collectors.toMap(e -> ((Integer) e[0]), e -> (String) e[1]));

Without java-8, you'll be able to do this in one line Commons collections, and the Closure class
List<Item> list;
#SuppressWarnings("unchecked")
Map<Key, Item> map = new HashMap<Key, Item>>(){{
CollectionUtils.forAllDo(list, new Closure() {
#Override
public void execute(Object input) {
Item item = (Item) input;
put(i.getKey(), item);
}
});
}};

like already said, in java-8 we have the concise solution by Collectors:
list.stream().collect(
groupingBy(Item::getKey)
)
and also, you can nest multiple group passing an other groupingBy method as second parameter:
list.stream().collect(
groupingBy(Item::getKey, groupingBy(Item::getOtherKey))
)
In this way, we'll have multi level map, like this: Map<key, Map<key, List<Item>>>

Many solutions come to mind, depending on what you want to achive:
Every List item is key and value
for( Object o : list ) {
map.put(o,o);
}
List elements have something to look them up, maybe a name:
for( MyObject o : list ) {
map.put(o.name,o);
}
List elements have something to look them up, and there is no guarantee that they are unique: Use Googles MultiMaps
for( MyObject o : list ) {
multimap.put(o.name,o);
}
Giving all the elements the position as a key:
for( int i=0; i<list.size; i++ ) {
map.put(i,list.get(i));
}
...
It really depends on what you want to achive.
As you can see from the examples, a Map is a mapping from a key to a value, while a list is just a series of elements having a position each. So they are simply not automatically convertible.

Here's a little method I wrote for exactly this purpose. It uses Validate from Apache Commons.
Feel free to use it.
/**
* Converts a <code>List</code> to a map. One of the methods of the list is called to retrive
* the value of the key to be used and the object itself from the list entry is used as the
* objct. An empty <code>Map</code> is returned upon null input.
* Reflection is used to retrieve the key from the object instance and method name passed in.
*
* #param <K> The type of the key to be used in the map
* #param <V> The type of value to be used in the map and the type of the elements in the
* collection
* #param coll The collection to be converted.
* #param keyType The class of key
* #param valueType The class of the value
* #param keyMethodName The method name to call on each instance in the collection to retrieve
* the key
* #return A map of key to value instances
* #throws IllegalArgumentException if any of the other paremeters are invalid.
*/
public static <K, V> Map<K, V> asMap(final java.util.Collection<V> coll,
final Class<K> keyType,
final Class<V> valueType,
final String keyMethodName) {
final HashMap<K, V> map = new HashMap<K, V>();
Method method = null;
if (isEmpty(coll)) return map;
notNull(keyType, Messages.getString(KEY_TYPE_NOT_NULL));
notNull(valueType, Messages.getString(VALUE_TYPE_NOT_NULL));
notEmpty(keyMethodName, Messages.getString(KEY_METHOD_NAME_NOT_NULL));
try {
// return the Method to invoke to get the key for the map
method = valueType.getMethod(keyMethodName);
}
catch (final NoSuchMethodException e) {
final String message =
String.format(
Messages.getString(METHOD_NOT_FOUND),
keyMethodName,
valueType);
e.fillInStackTrace();
logger.error(message, e);
throw new IllegalArgumentException(message, e);
}
try {
for (final V value : coll) {
Object object;
object = method.invoke(value);
#SuppressWarnings("unchecked")
final K key = (K) object;
map.put(key, value);
}
}
catch (final Exception e) {
final String message =
String.format(
Messages.getString(METHOD_CALL_FAILED),
method,
valueType);
e.fillInStackTrace();
logger.error(message, e);
throw new IllegalArgumentException(message, e);
}
return map;
}

A Java 8 example to convert a List<?> of objects into a Map<k, v>:
List<Hosting> list = new ArrayList<>();
list.add(new Hosting(1, "liquidweb.com", new Date()));
list.add(new Hosting(2, "linode.com", new Date()));
list.add(new Hosting(3, "digitalocean.com", new Date()));
//example 1
Map<Integer, String> result1 = list.stream().collect(
Collectors.toMap(Hosting::getId, Hosting::getName));
System.out.println("Result 1 : " + result1);
//example 2
Map<Integer, String> result2 = list.stream().collect(
Collectors.toMap(x -> x.getId(), x -> x.getName()));
Code copied from:
https://www.mkyong.com/java8/java-8-convert-list-to-map/

You can leverage the streams API of Java 8.
public class ListToMap {
public static void main(String[] args) {
List<User> items = Arrays.asList(new User("One"), new User("Two"), new User("Three"));
Map<String, User> map = createHashMap(items);
for(String key : map.keySet()) {
System.out.println(key +" : "+map.get(key));
}
}
public static Map<String, User> createHashMap(List<User> items) {
Map<String, User> map = items.stream().collect(Collectors.toMap(User::getId, Function.identity()));
return map;
}
}
For more details visit: http://codecramp.com/java-8-streams-api-convert-list-map/

I like Kango_V's answer, but I think it's too complex. I think this is simpler - maybe too simple. If inclined, you could replace String with a Generic marker, and make it work for any Key type.
public static <E> Map<String, E> convertListToMap(Collection<E> sourceList, ListToMapConverterInterface<E> converterInterface) {
Map<String, E> newMap = new HashMap<String, E>();
for( E item : sourceList ) {
newMap.put( converterInterface.getKeyForItem( item ), item );
}
return newMap;
}
public interface ListToMapConverterInterface<E> {
public String getKeyForItem(E item);
}
Used like this:
Map<String, PricingPlanAttribute> pricingPlanAttributeMap = convertListToMap( pricingPlanAttributeList,
new ListToMapConverterInterface<PricingPlanAttribute>() {
#Override
public String getKeyForItem(PricingPlanAttribute item) {
return item.getFullName();
}
} );

Apache Commons MapUtils.populateMap
If you don't use Java 8 and you don't want to use a explicit loop for some reason, try MapUtils.populateMap from Apache Commons.
MapUtils.populateMap
Say you have a list of Pairs.
List<ImmutablePair<String, String>> pairs = ImmutableList.of(
new ImmutablePair<>("A", "aaa"),
new ImmutablePair<>("B", "bbb")
);
And you now want a Map of the Pair's key to the Pair object.
Map<String, Pair<String, String>> map = new HashMap<>();
MapUtils.populateMap(map, pairs, new Transformer<Pair<String, String>, String>() {
#Override
public String transform(Pair<String, String> input) {
return input.getKey();
}
});
System.out.println(map);
gives output:
{A=(A,aaa), B=(B,bbb)}
That being said, a for loop is maybe easier to understand. (This below gives the same output):
Map<String, Pair<String, String>> map = new HashMap<>();
for (Pair<String, String> pair : pairs) {
map.put(pair.getKey(), pair);
}
System.out.println(map);

If you use Kotlin, there is an example:
listOf("one", "two").mapIndexed { i, it -> i to it }.toMap()

public class EmployeeDetailsFetchListToMap {
public static void main(String[] args) {
List<EmployeeDetailsFetch> list = new ArrayList<>();
list.add(new EmployeeDetailsFetch(1L, "vinay", 25000F));
list.add(new EmployeeDetailsFetch(2L, "kohli", 5000000F));
list.add(new EmployeeDetailsFetch(3L, "dhoni", 20000000F));
//adding id as key and map of id and student name
Map<Long, Map<Long, String>> map1 = list.stream()
.collect(
Collectors.groupingBy(
EmployeeDetailsFetch::getEmpId,
Collectors.toMap(
EmployeeDetailsFetch::getEmpId,
EmployeeDetailsFetch::getEmployeeName
)
)
);
System.out.println(map1);
//converting list into map of Student
//Adding id as Key and Value as Student into a map
Map<Long, EmployeeDetailsFetch> map = list.stream()
.collect(
Collectors.toMap(
EmployeeDetailsFetch::getEmpId,
EmployeeDetailsFetch -> EmployeeDetailsFetch
)
);
for(Map.Entry<Long, EmployeeDetailsFetch> m : map.entrySet()) {
System.out.println("key :" + m.getKey() + " Value : " + m.getValue());
}
}
}