HashMap put() Method in Java

07 Jul, 2022
The java.util.HashMap.put() method of HashMap is used to insert a mapping into a map. This means we can insert a specific key and the value it is mapping to into a particular map. If an existing key is passed then the previous value gets replaced by the new value. If a new pair is passed, then the pair gets inserted as a whole.
Syntax:  
Hash_Map.put(key, value)
Parameters: The method takes two parameters, both are of the Object type of the HashMap.  
key: This refers to the key element that needs to be inserted into the Map for mapping.
value: This refers to the value that the above key would map into.
Return Value: If an existing key is passed then the previous value gets returned. If a new pair is passed, then NULL is returned.Below programs are used to illustrate the working of java.util.HashMap.put() Method: Program 1: When passing an existing key.  
Java
// Java code to illustrate the put() methodimport java.util.*;  public class Hash_Map_Demo {    public static void main(String[] args)    {          // Creating an empty HashMap        HashMap<Integer, String> hash_map = new HashMap<Integer, String>();          // Mapping string values to int keys        hash_map.put(10, "Geeks");        hash_map.put(15, "4");        hash_map.put(20, "Geeks");        hash_map.put(25, "Welcomes");        hash_map.put(30, "You");          // Displaying the HashMap        System.out.println("Initial Mappings are: " + hash_map);          // Inserting existing key along with new value        String returned_value = (String)hash_map.put(20, "All");          // Verifying the returned value        System.out.println("Returned value is: " + returned_value);          // Displaying the new map        System.out.println("New map is: " + hash_map);    }}
Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
Returned value is: Geeks
New map is: {20=All, 25=Welcomes, 10=Geeks, 30=You, 15=4}
 
Program 2: When passing a new key.  
Java
// Java code to illustrate the put() methodimport java.util.*;  public class Hash_Map_Demo {    public static void main(String[] args)    {          // Creating an empty HashMap        HashMap<Integer, String> hash_map = new HashMap<Integer, String>();          // Mapping string values to int keys        hash_map.put(10, "Geeks");        hash_map.put(15, "4");        hash_map.put(20, "Geeks");        hash_map.put(25, "Welcomes");        hash_map.put(30, "You");          // Displaying the HashMap        System.out.println("Initial Mappings are: " + hash_map);          // Inserting existing key along with new value        String returned_value = (String)hash_map.put(50, "All");          // Verifying the returned value        System.out.println("Returned value is: " + returned_value);          // Displaying the new map        System.out.println("New map is: " + hash_map);    }}
Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
Returned value is: null
New map is: {50=All, 20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
 
Note: The same operation can be performed with any type of Mappings with variation and combination of different data types. 
simranarora5sos
Java - util package
Java-Collections
Java-Functions
Java-HashMap
Java
Java
Java-Collections
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