Java WeakHashMap keySet() Method

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Description

The Java WeakHashMap keySet() method is used to return 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.

Declaration

Following is the declaration for java.util.WeakHashMap.keySet() method.

public Set<K> keySet()

Parameters

NA

Return Value

The method call returns a set view of the keys contained in this map.

Exception

NA

Getting a Set View of Keys of WeakHashMap of Integer, Integer Pairs Example

The following example shows the usage of Java WeakHashMap keySet() method to get a Set view of keys of a Map. We've created a Map object of Integer,Integer pairs. Then few entries are added, map is printed. Using keySet() method, set is retrieved and printed.

package com.tutorialspoint;

import java.util.WeakHashMap;

public class WeakHashMapDemo {
   public static void main(String args[]) {
      
      // create hash map
      WeakHashMap<Integer,Integer> newmap = new WeakHashMap<>();

      // populate hash map
      newmap.put(1, 1);
      newmap.put(2, 2);
      newmap.put(3, 3); 

      System.out.println("Initial map elements: " + newmap);

      System.out.println("KeySet view: " + newmap.keySet());
   }
}

Output

Let us compile and run the above program, this will produce the following result.

Initial map elements: {3=3, 2=2, 1=1}
KeySet view: [3, 2, 1]

Getting a Set View of Keys of WeakHashMap of Integer, Strings Pairs Example

The following example shows the usage of Java WeakHashMap keySet() method to get a Set view of keys of a Map. We've created a Map object of Integer,String pairs. Then few entries are added, map is printed. Using keySet() method, set is retrieved and printed.

package com.tutorialspoint;

import java.util.WeakHashMap;

public class WeakHashMapDemo {
   public static void main(String args[]) {
      
      // create hash map
      WeakHashMap<Integer,String> newmap = new WeakHashMap<>();

      // populate hash map
      newmap.put(1, "tutorials");
      newmap.put(2, "point");
      newmap.put(3, "is best"); 

      System.out.println("Initial map elements: " + newmap);

      System.out.println("KeySet view: " + newmap.keySet());
   }
}

Output

Let us compile and run the above program, this will produce the following result.

Initial map elements: {3=is best, 2=point, 1=tutorials}
KeySet view: [3, 2, 1]

Getting a Set View of Keys of WeakHashMap of Integer, Object Pairs Example

The following example shows the usage of Java WeakHashMap keySet() method to get a Set view of keys of a Map. We've created a Map object of Integer,Student pairs. Then few entries are added, map is printed. Using keySet() method, set is retrieved and printed.

package com.tutorialspoint;

import java.util.WeakHashMap;

public class WeakHashMapDemo {
   public static void main(String args[]) {
      
      // create hash map
      WeakHashMap<Integer,Student> newmap = new WeakHashMap<>();

      // populate hash map
      newmap.put(1, new Student(1, "Julie"));
      newmap.put(2, new Student(2, "Robert"));
      newmap.put(3, new Student(3, "Adam"));

      System.out.println("Initial map elements: " + newmap);

      System.out.println("KeySet view: " + newmap.keySet());
   }
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
}

Output

Let us compile and run the above program, this will produce the following result.

Initial map elements: {3=[ 3, Adam ], 2=[ 2, Robert ], 1=[ 1, Julie ]}
KeySet view: [3, 2, 1]