write a program to check if a key exists in a HashMap in Java collection
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=Java, 2=JavaHashMap, 3=JavaPrograme}, key = 2
Output: true
Input: HashMap: {1=G, 2=e, 3=e, 4=k, 5=s}, key = 10
Output: false
Recommended: Please try your approach on {IDE} first, before moving on to the solution.
Using Iterator (Not Efficient):
Get the HashMap and the Key
Create an iterator to iterate over the HashMap using HashMap.iterate() method.
Iterate over the HashMap using the Iterator.hasNext() method.
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.
If the key matches, set the flag as true.
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 JavaHashMapExample {
public static void main(String[] args)
{
// Create a HashMap
HashMap<Integer, String>
map = new HashMap<>();
// Populate the HashMap
map.put(1, "Java");
map.put(2, "JavaHashMap");
map.put(3, "JavaPrograme");
// 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=Java, 2=JavaHashMap, 3=JavaPrograme}
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 JavaHashMapExample {
public static void main(String[] args)
{
try {
// Create a HashMap
HashMap<Integer, String>
map = new HashMap<>();
// Populate the HashMap
map.put(1, "Java");
map.put(2, "JavaHashMap");
map.put(null, "JavaPrograme");
// 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=JavaPrograme, 1=Java, 2=JavaHashMap}
java.lang.NullPointerException
Using HashMap.containsKey method(Efficient):
Get the HashMap and the Key
Check if the key exists in the HashMap or not using HashMap.containsKey() method. If the key exists, set the flag as true.
The flag value, contains the result.
Below is the implementation of the above approach:
Program 1:
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// Java program to check if a key exists
// in a HashMap or not
import java.util.*;
public class JavaHashMapExample {
public static void main(String[] args)
{
// Create a HashMap
HashMap<Integer, String>
map = new HashMap<>();
// Populate the HashMap
map.put(1, "Java");
map.put(2, "JavaHashMap");
map.put(null, "JavaPrograme");
// 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=JavaPrograme, 1=Java, 2=JavaHashMap}
Does key 2 exists: true
