A map cannot contain duplicate keys; each key can map to at most one value. EDIT. Umut Can Alaçam. Each time you look up sheep you get wooly mammal. Collection<String> all = map. Return Value: The method returns a set havin Feb 23, 2022 · If my understanding is correct you need to remove from the second map all entries that are contained in the first map. Using the keys of the common list and different list you can iterate through map, using equals you can compare the maps. Without using any collection classes. toMap (Employee::getId, Function. This is one solution to add objects with duplicate keys but Java 8 Stream. new Map(mappedArr). parseDouble(arr2[2]); Apr 28, 2021 · If you want to preserve order, obviously don't call . Apr 26, 2020 · To achieve what you actually ask for: Before your put line: while (data. You can simply use this: list. Create a LinkedHashSet from this ArrayList. I want to store the objects of Employee in a Map and the key would be the Employee name. Then remove the collection of keys from the map. Feb 11, 2022 · 1. no need to look at neighbours). This will remove the duplicates. The merge() function works as follows; if the specified key isn’t already associated with a value, or the value is null, it associates the key with the given value. Dec 11, 2018 · Approach: Get the ArrayList with duplicate values. getName (), Function. May 3, 2021 · The java. Next, Convert List of user-defined (custom) objects to Map and handling with the duplicate keys. Consider the below example where two Person instances are considered equal if both have the same id value. Dec 5, 2013 · This streams over the map's entry set, and collects it back into a map with the key and value swapped. Collectors; list. forEach(all::remove); System. Finally, multimap is put in the map and you find the MultiMap with duplicates added in the new Map too. As mentioned in the comment, maps do not store duplicated keys so you have to use Map<String, List<String>> instead Map<String, String>. edited Dec 14, 2018 at 13:19. Next, call stream. split(" "); String model=arr2[1]; Double distance=Double. remove("Age"); }); Since the map object is a reference, you can simply delete a key from the map without copying it into another map object. So its up to you to validate the map before trying to add the KEY and VALUE. forEach((key, value) -> map. Let’s start by creating an instance of MultiValuedMap: MultiValuedMap<String, String> map = new ArrayListValuedHashMap <>(); Copy. clear(); yourList. values(); new HashSet<>(all). The second (or third, or whatever) time you call put with a key that already exists in the map, it will simply override the value with a new one, regardless of whether it's a duplicate or not of the pre-existing one. This leaves only the duplicates in the collection. * A simple Java Program to check if HashMap contains a given key or value or not. It was worth telling you to improve your answer, and upvoting you :) It was worth telling you to improve your answer, and upvoting you :) Jul 25, 2020 · Convert List to Map in Java. You cannot remove duplicates from a HashMap because there are no duplicates in a HashMap in the first place. Side note: class Date is obsolete and not encouraged to be used. putAll(originalMap); We should note that put () and putAll () replace the values if there is a matching key. asList(9, 2, 2, 7, 6, 6, 5, 7); Then, let's collect the elements into a Map and count their Mar 8, 2016 · Interesting problem. How Do I go about it. The next step would be either a reduceByKey or groupByKey and filter. rulesMap. Create an ArrayList out of values() set. Create the stream of maps, then flatMap it to a stream of keys, then collect the keys into a Map counting how many of each key there is, then stream that EntrySet, Set<Map. Next, let’s see how we can add elements one at a time using the put method: map. Also after adding 10 objects in the Map. collect (Collectors. But a map that contains "Luke" will not return true if added "luke" and hence those answers wont work for this problem. stream(). addAll(set); Of course, this destroys the ordering of the elements in the ArrayList. Entry from a List. I want to remove those duplicate values. But in case you don't want to always map to a constant, the use of the "merge-function" might help: Jan 28, 2013 · String[] parts = rule. Set interface implementation such as HashSet or LinkedHashSet. If there are duplicate values, using groupingBy as suggested in another answer is a good suggestion, as each value of the inverted map will be a list of one or more elements. javaprogramto. List<Map<String, Object>> obtainAuditLog = pddBamControlDao. In toMap method we can pass key and value pairs as arguments. We can use this feature to find the distinct elements in the array and then find unique and duplicate elements using the simple add and remove operations. Remove duplicate elements from Arrays : Initially, there is a String [] Array with duplicate elements. An object that maps keys to values. time was introduced for such purposes Apr 10, 2020 · Anyway, without wasting any more of your time, here is the complete Java code to check if a key or value exists in a given Map or not. It is set up like this: LinkedHashMap<String, Integer> wordIndex = new LinkedHashMap<String, Integer>(); My goal is to remove any duplicates the user may enter. My idea is the following: Create a helper HashMap<String, DataRecord> where keys will be TUNs. stream () . findPaymentsByIds(pmtIds) . We’ve got a map that associates food items with food types: HashMap<String, String> foodItemTypeMap = new HashMap <>(); Nov 26, 2021 · Since this will just drop all objects with an already encountered id, just can simply use Map<Long, Employee> collect = employees. stream() Sep 12, 2018 · If you override the equals method accordingly you can do it this way in java 8+: import java. Using Stream and Set. toSet ()) method. It would require the stream to be sorted by the same property, to be able to take advantage of it. It is worth noting that, depending on the Map implementation you use, the iteration order Aug 29, 2021 · Java 8 toMap duplicate key. Jul 17, 2022 · 1. put( "fruits", "apple" ); Sep 20, 2023 · Alternatively, you can also count the occurrences of duplicate elements and keep that information in a map that contains the duplicate elements as keys and their frequency as values. toMap method with mergeFunction parameter where you can handle duplicated keys. identity (), (prev,next) -> prev)); to achieve the same. In case the duplicated value is found, there must be selected one key (k) from the two keys (k1 and k2) which hold these values, for this reason, assume the BinaryOperator<K> giving k from k1 and k2 is available. toList()) It's also achievable without overriding the equals method, but more verbose though: Mar 24, 2023 · Using Map to remove duplicates from an array of objects. With Java 8 a new IPA that resides in the package java. First, Let us convert List into Map. collect (toMap (p -> p. forEach. 2. out. In this article, You'll explore and learn how to convert List to Map in Java 8. groupingBy() to collect to a Map<Integer, List<Currency>> and in this case you could have multiple values by key or as alternative merge the duplicate keys with the toMap() overload, for example to keep the last entry : private static final Map<Integer, Currency Sep 13, 2020 · Iterate over a set of that collection, removing the first of each value encountered. stream. May 8, 2014 · The answer should be C: Take a look at the Map javadoc. May 24, 2018 · The code is generating an IllegalStateException: Duplicate key, if there are duplicate values in the list and while we are trying to extract map from list using java streams. To remove duplicate characters, you can pass the String itself as the second argument to the replace() method. merge, added in Java 8, which uses a user-provided BiFunction to merge values for duplicate keys. for the property Name (the same for the property Id etc. So used like this ". Map will eliminate the duplicate keys values, keeping only last inserted value of the same key. Set. ? I expect the output (1 , 7) (3, 7) You Map declaration looks like Map<String, Double> that means, that only one Double value can be mapped to String key. Map interface for entry removal using the key as an input. I have tags from posts, which is also array: tags: ['react', 'javascript', 'node'] and from map function I have output like this: node react javascript react javascript react javascript node Feb 7, 2014 · How it brings in your problem into picture, is by having a multimap which can contain duplicates. ): Set<String> nameSet = new HashSet<>(); Nov 9, 2022 · 2. replace() method. java-8 Mar 8, 2014 · The add() method on HashSet will return false if a value has already been added to the set. You can simply, change Double to List<Double> and map a key to the list, then your case look like: String[] arr2=line. The second ArrayList contains the elements with duplicates removed. duplicateKeyException Aug 18, 2020 · Output: Original list values : [A, B, C, C, B, A] newlist values are : [A, B, C] 3. if both a key and its value are the same). The method above uses this to detect that a duplicate has been found and then removes the duplicate from the HashMap by using the remove() method on the iterator. I'm creating a Linked HashMap that will read a string and integer from the user inputs. Using LinkedHashSet. 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. answered Jan 26, 2022 at 20:03. . Here we concatenate strings for duplicate keys: Sep 13, 2020 · Iterate over a set of that collection, removing the first of each value encountered. The program needs to accept requests in the form of keys, and return the respective values. Jan 8, 2024 · Conclusion. package com. In those cases, you will get a Duplicate key IllegalStateException. collect(. Second, streams are not able to detect whether they are sorted by a certain property. You can rewrite the above code to update a value in HashMap in just one line in Java 8 as shown below: map. While knowing this won't help us magically transform any array into an array of unique values, certain use cases Dec 10, 2020 · 2. Finally, store those elements into another new Set using collect (Collectors. identity())); Wow, it just took one line to convert a list An object that maps keys to values. May 17, 2010 · The correct answer for Java is use a Set. 1. I have a spatial index of objects where I want to find all objects between a start/end longitude. In the below example, key is the string and value is the length of string. Jul 24, 2012 · There wont be any duplicate Keys . Oct 1, 2015 · The Java 8 addition is great because this is one of the recent good changes on the Map API people need to know about. putIfAbsent(key, value) != null) key += "_"; Jan 6, 2020 · I have a map Map<K, V> and my goal is to remove the duplicated values and output the very same structure Map<K, V> again. Next, use LinkedHashSet to remove the duplicates and preserve the order as in the original list. identity(), email -> processEmails(email), (list1, list2) -> list1 // or { list1. keySet() Parameters: The method does not take any parameter. Finding Distinct Items by Multiple Fields. More generally using streams here is complicated. In List to Map conversion, we will see how to maintain unique keys. Map<String, List<String>> map = emails. I can't think of a particularly neat way of doing it but here's a potential solution using Java 8 - I'm pretty sure it could be simplified somewhat. sorted, which messes with order. stringObjectMap. Oct 14, 2015 · In Java 8 you can do: e. toMap(), which handles values having the same key: Map<BgwContract, List<Fee>> bgwContractFeeMap = bgwContractList. HashMap. Modifying the map while iterating is usually frowned upon. If you already have a List<Customer> and want to de duplicate it. 2. Feb 12, 2018 · It is working with Java 7 but can't find a way to do it in Java 8. Also, find the commented code in the main method, which exhibits the fact that Map cannot hold duplicates. Remove Duplicate Custom Objects. Using Set implemented classes. IllegalStateException: Duplicate key 4 (attempted merging values 16 and 16) Aug 24, 2021 · This was a very good 1 liner fix for duplicate keys. * in Java. First iteration joins common key/value pairs from map1 and map2 together and adds them to the resulting map or adds unique key/value pairs in map1 to the resulting map. java8. getOrDefault( key, 0) + 1 ); May 15, 2015 · Then, you can use the reduceByKey or reduce operations to eliminate duplicates. The Stream API, which was unveiled in Java 8 to offer a more practical method of interacting with collections, includes this action. toMap () First create a list with string values and convert it into Stream using stream () method. If the key is a duplicate, then append all "parts" of this duplicate zone into the existing zone's parts list. println(new HashSet<>(all)); Prints [a, b] Mar 3, 2021 · There are basically two methods to remove the duplicate entries from the collection: Now Let’s see the implementation using the java program to remove the duplicate entries by using both the methods one by one:-. Update: After overriding equals() you can do the following: List<Customer> distinctCustomers = customersList. List and just add the objects to the Set. toMap(Payment::getAbcId, Payment::getPaymentId)); But then I reallize this could have duplicate keys, so I want it to return a. Let’s see an example: Map<String, String> initDevMap() {. The problem originated when I wrote some code in python using SortedContainers. Streams like it if the operation(s) you want to perform on each element within a stream are independent (do not need to look at anything except the one element being considered, i. If the String already appears in the map, increment its key by one, otherwise, add it to the map. HashSet calls put(K,V) on the internal HashMap which just inserts the new object overwriting the old entry if duplicate. Map interface. Otherwise, it replaces the value with the results of the given remapping function. The same way you remove duplicates Aug 18, 2020 · Output: Original list values : [A, B, C, C, B, A] newlist values are : [A, B, C] 3. put("The", 0); wordIndex. Feb 28, 2015 · The pramodh's answer is good if you want to map your value to 1. You don't need the stream API, creating a Set will elimnate duplicates: Set<Map<String, Object>> set = new HashSet<> (list); Provided that, equals map has the same keys, and the values associated to that keys in the Maps implements equals and hashCode. I want to retrieve the 8th object that was entered. populateAuditLogMap(); I think you should provide more details, maybe some example of what you think duplicate is. asList(9, 2, 2, 7, 6, 6, 5, 7); Then, let's collect the elements into a Map and count their Dec 11, 2009 · Maybe you can iterate over the map looking for the keys to remove and storing them in a separate collection. com Dec 10, 2016 · Remove duplicates from java list without using sets. The String. If it returns false then it means that there are duplicates present in the Original Arrays. toMap ()) method to convert list to map. Map<Long, Integer> abcIdToPmtId = paymentController. Then print them as a set showing the duplicates. Just check the below method works for you! Oct 29, 2009 · They just always execute the add() on their internal structure which holds the set elements and let that object handle the duplication case. For example: put("123", 1); Now let's assume that you have "123" again, you should get the count of the key and add one to it: Feb 1, 2018 · Use java. I. Feb 20, 2012 · I'm creating a program that needs to store key-value pairs. I want to pick the (Key,Value) pair which has duplicate values. LinkedHashSet is a collection api Set interface. If I get the Duplicate (Key,Value) as another Hashmap it would be great. Jan 8, 2024 · In this tutorial, we’ll explore removing duplicate values from a HashMap. collect(Collectors. Sep 17, 2014 · When you try to add the KEY which is already available in the hashmap, then it will override the previous KEY's VALUE and adds the new VALUE. put("name", "MyName"); Dec 19, 2021 · The method identity() makes the mapper use the Integer element itself as the key of the map entry. So, we will create a custom Predicate for this purpose. This operation will be called every time when same keys appears. The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys Aug 2, 2022 · Use this another version of toMap(keyMapper,valueMapper,mergeFunction) which requires the third argument, mergeFunction - a function responsible for resolving duplicates. Summary: HashMap allows duplicate values, but not keys. Here is some code to get you started: return "{0}{1}{2}". If we define merge function in collect method, then the problem got resolved. Let’s discuss one-by-one in detail with example/explanation. Collectors. Using Method remove (Object key) Let’s try it out using a simple example. As always, the complete source code for the examples is available over on GitHub. 46 2. Output: 2. The same syntax can be used to remove the duplicate objects from List. May 16, 2014 · 3. Only Duplicate values can occur. First, the OP did never state that the stream is sorted at all. // TODO: exception when duplicate key in map. Finally, Sort and collect the Map from List. so with something like this you can achieve the no duplicates requirement (Which is basically the implementation of HashSet): String testString = "foo"; Feb 8, 2018 · 1. If you can not or don't want to override the equals method, you can filter the stream in the following way for any property, e. (check difference between Map and object) new Map(entry array just mapped above) Third we use the map values to retrieve the original items, but this time without duplicates. You need to pass a merge function to Collectors. frequency(e, s) > 1); If !Java 8 you can create a HashMap<String, Integer>. identity () for the valueMapper: Map<String, Person> myMap = persons. Syntax: hash_map. Use distinct () method of Stream API to remove duplicate String elements and then invoke toArray (); method to convert/result into Object [] Array. In this article, we’ve explored two approaches to handling duplicated keys when producing a Map result using Stream API: groupingBy () – Create a Map result in the type Map<Key, List<Value>>. * Using the String. e. values() Oct 7, 2014 · 2. The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys Sep 10, 2011 · 2. To do so, we need to be very careful about the object’s equals() method, because it will decide if an object is duplicate or unique. util. Using combination of both List & Set implemented classes. merge operates on individual keys and values, so you'll need to use a loop or Map. toMap(this::getRequestId, ov -> ov, (ov, ov2) -> ov));" Apr 13, 2017 · Ways to remove duplicate elements from Arrays: Using List implemented classes. containsKey(key)) key += "_"; data. Sometimes when you use a key other than the primary key it is possible that there might be duplicate records. format(x[0],x[2],x[3]) Now, you have a key-value RDD that is keyed by columns 1,3 and 4. You can use finalmapnode. println(new HashSet<>(all)); Prints [a, b] Nov 29, 2010 · If you need more control over how values are combined, you can use Map. merge(key, value, (oldValue, newValue Apr 16, 2015 · If you can make use of equals, then filter the list by using distinct within a stream (see answers above). Sep 20, 2023 · Alternatively, you can also count the occurrences of duplicate elements and keep that information in a map that contains the duplicate elements as keys and their frequency as values. Oct 15, 2008 · The easiest way to remove repeated elements is to add the contents to a Set (which will not allow duplicates) and then add the Set back to the ArrayList: Set<String> set = new HashSet<>(yourList); yourList. . addAll Mar 29, 2024 · In this article, we’ve explored various ways to find map keys with duplicate values in a Set, including a classic loop-based solution, Java 8-based solutions, and approaches using Guava. The problem is that there are sometimes multiple values for each key, and the map class doesn't allow for duplicate keys. of(E elements) throws IllegalArgumentException if any Dec 30, 2020 · I'm trying to remove duplicates in the array. To achieve what you actually ask for: Before your put line: while (data. mapTo () – Allows us to implement the merge logic in the merge function. Java Set class stores only the distinct elements. answered Jan 19, 2021 at 11:56. toList()); Sep 29, 2021 · I am creating a Map<String, Object> using the JavaStreams within my application and then I pass the value to the Map with duplicate keys then it throws the following error: java. // using lambda newMap. Associates the specified value with the specified key in this map (optional operation). Collectors. @paweloque For Comparing two Map Objects in java, you can add the keys of a map to list and with those 2 lists you can use the methods retainAll() and removeAll() and add them to another common keys list and different keys list. Jan 8, 2024 · Instead of iterating through all of the entries, we can use the putAll () method, which shallow-copies all of the mappings in one step: HashMap<String, Employee> shallowCopy = new HashMap <>(); shallowCopy. toMap( Function. Then, it doesn’t matter how Employee’s hashCode and equals are implemented. Using HashSet. Jan 26, 2022 · 2. How to remove duplicates from a list of object. This does assume there aren't any duplicate values in the map. keySet() method in Java is used to create a set out of the key elements contained in the hash map. distinct(). e. Removing Duplicates Using LinkedHashSet. Mar 14, 2022 · Java does not have direct support for finding such distinct items from the Stream where items should be distinct by multiple fields. Set instead of java. Suppose there is a class Employee with name, sal and dob as attributes. Aug 1, 2019 · It is a Java bug fixed in Java 9. distinct () Method. Jan 10, 2019 · 6. filter () method by adding elements into newly created HashSet object using add () method. The distinct () operation in Java 8 is mostly related to streams and is employed to remove duplicate elements from the stream. If you don't find a way to configure the ObjectMapper to filter out duplicate attributes, you can serialize the problematic object to JSON, then serialize the JSON to a Map object, merge duplicate attributes and serialize it to JSON again: Map<String, String> objectWithDuplicates = new HashMap<>(); map. putIfAbsent(key, value) != null) key += "_"; Dec 28, 2023 · In Java, there are a few different ways to remove duplicate characters from a String. Remove duplicated key Map. it will be ignored as removing the key and adding back the same key (the dummy value is the same) would not make any sense at all. Below given is a function that accepts varargs parameters and returns a Predicate instance. 1. Convert this LinkedHashSet back to Arraylist. @DanielEarwicker this question is about "distinct by property". It keeps the first value I had the object in the stream as the value itself. put( "Tom", "Linux" ); Jul 25, 2020 · Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand Mar 7, 2014 · I found a way to get TreeSet to store duplicate keys. put("Hello", 6); Apr 22, 2022 · For finding duplicates, use Stream. tomap May 17, 2018 · Here is an example using iteration of both maps. put(parts[0], parts[1]); // Put the two parts into the map. split("::="); // split the line into non-terminal and terminal. To solve your issue, either use Collectors. Here is an example: wordIndex. It basically returns a set view of the keys or we can create a new set and store the key elements in them. Thus, when you call it with duplicate entries, it always throws an IllegalStateException: Exception in thread "main" java. To sum up, you just have to use Collectors. <E> java. If the map previously contained a mapping for the key, the old value is replaced by the specified value. A not-so-well-known fact is that the Map data structure maintains key uniqueness, meaning that there can be no more than one key-value pair with the same key in a given Map. Jan 8, 2024 · We can add elements using the put and putAll methods. stream() . Jan 4, 2020 · Then, the follwing code will remove duplicates: List<DataRecord> noDuplicatesList = new ArrayList<>(new HashSet<>(transactionList)); When you can't override the equals method, you need to find a workaround. areus. Everything works fine, but now my assignment says that I need to throw an exception if the key of any line is duplicated (occurring more than once). This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface. This idea may be suspect if the map is very large. Java 8 solution using Map#merge. See this post for more info. IllegalStateException: Duplicate key namespace:localName (attempted merging values [ONE] and [TWO]) at java. removeIf(s -> Collections. you can use collection as value against same key. Entry>, filter by the count of the occurrence of the keys, map that to the key values and collect to a list. An array is indexed by an integer, and can have duplicate values, arr[2]=5 ; arr[7]=5; A hash can also store duplicate values, but the keys must be unique: Adam{age}=21; Bill{age}=21; Oct 22, 2013 · Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand Map does not supports duplicate keys. identity (), // or p -> p (p1, p2) -> { /* do logic */ })); But as you can see the resulting map values are Person objects, if you This code is very simple and easy to read, let's now see the Java 8 version to find out whether Java 8 really makes your life easy when it comes to writing day to day code: Map< String, Course > result = listOfCourses. First step is to iterate through original String [] Array with duplicates. 2 Answers. Introduction to the Problem. Dec 16, 2020 · Or, you can try this. g. The name can be duplicate. So there might be one abc for several payments, now I have: //find abc id for each payment id. Using Java 8 Streams. Dec 10, 2022 · This method returns the value to which the specified key is mapped, or default value, if this map contains no mapping for the key. See full list on baeldung. Second iteration grabs anything leftover in map2 that didnt match map1 and adds them to the resulting map. As a side note, for every duplicate key, since the value generated for our entry in HashSet is some random/dummy value, the key is not replaced at all. Mar 25, 2019 · I want to stream z and do the following: construct a map out of z with the following conditions: if the key (based on the "Id" of the Zone) is new, then create an entry in the map with the key and the Zone. containsKey(KEY); to validate and then add the VALUES. Set<Customer> s = new HashSet<Customer> (listCustomer); Otherise just use a Set implemenation HashSet, TreeSet directly and skip the List construction phase. Jan 21, 2019 · 2. arraylist. Collect List to Map using Collectors. base/java. Introduction. HashMap allows multiple keys to have the same value, making duplicates inevitable in certain scenarios. replace() method can be used to replace all occurrences of a specified character with another character. I'm still not sure we understand the question, though, as it's not at all clear how you expected to generate the block titled "Output", or what you intend to do with it. A dictionary contains words and definitions, so sheep="wooly mammal" is a valid assignment. This will assure you don't have any duplicate objects in the Set. Documentation Jan 8, 2024 · We can use the methods provided by the java. Let's create a List of Integer type: List<Integer> list = Arrays. if you want to access the whole person object in the merge function then pass Function. The longitudes could be duplicates but I still need the ability to efficiently add/remove specific objects from Jun 17, 2020 · 1. toMap(Course::getTitle , Function . Sorted by: 2. In Java 8 I can create key, value pair but not able to create a list if a type is same. Dec 4, 2018 · Below is the code snippet. In the code Jan 8, 2024 · Java 8 adds a new merge() function into the java. In a Map multiple duplicate entries will be there for one value. You can do these two lines in one: while (data. put(key, value); This will keep on checking the map to see if key already exists, and if it does, it adds the _ to the end, and tries again. lang. Note: Map keeps the order of insertion. Dec 7, 2014 · You still won't have duplicates in the HashMap, because, as two other posters have pointed out, you can't get duplicate keys in a Map. */ public class ContainsKeyExample { public static void main (String[] args) { // let's create a Map which maps id to Feb 3, 2018 · Update: It might be different from possible duplicate because the possible duplicate's answers do show how to use distinct() with property using a map. put ( key, map . Map<String, String> devMap = new HashMap <>(); devMap. forEach(stringObjectMap -> {. uu av lx yu xa pq ci et uw bf