How to check if a key exists in a HashMap in Java

Given a HashMap and a key in Java, the task is to check if this key exists in the HashMap or not.

Examples:

Input: HashMap: {1=Geeks, 2=ForGeeks, 3=GeeksForGeeks}, key = 2
Output: true

Input: HashMap: {1=G, 2=e, 3=e, 4=k, 5=s}, key = 10
Output: false

1. Using Iterator (Not Efficient):
   1. Get the HashMap and the Key
   2. Create an iterator to iterate over the HashMap using HashMap.iterate() method.
   3. Iterate over the HashMap using the Iterator.hasNext() method.
   4. While iterating, check for the key at that iteration to be equal to the key specified. The entry key of the Map can be obtained with the help of entry.getKey() method.
   5. If the key matches, set the flag as true.
   6. The flag value after iterating, contains the result.

   Below is the implementation of the above approach:

   Program 1:

   // Java program to check if a key exists
   // in a HashMap or not
   
   import java.util.*;
   
   public class GFG {
       public static void main(String[] args)
       {
   
           // Create a HashMap
           HashMap<Integer, String>
               map = new HashMap<>();
   
           // Populate the HashMap
           map.put(1, "Geeks");
           map.put(2, "ForGeeks");
           map.put(3, "GeeksForGeeks");
   
           // Get the key to be removed
           int keyToBeChecked = 2;
   
           // Print the initial HashMap
           System.out.println("HashMap: "
                              + map);
   
           // Get the iterator over the HashMap
           Iterator<Map.Entry<Integer, String> >
               iterator = map.entrySet().iterator();
   
           // flag to store result
           boolean isKeyPresent = false;
   
           // Iterate over the HashMap
           while (iterator.hasNext()) {
   
               // Get the entry at this iteration
               Map.Entry<Integer, String>
                   entry
                   = iterator.next();
   
               // Check if this key is the required key
               if (keyToBeChecked == entry.getKey()) {
   
                   isKeyPresent = true;
               }
           }
   
           // Print the result
           System.out.println("Does key "
                              + keyToBeChecked
                              + " exists: "
                              + isKeyPresent);
       }
   }
   Output:

   HashMap: {1=Geeks, 2=ForGeeks, 3=GeeksForGeeks}
   Does key 2 exists: true
   

   Program 2: To show why this method is not suggested. If the HashMap contains null values, then this method will throw NullPointerException. This makes this method a non suggestable method to use.

   // Java program to check if a key exists
   // in a HashMap or not
   
   import java.util.*;
   
   public class GFG {
       public static void main(String[] args)
       {
   
           try {
               // Create a HashMap
               HashMap<Integer, String>
                   map = new HashMap<>();
   
               // Populate the HashMap
               map.put(1, "Geeks");
               map.put(2, "ForGeeks");
               map.put(null, "GeeksForGeeks");
   
               // Get the key to be removed
               int keyToBeChecked = 2;
   
               // Print the initial HashMap
               System.out.println("HashMap: "
                                  + map);
   
               // Get the iterator over the HashMap
               Iterator<Map.Entry<Integer, String> >
                   iterator = map.entrySet().iterator();
   
               // flag to store result
               boolean isKeyPresent = false;
   
               // Iterate over the HashMap
               while (iterator.hasNext()) {
   
                   // Get the entry at this iteration
                   Map.Entry<Integer, String>
                       entry
                       = iterator.next();
   
                   // Check if this key is the required key
                   if (keyToBeChecked == entry.getKey()) {
   
                       isKeyPresent = true;
                   }
               }
   
               // Print the result
               System.out.println("Does key "
                                  + keyToBeChecked
                                  + " exists: "
                                  + isKeyPresent);
           }
           catch (Exception e) {
               System.out.println(e);
           }
       }
   }
   Output:

   HashMap: {null=GeeksForGeeks, 1=Geeks, 2=ForGeeks}
   java.lang.NullPointerException
   

2. Using HashMap.containsKey method(Efficient):
   1. Get the HashMap and the Key
   2. Check if the key exists in the HashMap or not using HashMap.containsKey() method. If the key exists, set the flag as true.
   3. The flag value, contains the result.

   Below is the implementation of the above approach:

   Program 1:

   // Java program to check if a key exists
   // in a HashMap or not
   
   import java.util.*;
   
   public class GFG {
       public static void main(String[] args)
       {
   
           // Create a HashMap
           HashMap<Integer, String>
               map = new HashMap<>();
   
           // Populate the HashMap
           map.put(1, "Geeks");
           map.put(2, "ForGeeks");
           map.put(null, "GeeksForGeeks");
   
           // Get the key to be removed
           int keyToBeChecked = 2;
   
           // Print the initial HashMap
           System.out.println("HashMap: "
                              + map);
   
           // Check is key exists in the Map
           boolean isKeyPresent = map.containsKey(keyToBeChecked);
   
           // Print the result
           System.out.println("Does key "
                              + keyToBeChecked
                              + " exists: "
                              + isKeyPresent);
       }
   }
   Output:

   HashMap: {null=GeeksForGeeks, 1=Geeks, 2=ForGeeks}
   Does key 2 exists: true