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WeakHashMap putall() Method in Java

Last Updated : 20 Jul, 2018
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The java.util.WeakHashMap.putAll() is an inbuilt method of WeakHashMap class that is used for the copy operation. The method copies all of the elements i.e., the mappings, from one map into another. Syntax: 
new_weakhash_map.putAll(exist_weakhash_map)
Parameters: The method takes one parameter exist_weakhash_map that refers to the existing map we want to copy from. Return Value: The method does not return any values. Exception: The method throws NullPointerException if the map we want to copy from is NULL. Below programs illustrates the working of java.util.WeakHashMap.putAll() method: Program 1: Mapping String Values to Integer Keys. Java 
// Java code to illustrate the putAll() method
import java.util.*;

public class Weak_Hash_Map_Demo {
    public static void main(String[] args)
    {

        // Creating an empty WeakHashMap
        Map<Integer, String> weak_hash = new 
                      WeakHashMap<Integer, String>();

        // Mapping string values to int keys
        weak_hash.put(10, "Geeks");
        weak_hash.put(15, "4");
        weak_hash.put(20, "Geeks");
        weak_hash.put(25, "Welcomes");
        weak_hash.put(30, "You");

        // Displaying the WeakHashMap
        System.out.println("Initial Mappings are: " + 
                                           weak_hash);

        // Creating a new weakhash map and copying
        Map<Integer, String> new_weakhash_map = new 
                        WeakHashMap<Integer, String>();
        new_weakhash_map.putAll(weak_hash);

        // Displaying the final WeakHashMap
        System.out.println("The new map: " + 
                                    new_weakhash_map);
    }
}
Output: 
Initial Mappings are: {30=You, 15=4, 10=Geeks, 25=Welcomes, 20=Geeks}
The new map: {15=4, 30=You, 10=Geeks, 25=Welcomes, 20=Geeks}
Program 2: Mapping Integer Values to String Keys. Java 
// Java code to illustrate the putAll() method
import java.util.*;

public class WeakHash_Map_Demo {
    public static void main(String[] args)
    {

        // Creating an empty WeakHashMap
        Map<String, Integer> weak_hash = new 
                      WeakHashMap<String, Integer>();

        // Mapping int values to string keys
        weak_hash.put("Geeks", 10);
        weak_hash.put("4", 15);
        weak_hash.put("Geeks", 20);
        weak_hash.put("Welcomes", 25);
        weak_hash.put("You", 30);

        // Displaying the WeakHashMap
        System.out.println("Initial Mappings are: " + 
                                          weak_hash);

        // Creating a new weakhash map and copying
        Map<String, Integer> new_weakhash_map = new 
                      WeakHashMap<String, Integer>();
        new_weakhash_map.putAll(weak_hash);

        // Displaying the final WeakHashMap
        System.out.println("The new map: " + 
                                   new_weakhash_map);
    }
}
Output: 
Initial Mappings are: {Welcomes=25, 4=15, You=30, Geeks=20}
The new map: {Welcomes=25, 4=15, You=30, Geeks=20}
Note: The same operation can be performed with any type of Mappings with variation and combination of different data types.

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chinmoy lenka
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