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Java Method Overloading Comprehensive Java HashMap Methods

Java HashMap putAll() usage and example

Java HashMap Methods

Java HashMap putAll() method will put all keys/The value mappings from the specified Map are inserted into the HashMap.

The syntax of the putAll() method is:

hashmap.putAll(Map m)

putAll() parameter

map - 　A map containing the mappings to be inserted into the hashmap

putAll() return value

The putAll() method does not return any value.

Note: If the map contains any keys that already exist in the hash map. Then, the new value associated with the key will replace the previous value in the hashmap.

Example1: Java HashMap putAll()

Example

import java.util.HashMap;
class Main {
　　public static void main(String[] args){
　　　　//Create a HashMap
　　　　HashMap<String, Integer> primeNumbers = new HashMap<>();
　　　　//Add the mapping to the HashMap
　　　　primeNumbers.put("Two",　2);
　　　　primeNumbers.put("Three",　3);
　　　　System.out.println("Prime Numbers: "　+　primeNumbers);
　　　　//Create another HashMap
　　　　HashMap<String, Integer> numbers = new HashMap<>();
　　　　numbers.put("One",　1);
　　　　numbers.put("Four",　4);
　　　　//Add all mappings from primeNumbers to numbers
　　　　numbers.putAll(primeNumbers);
　　　　System.out.println("Numbers: "　+　numbers);
　　}
}

Output Result

Prime Numbers: {Two=2, Three=3}
Numbers: {One=1, Four=4, Two=2, Three=3}

In the above example, we created two hash maps named primeNumbers and numbers. Note this line,

numbers.putAll(primeNumbers);

Here, the putAll() method adds all mappings from primeNumbers to numbers.

Note：We have used the put() method to add a single mapping to the hashmap. For more information, please visitJava HashMap put()。

Example2：Insert the mapping from TreeMap to HashMap

Example

import java.util.HashMap;
import java.util.TreeMap;
class Main {
　　public static void main(String[] args){
　　　　//Create a String type TreeMap
　　　　TreeMap<String, String> treemap = new TreeMap<>();
　　　　//Add the mapping to the TreeMap
　　　　treemap.put("A", "Apple");
　　　　treemap.put("B", "Ball");
　　　　treemap.put("C", "Cat");
　　　　System.out.println("TreeMap: "　+　treemap);
　　　　//Create a HashMap
　　　　HashMap<String, String> hashmap = new HashMap<>();
　　　　//Add the mapping to the HashMap
　　　　hashmap.put("Y", "Yak");
　　　　hashmap.put("Z", "Zebra");
　　　　System.out.println("Initial HashMap:",　+　hashmap);
　　　　//　Add all mappings from TreeMap to HashMap
　　　　hashmap.putAll(treemap);
　　　　System.out.println("Updated HashMap:",　+　hashmap);
　　}
}

Output Result

TreeMap: {A=Apple, B=Ball, C=Cat}
Initial HashMap: {Y=Yak, Z=Zebra}
Updated HashMap: {A=Apple, B=Ball, C=Cat, Y=Yak, Z=Zebra}

In the above example, we created a TreeMap and a HashMap. Note this line,

hashmap.putAll(treemap);

　 Here, we use the putAll() method to add all mappings from TreeMap to HashMap.　

Java HashMap Methods

Java Method Overloading Comprehensive Java HashMap Methods