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