How to check if a key exists in a HashMap in Java
Last Updated :
11 Dec, 2018
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
- 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:
import java.util.*;
public class GFG {
public static void main(String[] args)
{
HashMap<Integer, String>
map = new HashMap<>();
map.put( 1 , "Geeks" );
map.put( 2 , "ForGeeks" );
map.put( 3 , "GeeksForGeeks" );
int keyToBeChecked = 2 ;
System.out.println( "HashMap: "
+ map);
Iterator<Map.Entry<Integer, String> >
iterator = map.entrySet().iterator();
boolean isKeyPresent = false ;
while (iterator.hasNext()) {
Map.Entry<Integer, String>
entry
= iterator.next();
if (keyToBeChecked == entry.getKey()) {
isKeyPresent = true ;
}
}
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.
import java.util.*;
public class GFG {
public static void main(String[] args)
{
try {
HashMap<Integer, String>
map = new HashMap<>();
map.put( 1 , "Geeks" );
map.put( 2 , "ForGeeks" );
map.put( null , "GeeksForGeeks" );
int keyToBeChecked = 2 ;
System.out.println( "HashMap: "
+ map);
Iterator<Map.Entry<Integer, String> >
iterator = map.entrySet().iterator();
boolean isKeyPresent = false ;
while (iterator.hasNext()) {
Map.Entry<Integer, String>
entry
= iterator.next();
if (keyToBeChecked == entry.getKey()) {
isKeyPresent = true ;
}
}
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
- 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:
import java.util.*;
public class GFG {
public static void main(String[] args)
{
HashMap<Integer, String>
map = new HashMap<>();
map.put( 1 , "Geeks" );
map.put( 2 , "ForGeeks" );
map.put( null , "GeeksForGeeks" );
int keyToBeChecked = 2 ;
System.out.println( "HashMap: "
+ map);
boolean isKeyPresent = map.containsKey(keyToBeChecked);
System.out.println( "Does key "
+ keyToBeChecked
+ " exists: "
+ isKeyPresent);
}
}
|
Output:
HashMap: {null=GeeksForGeeks, 1=Geeks, 2=ForGeeks}
Does key 2 exists: true
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