In this article, we will discuss how to get all keys of a HashMap or implementation classes of Map interface like LinkedHashMap or TreeMap
1. Map implementation classes:
- HashMap –> retrieves keys in random-order
- LinkedHashMap –> retrieves keys as per insertion-order
- TreeMap –> retrieves keys as per some sorting-order
2. To get all keys of HashMap :
- use keyset() method of Map interface
- which returns Set of Keys
Syntax:
Set<String> set = hashMap.keySet();
GetAllKeysOfHashMap.java
package in.bench.resources.java.map;
import java.util.HashMap;
import java.util.Set;
public class GetAllKeysOfHashMap {
public static void main(String[] args) {
// creating HashMap object of type <String, String>
HashMap<String, String> hm = new HashMap<String, String>();
// adding key-value pairs to HashMap object
hm.put("Google", "Sundar Pichai");
hm.put("Facebook", "Mark Zuckerberg");
hm.put("LinkedIn", "Reid Hoffman");
hm.put("Apple", "Steve Jobs");
hm.put("Microsoft", "Bill Gates");
System.out.println("all Key-Value pairs:\n\n" + hm + "\n\n");
System.out.println("List of all keys using keySet(): \n");
// Iterating keys using keySet()
Set<String> companies = hm.keySet();
for(String company : companies) {
System.out.println(company);
}
}
}
Output:
all Key-Value pairs:
{Microsoft=Bill Gates, Facebook=Mark Zuckerberg, Apple=Steve Jobs,
LinkedIn=Reid Hoffman, Google=Sundar Pichai}
List of all keys using keySet():
Microsoft
Facebook
Apple
LinkedIn
Google
Note: We can also print values corresponding to keys retrieved using keySet() method
Related Articles:
- How to get all keys of a HashMap
- How to get all values of a HashMap
- How to get all Entries or Key-Value pairs of HashMap
- How to get size or length of HashMap
- How to check whether a particular key is present in HashMap
- How to check whether a particular value is present in HashMap
- How to check whether HashMap is empty or not ?
- Adding one HashMap to another HashMap using putAll method
- How to delete an entry of HashMap
- How to delete all entries of HashMap
- How to remove an entry from HashMap by comparing values in Java 8
- How to remove an entry from HashMap by comparing keys in Java 8
- Various ways to iterate through HashMap
- To reverse order the LinkedHashMap contents
- Map: How ConcurrentModificationException can be handled in Java
References:
- https://docs.oracle.com/javase/tutorial/collections/intro/
- https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Collection.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Map.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Map.Entry.html
- https://docs.oracle.com/javase/tutorial/collections/interfaces/map.html
- https://docs.oracle.com/javase/7/docs/api/java/util/HashMap.html
- https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html
- https://docs.oracle.com/javase/tutorial/collections/interfaces/set.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Set.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Iterator.html
- https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html
- http://docs.oracle.com/javase/6/docs/api/java/util/LinkedHashMap.html
- https://docs.oracle.com/javase/8/docs/api/java/util/LinkedHashMap.html
- https://docs.oracle.com/javase/7/docs/api/java/util/TreeMap.html
- https://docs.oracle.com/javase/8/docs/api/java/util/TreeMap.html
Happy Coding !!
Happy Learning !!
