HashMap Class Methods in Java with Examples | Set 1 (put(), get(), isEmpty() and size())
In this post more methods are discussed.
- keySet(): java.util.HashMap.keySet() It returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa.
Syntax: public Set keySet() Return: a set view of the keys contained in this map
- values(): java.util.HashMap.values() It returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa.
Syntax: public Collection values() Return: a collection view of the values contained in this map
- containsKey(): java.util.HashMap.containsKey() It returns true if this map maps one or more keys to the specified value.
Syntax: public boolean containsValue(Object value) Parameters: value - value whose presence in this map is to be tested Return: true if this map maps one or more keys to the specified value
Implementation:
// Java program illustrating usage of HashMap class methods// keySet(), values(), containsKey()import java.util.*;public class NewClass{ public static void main(String args[]) { // 1 Creation of HashMap HashMap<String, String> Geeks = new HashMap<>(); // 2 Adding values to HashMap as ("keys", "values") Geeks.put("Language", "Java"); Geeks.put("Platform", "Geeks For geeks"); Geeks.put("Code", "HashMap"); Geeks.put("Learn", "More"); // 3 containsKey() method is to check the presence // of a particluar key // Since 'Code' key present here, the condition is true if (Geeks.containsKey("Code")) System.out.println("Testing .containsKey : " + Geeks.get("Code")); // 4 keySet() method returns all the keys in HashMap Set<String> Geekskeys = Geeks.keySet(); System.out.println("Initial keys : " + Geekskeys); // 5 values() method return all the values in HashMap Collection<String> Geeksvalues = Geeks.values(); System.out.println("Initial values : " + Geeksvalues); // Adding new set of key-value Geeks.put("Search", "JavaArticle"); // Again using .keySet() and .values() methods System.out.println("New Keys : " + Geekskeys); System.out.println("New Values: " + Geeksvalues); }} |
Output:
Testing .containsKey : HashMap Initial keys : [Language, Platform, Learn, Code] Initial values : [Java, Geeks For geeks, More, HashMap] New Keys : [Language, Platform, Search, Learn, Code] New Values: [Java, Geeks For geeks, JavaArticle, More, HashMap]
Syntax: public Set<Map.Entry> entrySet() Return: complete set of keys and values
It is very nice way to assign values to the keys that are not yet mapped, without interfering with the already present set of keys and values.
Syntax: default V getOrDefault(Object key,V defaultValue) Parameters: key - the key whose mapped value we need to return defaultValue - the default for the keys present in HashMap Return: mapping the unmapped keys with the default value.
1st method accepts set of key and value which will replace the already present value of the key with the new value passed in the argument. If no such set is present replace() method will do nothing.
Meanwhile 2nd method will only replace the already present set of key-old_value if the key and old_Value are found in the HashMap.
Syntax:
replace(k key, v value)
or
replace(k key, v oldvalue, newvalue)
Parameters:
key - key in set with the old value.
value - new value we want to be with the specified key
oldvalue - old value in set with the specified key
newvalue - new value we want to be with the specified key
Return:
True - if the value is replaced
Null - if there is no such set present
Syntax: public V putIfAbsent(key, value) Parameters: key - key with which the specified value is associates. value - value to associates with the specified key.
// Java Program illustrating HashMap class methods(). // entrySet(), getOrDefault(), replace(), putIfAbsentimport java.util.*;public class NewClass{ public static void main(String args[]) { // Creation of HashMap HashMap<String, String> Geeks = new HashMap<>(); // Adding values to HashMap as ("keys", "values") Geeks.put("Language", "Java"); Geeks.put("Code", "HashMap"); Geeks.put("Learn", "More"); // .entrySet() returns all the keys with their values present in Hashmap Set<Map.Entry<String, String>> mappingGeeks = Geeks.entrySet(); System.out.println("Set of Keys and Values using entrySet() : "+mappingGeeks); System.out.println(); // Using .getOrDefault to access value // Here it is Showing Default value as key - "Code" was already present System.out.println("Using .getorDefault : " + Geeks.getOrDefault("Code","javaArticle")); // Here it is Showing set value as key - "Search" was not present System.out.println("Using .getorDefault : " + Geeks.getOrDefault("Search","javaArticle")); System.out.println(); // .replace() method replacing value of key "Learn" Geeks.replace("Learn", "Methods"); System.out.println("working of .replace() : "+mappingGeeks); System.out.println(); /* .putIfAbsent() method is placing a new key-value as they were not present initially*/ Geeks.putIfAbsent("cool", "HashMap methods"); System.out.println("working of .putIfAbsent() : "+mappingGeeks); /* .putIfAbsent() method is not doing anything as key-value were already present */ Geeks.putIfAbsent("Code", "With_JAVA"); System.out.println("working of .putIfAbsent() : "+mappingGeeks); }} |
Output:
Set of Keys and Values using entrySet() : [Language=Java, Learn=More, Code=HashMap] Using .getorDefault : HashMap Using .getorDefault : javaArticle working of .replace() : [Language=Java, Learn=Methods, Code=HashMap] working of .putIfAbsent() : [Language=Java, cool=HashMap methods, Learn=Methods, Code=HashMap] working of .putIfAbsent() : [Language=Java, cool=HashMap methods, Learn=Methods, Code=HashMap]
// Java Program illustrating remove() method using Iterator. import java.util.*;public class NewClass{ public static void main(String args[]) { // Creation of HashMap HashMap<String, String> Geeks = new HashMap<>(); // Adding values to HashMap as ("keys", "values") Geeks.put("Language", "Java"); Geeks.put("Platform", "Geeks For geeks"); Geeks.put("Code", "HashMap"); // .entrySet() returns all the keys with their values present in Hashmap Set<Map.Entry<String, String>> mappingGeeks = Geeks.entrySet(); System.out.println("Set of Keys and Values : "+mappingGeeks); System.out.println(); // Creating an iterator System.out.println("Use of Iterator to remove the sets."); Iterator<Map.Entry<String, String>> geeks_iterator = Geeks.entrySet().iterator(); while(geeks_iterator.hasNext()) { Map.Entry<String, String> entry = geeks_iterator.next(); // Removing a set one by one using iterator geeks_iterator.remove(); // right way to remove entries from Map, // avoids ConcurrentModificationException System.out.println("Set of Keys and Values : "+mappingGeeks); } }} |
Output:
Set of Keys and Values : [Language=Java, Platform=Geeks For geeks, Code=HashMap] Use of Iterator to remove the sets. Set of Keys and Values : [Platform=Geeks For geeks, Code=HashMap] Set of Keys and Values : [Code=HashMap] Set of Keys and Values : []
Advantage:
If we use for loop, it get translated to Iterator internally but without using Iterator explicitly we can’t remove any entry during Iteration.On doing so, Iterator may throw ConcurrentModificationException. So, we use explicit Iterator and while loop to traverse.
Reference:
https://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html
This article is contributed by Mohit Gupta. If you like Lazyroar and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the Lazyroar main page and help other Geeks.
