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We counsel studying this educational, in the collection indexed in the left menu. Java is an object orientated language and some concepts could also be new. Take breaks when wanted, and move over the examples as time and again as needed.Yes, the Google Maps APIs can now be utilized in Desktop programs. Check out these Stack Overflow threads: Google Map in JAVA Swing. Embedding Gecko/Webkit in Java. Webkit browser in a Java app. Rendering webpages with WebKit in Java. You can also see the tutorail of using Maps in Java Desktop Application.Browse and obtain Minecraft Java Maps via the Planet Minecraft group.Java Maps Java Location Map. Full measurement. Online Map of Java. Large detailed vacationer map of Java. 4295x3158 / 4,83 Mb Go to Map. Java on the World Map. 1500x752 / 241 Kb Go to Map. About Java: The Facts: Provinces: Banten, Special Capital Region of Jakarta, West Java, Central Java, East Java, Yogyakarta Special Region.
DetailsWritten by means of Nam Ha Minh Last Updated on 14 June 2019   |   Print E-mailThis instructional helps you realize and master Map - a member in the Java Collections Framework. You will know about: A Map is an object that maps keys to values, or is a selection of attribute-value pairs. It models the serve as abstraction in arithmetic. The following picture illustrates a map:Note that a Map is not thought to be to be a true collection, as the Map interface does no longer prolong the Collection interface. Instead, it starts an impartial department in the Java Collections Framework, as shown in the following diagram: Characteristics of a Map:Because a Map isn't a real assortment, its characteristics and behaviors are other than the opposite collections like List or Set.A Map cannot comprise replica keys and each key can map to at maximum one value. Some implementations allow null key and null value (HashMap and LinkedHashMap) but some does now not (TreeMap).The order of a map depends on explicit implementations, e.g TreeMap and ConnectedHashMap have predictable order, while HashMap does now not. Why and When Use Maps:Maps are completely for key-value affiliation mapping such as dictionaries. Use Maps when you wish to have to retrieve and update parts by keys, or perform lookups by way of keys. Some examples:A map of error codes and their descriptions.A map of zip codes and towns.A map of managers and employees. Each supervisor (key) is related to a listing of staff (value) he manages.A map of classes and scholars. Each magnificence (key) is associated with a listing of students (value).This instructional supplies code examples around the three primary implementations of Map which are described under. In the inheritance tree of the Map interface, there are a number of implementations however handiest Three major, commonplace, and basic goal implementations - they're HashMap and LinkedHashMap and TreeMap. Let’s see the characteristics and behaviors of each and every implementation:HashMap: this implementation makes use of a hash desk as the underlying data structure. It implements the entire Map operations and permits null values and one null key. This elegance is roughly equivalent to Hashtable - a legacy knowledge construction ahead of Java Collections Framework, however it's not synchronized and permits nulls. HashMap does no longer ensure the order of its key-value elements. Therefore, consider to make use of a HashMap when order does not matter and nulls are applicable. ConnectedHashMap: this implementation makes use of a hash desk and a linked checklist because the underlying data structures, thus the order of a ConnectedHashMap is predictable, with insertion-order because the default order. This implementation additionally allows nulls like HashMap. So consider using a ConnectedHashMap when you want a Map with its key-value pairs are sorted via their insertion order. TreeMap: this implementation makes use of a red-black tree because the underlying information construction. A TreeMap is looked after in step with the herbal ordering of its keys, or by means of a Comparator supplied at advent time. This implementation does no longer allow nulls. So believe the usage of a TreeMap when you need a Map types its key-value pairs by the herbal order of the keys (e.g. alphabetic order or numeric order), or via a custom order you specify.So a long way you have got understood the important thing variations of the 3 main Map’s implementations. And the code examples in this educational are around them.Now, let’s see find out how to use Map for your daily coding. Creating a HashMap:Always use interface sort (Map), generics and diamond operator to declare a new map. The following code creates a HashMap:Map<Integer, String> mapHttpErrors = new HashMap<>(); mapHttpErrors.put(200, "OK"); mapHttpErrors.put(303, "See Other"); mapHttpErrors.put(404, "Not Found"); mapHttpErrors.put(500, "Internal Server Error"); System.out.println(mapHttpErrors);This maps HTTP standing codes to their descriptions. Output:404=Not Found, 500=Internal Server Error, 200=OK, 303=See OtherAs you can see in the output, a HashMap does no longer impose any order on its key-value parts.You can create a brand new Map that copies parts from an existing map. For instance:Map<Integer, String> mapErrors = new HashMap<>(mapHttpErrors);The map mapErrors is created with initial components copied from the map mapHttpErrors. Creating a LinkedHashMap:The following code creates a LinkedHashMap that maps telephone numbers with contact names:Map<String, String> mapContacts = new LinkedHashMap<>(); mapContacts.put("0169238175", "Tom"); mapContacts.put("0904891321", "Peter"); mapContacts.put("0945678912", "Mary"); mapContacts.put("0981127421", "John"); System.out.println(mapContacts);Output:0169238175=Tom, 0904891321=Peter, 0945678912=Mary, 0981127421=JohnAs you'll be able to see, the LinkedHashMap maintains its elements by their insertion order. Creating a TreeMap:The following code creates a TreeMap that maps report extensions to programming languages:Map<String, String> mapLang = new TreeMap<>(); mapLang.put(".c", "C"); mapLang.put(".java", "Java"); mapLang.put(".pl", "Perl"); mapLang.put(".cs", "C#"); mapLang.put(".php", "PHP"); mapLang.put(".cpp", "C++"); mapLang.put(".xml", "XML"); System.out.println(mapLang);Output:.c=C, .cpp=C++, .cs=C#, .java=Java, .php=PHP, .pl=Perl, .xml=XMLAs you'll be able to see, the TreeMap sorts its keys by means of their natural ordering, which is the alphabetical order on this case. The basic operations of a Map are affiliation (put), look up (get), checking (containsKeyand containsValue), amendment (removeand exchange) and cardinality (measurement and isEmpty). Associating a value with a key:The put(Okay, V) method buddies the required price V with the required key Ok. If the map already accommodates a mapping for the key, the outdated price is changed by way of the required cost:Map<Integer, String> mapHttpErrors = new HashMap<>(); mapHttpErrors.put(400, "Bad Request"); mapHttpErrors.put(304, "Not Modified"); mapHttpErrors.put(200, "OK"); mapHttpErrors.put(301, "Moved Permanently"); mapHttpErrors.put(500, "Internal Server Error"); Getting a value associated with a specified key:The get(Object key) way returns the price related to the required key, or returns null if the map incorporates no mapping for the key. Given the map in the previous instance:String status301 = mapHttpErrors.get(301); System.out.println("301: " + status301);Output:301: Moved Permanently Checking if the map comprises a specified key:The manner containsKey(Object key) returns true if the map contains a mapping for the required key. For instance:if (mapHttpErrors.containsKey("200")) System.out.println("Http status 200"); Output:Found: Http standing 200 Checking if the map comprises a specified cost:The way containsValue(Object price) returns true if the map accommodates a number of keys associated with the desired cost. For example:if (mapHttpErrors.containsValue("OK")) System.out.println("Found status OK"); Output:Found status OK Removing a mapping from the map:The take away(Object key) method removes the mapping for a key from the map whether it is provide (we care about only the important thing, and the worth does no longer subject). This means returns the value to which the map in the past associated the important thing, or null if the map doesn’t contain mapping for the important thing. Here’s an instance:String removedValue = mapHttpErrors.take away(500); if (removedValue != null) System.out.println("Removed value: " + removedValue); Output:Removed value: Internal Server ErrorSimilarly, the remove(Object key, Object cost) manner eliminates the mapping of a specified key and specified cost, and returns true if the value used to be removed. This approach comes in handy in case we really care about the key and worth to be got rid of.I recommend you to learn this well-know Java assortment guide to learn in-depth about Java collections framework. Replacing a value related to a specified key:The exchange(Okay key, V value)approach replaces the entry for the desired key only whether it is recently mapping to a couple price. This manner returns the former price associated with the required key. Here’s an example:System.out.println("Map before: " + mapHttpErrors); mapHttpErrors.replace(304, "No Changes"); System.out.println("Map after: " + mapHttpErrors);Output:Map ahead of: 400=Bad Request, 304=Not Modified, 200=OK, 301=Moved Permanently Map after: 400=Bad Request, 304=No Changes, 200=OK, 301=Moved PermanentlySimilarly, the exchange(Okay key, V oldValue, V newValue) method replaces the access for the required key best if it is these days mapping to the required value. This way returns true if the worth was changed. Useful in case we need to change exactly a key-value mapping. Getting the scale of the map:The size()approach returns the choice of key-value mappings in this map. For example:int size = mapHttpErrors.measurement();Output:Number of HTTP status code: 5 Checking if the map is empty:The isEmpty() way returns true if the map accommodates no key-value mappings. For instance:if (mapHttpErrors.isEmpty()) System.out.println("No Error"); else System.out.println("Have HTTP Errors"); Output:Have HTTP Errors As a Map isn't a true assortment, there is no direct means for iterating over a map. Instead, we will iterate over a map the use of its assortment views. Any Map’s implementation has to give you the following 3 Collection view methods:keySet(): returns a Set view of the keys contained within the map. Hence we will iterate over the keys of the map as proven in the following example:Map<String, String> mapCountryCodes = new HashMap<>(); mapCountryCodes.put("1", "USA"); mapCountryCodes.put("44", "United Kingdom"); mapCountryCodes.put("33", "France"); mapCountryCodes.put("81", "Japan"); Set<String> setCodes = mapCountryCodes.keySet(); Iterator<String> iterator = setCodes.iterator(); whilst (iterator.hasNext()) String code = iterator.subsequent(); String nation = mapCountryCodes.get(code); System.out.println(code + " => " + country); Output:44 => United Kingdom 33 => France 1 => USA 81 => Japan values(): returns a number of values contained within the map. Thus we will iterate over values of the map like this:Collection<String> nations = mapCountryCodes.values(); for (String country : countries) System.out.println(nation); Output:United Kingdom France USA Japan accessSet(): returns a Set view of the mappings contained in this map. Therefore we can iterate over mappings within the map like this:Set<Map.Entry<String, String>> entries = mapCountryCodes.accessSet(); for (Map.Entry<String, String> entry : entries) String code = entry.getKey(); String country = access.getValue(); System.out.println(code + " => " + country); Output:44 => United Kingdom 33 => France 1 => USA 81 => JapanSince Java 8 with Lambda expressions and the forEach() statement, iterating over a Map is as easy as:mapCountryCodes.forEach( (code, country) -> System.out.println(code + " => " + nation));Output:44 => United Kingdom 33 => France 1 => USA 81 => JapanFor more details about the different methods of collection iteration, learn this article: The 4 Methods for Iterating Collections in Java. There are two bulk operations with maps: clear() and putAll().The transparent() method eliminates all mappings from the map. The map will be empty after this method returns. For instance:mapHttpErrors.transparent(); System.out.println("Is map empty? " + mapHttpErrors.isEmpty());Output:Is map empty? trueThe putAll(Map<K, V> m) means copies the entire mappings from the desired map to this map. Here’s an instance:Map<Integer, String> countryCodesEU = new HashMap<>(); countryCodesEU.put(44, "United Kingdom"); countryCodesEU.put(33, "France"); countryCodesEU.put(49, "Germany"); Map<Integer, String> countryCodesWorld = new HashMap<>(); countryCodesWorld.put(1, "United States"); countryCodesWorld.put(86, "China"); countryCodesWorld.put(82, "South Korea"); System.out.println("Before: " + countryCodesWorld); countryCodesWorld.putAll(countryCodesEU); System.out.println("After: " + countryCodesWorld);Output:Before: 1=United States, 82=South Korea, 86=China After: 1=United States, 33=France, 49=Germany, 82=South Korea, 86=China, 44=United Kingdom Unlike the legacy Hashtable which is synchronized, the HashMap, TreeMap and ConnectedHashMap don't seem to be synchronized. If thread-safe is priority, imagine using ConcurrentHashMap rather than HashMap. Or we will use the Collections.synchronizedMap() software manner that returns a synchronized (thread-safe) map subsidized by way of the specified map. For example:Map<Integer, String> map = Collections.synchronizedMap(new HashMap<>());And remember we need to manually synchronize the map when iterating over any of its assortment views:Set<Integer> keySet = map.keySet(); synchronized (map) Iterator<Integer> iterator = keySet.iterator(); while (iterator.hasNext()) Integer key = iterator.subsequent(); String value = map.get(key); If you use a kind of SortedMap, e.g. TreeMap, believe the usage of the more particular method Collections.synchronizedSortedMap(). NOTE: If you employ your individual sort for the important thing and value (e.g. Student or Employee), the important thing class and value class must enforce the equals() and hashCode() methods properly so that the map can glance up them appropriately. API References Related Map Tutorials: Other Java Collections Tutorials: That's a comprehensive and great detailed educational about Java map. I hope you snatch something new and experience finding out. If you will have time and finances, I like to recommend you to take this Java masterclass course to be told Java programming in-depth. About the Author:Nam Ha Minh is certified Java programmer (SCJP and SCWCD). He began programming with Java in the time of Java 1.Four and has been falling in love with Java since then. Make buddy with him on Facebook and watch his Java videos you YouTube.Add comment