In previous articles, we have discussed various ways to iterate through Map but those are with String object only i.e.; both keys and values are in String-type only
What if we want to iterate through HashMap of ArrayList ?
- where keys are in either String/Integer type
- values are ArrayList of String type
- or some other type of our interest like Double, Integer or Float, etc.
Various ways to iterate through Map:
- Using keySet(); method and for-each loop
- Using keySet(); method and Iterator interface
- Using entrySet(); method and for-each loop
- Using entrySet(); method and Iterator interface
- Using forEach(); in Java 1.8 version
- Read different ways to iterate Map Entry
- Read how to iterate Map using Iterable.forEach() in Java 8
Various ways to iterate over HashMap of ArrayList in Java
We will limit our code to 3 demo examples i.e.,
- Using keySet(); and enhanced for-each loop
- Using entrySet(); and Iterator interface
- Using forEach in Java 1.8 version
Let us move forward and discuss all possible ways to iterate HashMap of ArrayList of (String) type
Way 1: Get keys using keySet() method of Map and iterate using enhanced for-loop
IteratingHashMapUsingKeySetAndForLoop.java
package in.bench.resources.iterating.hashmap.of.arraylist;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
public class IteratingHashMapUsingKeySetAndForLoop {
public static void main(String[] args) {
// create HashMap of Continent & list of Top Countries
Map<String, List<String>> continentTopContries =
new HashMap<String, List<String>>();
// create ArrayList-1
// for adding top countries of Asian Continent
List<String> topCountriesOfAsiaContinent =
new ArrayList<String>();
// add top countries of Asian continent
topCountriesOfAsiaContinent.add("India");
topCountriesOfAsiaContinent.add("China");
topCountriesOfAsiaContinent.add("Russia");
topCountriesOfAsiaContinent.add("Japan");
// put 1st entry as Asia and its list of Top countries
continentTopContries.put("Asia",
topCountriesOfAsiaContinent);
// create ArrayList-2
// for adding top countries of Africa Continent
List<String> topCountriesOfAfricaContinent =
new ArrayList<String>();
// add top countries of African continent
topCountriesOfAfricaContinent.add("Libya");
topCountriesOfAfricaContinent.add("Zimbabwe");
topCountriesOfAfricaContinent.add("Nigeria");
topCountriesOfAfricaContinent.add("Kenya");
// put 2nd entry as Africa and its list of Top countries
continentTopContries.put("Africa",
topCountriesOfAfricaContinent);
// Way 1: Get keySet() and
// Iterate using for-each loop for ArrayList
System.out.println("Way 1: Get keySet()"
+ " and Iterate using for-each loop");
// get keySet() into Set
Set<String> setOfKeySet = continentTopContries.keySet();
// for-each loop
for(String key : setOfKeySet) {
System.out.println("\nContinent name : " + key
+ "\nList of Top Countries of " + key + ":");
for(String country : continentTopContries.get(key)) {
System.out.println("\t\t\t\t" + country);
}
}
}
}
Output:
Way 1: Get keySet() and Iterate using for-each loop for ArrayList Continent name : Asia List of Top Countries of Asia: India China Russia Japan Continent name : Africa List of Top Countries of Africa: Libya Zimbabwe Nigeria Kenya
Way 2: Get entries using entrySet() method of Map and iterate using Iterator interface
IteratingHashMapUsingEntrySetAndIterator.java
package in.bench.resources.iterating.hashmap.of.arraylist;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.Map.Entry;
public class IteratingHashMapUsingEntrySetAndIterator {
public static void main(String[] args) {
// create HashMap of Continent and list of Top Countries
Map<String, List<String>> continentTopContries =
new HashMap<String, List<String>>();
// create ArrayList-1
// for adding top countries of Asian Continent
List<String> topCountriesOfAsiaContinent =
new ArrayList<String>();
// add top countries of Asian continent
topCountriesOfAsiaContinent.add("India");
topCountriesOfAsiaContinent.add("China");
topCountriesOfAsiaContinent.add("Russia");
topCountriesOfAsiaContinent.add("Japan");
// put 1st entry as Asia and its list of Top countries
continentTopContries.put("Asia",
topCountriesOfAsiaContinent);
// create ArrayList-2
// for adding top countries of Africa Continent
List<String> topCountriesOfAfricaContinent =
new ArrayList<String>();
// add top countries of African continent
topCountriesOfAfricaContinent.add("Libya");
topCountriesOfAfricaContinent.add("Zimbabwe");
topCountriesOfAfricaContinent.add("Nigeria");
topCountriesOfAfricaContinent.add("Kenya");
// put 2nd entry as Africa and its list of Top countries
continentTopContries.put("Africa",
topCountriesOfAfricaContinent);
// Way 2: Get entrySet() and
// Iterate using Iterator interface for ArrayList
System.out.println("Way 2: Get entrySet() and Iterate "
+ "using Iterator interface for ArrayList");
// get entrySet() into Set
Set<Map.Entry<String, List<String>>> entrySet =
continentTopContries.entrySet();
// Collection Iterator
Iterator<Entry<String, List<String>>> iterator =
entrySet.iterator();
while(iterator.hasNext()) {
Entry<String, List<String>> entry = iterator.next();
System.out.println("\nContinent name : "
+ entry.getKey()
+ "\nList of Top Countries of "
+ entry.getKey() + ":");
for(String country : entry.getValue()) {
System.out.println("\t\t\t\t" + country);
}
}
}
}
Output:
Way 2: Get entrySet() and Iterate using Iterator interface for ArrayList Continent name : Asia List of Top Countries of Asia: India China Russia Japan Continent name : Africa List of Top Countries of Africa: Libya Zimbabwe Nigeria Kenya
Note:
- keySet(); and entrySet(); methods of Map from Java 1.2 version
- Iterator interface for iterating comes from Java 1.2 version
- Whereas enhanced for-each loop introduced in Java 1.5 version
Way 3: Using Iterable.forEach() in Java 8 version
IteratingHashMapUsingJava8ForEach.java
package in.bench.resources.iterating.hashmap.of.arraylist;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class IteratingHashMapUsingJava8ForEach {
public static void main(String[] args) {
// create HashMap of Continent and list of Top Countries
Map<String, List<String>> continentTopContries =
new HashMap<String, List<String>>();
// create ArrayList-1
// for adding top countries of Asian Continent
List<String> topCountriesOfAsiaContinent =
new ArrayList<String>();
// add top countries of Asian continent
topCountriesOfAsiaContinent.add("India");
topCountriesOfAsiaContinent.add("China");
topCountriesOfAsiaContinent.add("Russia");
topCountriesOfAsiaContinent.add("Japan");
// put 1st entry as Asia and its list of Top countries
continentTopContries.put("Asia",
topCountriesOfAsiaContinent);
// create ArrayList-2
// for adding top countries of Africa Continent
List<String> topCountriesOfAfricaContinent =
new ArrayList<String>();
// add top countries of African continent
topCountriesOfAfricaContinent.add("Libya");
topCountriesOfAfricaContinent.add("Zimbabwe");
topCountriesOfAfricaContinent.add("Nigeria");
topCountriesOfAfricaContinent.add("Kenya");
// put 2nd entry as Africa and its list of Top countries
continentTopContries.put("Africa",
topCountriesOfAfricaContinent);
// Way 3: Using Iterable.forEach() in Java 8 version
System.out.println("Way 3: Using Iterable.forEach()"
+ " in Java 8 version\n");
// Iterating Map/Hashtable using forEach() in Java 8
continentTopContries.forEach(
(key, value)->System.out.println(
"Continent name : " + key + "\t\t"
+ "List of Top Countries : " + value));
}
}
Output:
Way 3: Using Iterable.forEach() in Java 8 version Continent name : Asia List of Top Countries : [India, China, Russia, Japan] Continent name : Africa List of Top Countries : [Libya, Zimbabwe, Nigeria, Kenya]
From above example, HashMap
- allows only unique keys to be stored
- null object is allowed; but maximum of one
- while iterating, keys are retrieved in random-order
Happy Coding !!
Happy Learning !!