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WeakHashMap keySet() method in Java

Last Updated : 20 Jul, 2018
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The java.util.WeakHashMap.keySet() method in Java is used to create a set out of the key elements contained in the hash map. It basically returns a set view of the keys or we can create a new set and store the key elements in them. Syntax: 
Weak_Hash_Map.keySet()
Parameters: The method does not take any parameter. Return Value: The method returns a set having the keys of the weak hash map. Below programs are used to illustrate the working of java.util.WeakHashMap.keySet() Method: Program 1: Mapping String Values to Integer Keys. Java 
// Java code to illustrate the keySet() 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);

        // Using keySet() to get the set view of keys
        System.out.println("The set is: " + 
                                  weak_hash.keySet());
    }
}
Output: 
Initial Mappings are: {30=You, 15=4, 10=Geeks, 25=Welcomes, 20=Geeks}
The set is: [30, 15, 10, 25, 20]
Program 2: Mapping Integer Values to String Keys. Java 
// Java code to illustrate the keySet() method
import java.util.*;

public class Weak_Hash_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);

        // Using keySet() to get the set view of keys
        System.out.println("The set is: " + 
                                  weak_hash.keySet());
    }
}
Output: 
Initial Mappings are: {Welcomes=25, 4=15, You=30, Geeks=20}
The set is: [Welcomes, 4, You, Geeks]
Note: The same operation can be performed with any type of Mappings with variation and combination of different data types.

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