In this article, we will discuss different ways to iterate through List of HashMap. Already, in earlier articles, we have discussed following topics
- Iterating over ArrayList – 5 ways
- Iterating over ArrayList in Java 8
- Iterating over HashMap – 5 ways
- Iterating over HashMap in Java 8
- Iterating over Hashtable post Java 8 release
- Iterating over HashMap of ArrayList
Now, we will iterate through List of HashMap i.e.; a list containing HashMap with
- key being String-type
- values being String-type only
Different ways to iterate through List:
- Regular/Standard/Traditional for-loop
- Enhanced for-loop, introduced in Java 1.5 version
- Iterating using Iterator of Collection interface
- Iterating using ListIterator of List interface
- Iterating List using forEach() in Java 1.8 version
Different 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
Ways to iterate over List of HashMap in Java:
We can pick any combination from above listed iterating ways, but we will limit our code to 3 demo examples i.e.,
- Using Iterator interface and entrySet() method of Map interface
- Using enhanced for-loop and keySet() method of Map interface
- Using forEach() method in Java 8 using Method reference
Let us move forward and discuss all possible ways to iterate List of HashMap of (String, String) type
1. Iterate using Iterator interface and loop through Map.entrySet()
- Outer-List –> iterated using Iterator interface
- Inner-Map –> iterated using enhanced for-loop, after getting entrySet();
IteratingArrayListUsingIteratorAndEntrySet.java
package in.bench.resources.string.methods;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
public class IteratingArrayListUsingIteratorAndEntrySet {
public static void main(String[] args) {
// HashMap 1: input preparation for WEST India Region
Map<String, String> westIndiaStateCapital =
new HashMap<String, String>();
westIndiaStateCapital.put("Maharashtra", "Mumbai");
westIndiaStateCapital.put("Gujarat", "Gandhinagar");
westIndiaStateCapital.put("Rajasthan", "Jaipur");
westIndiaStateCapital.put("Goa", "Panaji");
// HashMap 2: input preparation for SOUTH India Region
Map<String, String> southIndiaStateCapital =
new HashMap<String, String>();
southIndiaStateCapital.put("Andhra Pradesh", "Amaravati");
southIndiaStateCapital.put("Karnataka", "Bengaluru");
southIndiaStateCapital.put("Kerala","Thiruvananthapuram");
southIndiaStateCapital.put("Tamil Nadu", "Chennai");
southIndiaStateCapital.put("Telangana", "Hyderabad");
// HashMap 3: input preparation for EAST India Region
Map<String, String> eastIndiaStateCapital =
new HashMap<String, String>();
eastIndiaStateCapital.put("Bihar", "Patna");
eastIndiaStateCapital.put("Jharkhand", "Ranchi");
eastIndiaStateCapital.put("West Bengal", "Kolkata");
eastIndiaStateCapital.put("Odisha", "Bhubaneswar");
// HashMap 4: input preparation for SOUTH India Region
Map<String, String> northIndiaStateCapital =
new HashMap<String, String>();
northIndiaStateCapital.put("Delhi", "New Delhi");
northIndiaStateCapital.put("Himachal Pradesh", "Shimla");
northIndiaStateCapital.put("Jammu n Kashmir", "Srinagar");
northIndiaStateCapital.put("Punjab", "Chandigarh");
// HashMap 5: input preparation for CENTRAL India Region
Map<String, String> centralIndiaStateCapital =
new HashMap<String, String>();
centralIndiaStateCapital.put("Madhya Pradesh", "Bhopal");
centralIndiaStateCapital.put("Uttar Pradesh", "Lucknow");
centralIndiaStateCapital.put("Chhattisgarh", "Raipur");
// create ArrayList and insert all 5 HashMap into it
List<Map<String, String>> listOfIndianRegions =
new ArrayList<Map<String, String>>();
// add all 5 regions to AL
listOfIndianRegions.add(westIndiaStateCapital);
listOfIndianRegions.add(southIndiaStateCapital);
listOfIndianRegions.add(eastIndiaStateCapital);
listOfIndianRegions.add(northIndiaStateCapital);
listOfIndianRegions.add(centralIndiaStateCapital);
// temp variable to indicate zone/region
int regionIndex = 1;
// Way 1: Iterate using for-each and
// loop through Map using entrySet() method
System.out.println("Way 1: Iterate using for-each and"
+ " loop through Map using entrySet() method");
// get Iterator for looping through AL
Iterator<Map<String, String>> iterator =
listOfIndianRegions.iterator();
// iterate AL using while-loop
while(iterator.hasNext()) {
System.out.println("\nIndia Region - " + regionIndex);
System.out.println("=========================="
+ "========================");
Map<String, String> region = iterator.next();
// getting entrySet() into Set
Set<Entry<String, String>> entrySet =
region.entrySet();
// for-each loop
for(Entry<String, String> entry : entrySet) {
System.out.println("State : " + entry.getKey()
+ "\tCapital : " + entry.getValue());
}
// increment region index by 1
regionIndex++;
}
}
}
Output:
Way 1: Iterate using for-each and loop through Map using entrySet() method
India Region - 1
==================================================
State : Gujarat Capital : Gandhinagar
State : Goa Capital : Panaji
State : Maharashtra Capital : Mumbai
State : Rajasthan Capital : Jaipur
India Region - 2
==================================================
State : Karnataka Capital : Bengaluru
State : Telangana Capital : Hyderabad
State : Andhra Pradesh Capital : Amaravati
State : Tamil Nadu Capital : Chennai
State : Kerala Capital : Thiruvananthapuram
India Region - 3
==================================================
State : Jharkhand Capital : Ranchi
State : Bihar Capital : Patna
State : Odisha Capital : Bhubaneswar
State : West Bengal Capital : Kolkata
India Region - 4
==================================================
State : Delhi Capital : New Delhi
State : Himachal Pradesh Capital : Shimla
State : Jammu n Kashmir Capital : Srinagar
State : Punjab Capital : Chandigarh
India Region - 5
==================================================
State : Madhya Pradesh Capital : Bhopal
State : Uttar Pradesh Capital : Lucknow
State : Chhattisgarh Capital : Raipur
2. Iterate using enhanced for-loop and loop through Map.keySet()
- Outer List –> iterated using enhanced for-loop
- Inner Map –> iterated using Iterator interface, after getting keySet()
IteratingArrayListUsingForLoopAndKeySet.java
package in.bench.resources.list.of.hashmap;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
public class IteratingArrayListUsingForLoopAndKeySet {
public static void main(String[] args) {
// HashMap 1: input preparation for WEST India Region
Map<String, String> westIndiaStateCapital =
new HashMap<String, String>();
westIndiaStateCapital.put("Maharashtra", "Mumbai");
westIndiaStateCapital.put("Gujarat", "Gandhinagar");
westIndiaStateCapital.put("Rajasthan", "Jaipur");
westIndiaStateCapital.put("Goa", "Panaji");
// HashMap 2: input preparation for SOUTH India Region
Map<String, String> southIndiaStateCapital =
new HashMap<String, String>();
southIndiaStateCapital.put("Andhra Pradesh", "Amaravati");
southIndiaStateCapital.put("Karnataka", "Bengaluru");
southIndiaStateCapital.put("Kerala","Thiruvananthapuram");
southIndiaStateCapital.put("Tamil Nadu", "Chennai");
southIndiaStateCapital.put("Telangana", "Hyderabad");
// HashMap 3: input preparation for EAST India Region
Map<String, String> eastIndiaStateCapital =
new HashMap<String, String>();
eastIndiaStateCapital.put("Bihar", "Patna");
eastIndiaStateCapital.put("Jharkhand", "Ranchi");
eastIndiaStateCapital.put("West Bengal", "Kolkata");
eastIndiaStateCapital.put("Odisha", "Bhubaneswar");
// HashMap 4: input preparation for SOUTH India Region
Map<String, String> northIndiaStateCapital =
new HashMap<String, String>();
northIndiaStateCapital.put("Delhi", "New Delhi");
northIndiaStateCapital.put("Himachal Pradesh", "Shimla");
northIndiaStateCapital.put("Jammu n Kashmir", "Srinagar");
northIndiaStateCapital.put("Punjab", "Chandigarh");
// HashMap 5: input preparation for CENTRAL India Region
Map<String, String> centralIndiaStateCapital =
new HashMap<String, String>();
centralIndiaStateCapital.put("Madhya Pradesh", "Bhopal");
centralIndiaStateCapital.put("Uttar Pradesh", "Lucknow");
centralIndiaStateCapital.put("Chhattisgarh", "Raipur");
// create ArrayList and insert all 5 HashMap into it
List<Map<String, String>> listOfIndianRegions =
new ArrayList<Map<String, String>>();
// add all 5 regions to AL
listOfIndianRegions.add(westIndiaStateCapital);
listOfIndianRegions.add(southIndiaStateCapital);
listOfIndianRegions.add(eastIndiaStateCapital);
listOfIndianRegions.add(northIndiaStateCapital);
listOfIndianRegions.add(centralIndiaStateCapital);
// temp variable to indicate zone/region
int regionIndex = 1;
// Way 2: Iterate using Iterator interface and
// loop through Map using keyset() method
System.out.println("Way 2: Iterate using"
+ " Iterator interface and "
+ "loop through Map using keyset() method");
for(Map<String, String> region : listOfIndianRegions) {
System.out.println("\nIndia Region - " + regionIndex);
System.out.println("============================"
+ "======================");
// get entrySet() into Set
Set<String> setOfIndianStates = region.keySet();
// Collection Iterator
Iterator<String> iterator =
setOfIndianStates.iterator();
// iterate using while-loop after getting Iterator
while(iterator.hasNext()) {
String key = iterator.next();
System.out.println("State : " + key
+ "\tCapital : " + region.get(key));
}
// increment region index by 1
regionIndex++;
}
}
}
Output:
Way 2: Iterate using Iterator interface and loop through Map using keyset() method
India Region - 1
==================================================
State : Gujarat Capital : Gandhinagar
State : Goa Capital : Panaji
State : Maharashtra Capital : Mumbai
State : Rajasthan Capital : Jaipur
India Region - 2
==================================================
State : Karnataka Capital : Bengaluru
State : Telangana Capital : Hyderabad
State : Andhra Pradesh Capital : Amaravati
State : Tamil Nadu Capital : Chennai
State : Kerala Capital : Thiruvananthapuram
India Region - 3
==================================================
State : Jharkhand Capital : Ranchi
State : Bihar Capital : Patna
State : Odisha Capital : Bhubaneswar
State : West Bengal Capital : Kolkata
India Region - 4
==================================================
State : Delhi Capital : New Delhi
State : Himachal Pradesh Capital : Shimla
State : Jammu n Kashmir Capital : Srinagar
State : Punjab Capital : Chandigarh
India Region - 5
==================================================
State : Madhya Pradesh Capital : Bhopal
State : Uttar Pradesh Capital : Lucknow
State : Chhattisgarh Capital : Raipur
Note:
- keySet(); and entrySet(); methods of Map interface 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
3. Using Java 8’s Iterable.forEach() method
- forEach(System.out::println); –> this line prints entire output in console (in {key-value} pair format)
IteratingArrayListUsingJava8ForEach.java
package in.bench.resources.list.of.hashmap;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class IteratingArrayListUsingJava8ForEach {
public static void main(String[] args) {
// HashMap 1: input preparation for WEST India Region
Map<String, String> westIndiaStateCapital =
new HashMap<String, String>();
westIndiaStateCapital.put("Maharashtra", "Mumbai");
westIndiaStateCapital.put("Gujarat", "Gandhinagar");
westIndiaStateCapital.put("Rajasthan", "Jaipur");
westIndiaStateCapital.put("Goa", "Panaji");
// HashMap 2: input preparation for SOUTH India Region
Map<String, String> southIndiaStateCapital =
new HashMap<String, String>();
southIndiaStateCapital.put("Andhra Pradesh", "Amaravati");
southIndiaStateCapital.put("Karnataka", "Bengaluru");
southIndiaStateCapital.put("Kerala","Thiruvananthapuram");
southIndiaStateCapital.put("Tamil Nadu", "Chennai");
southIndiaStateCapital.put("Telangana", "Hyderabad");
// HashMap 3: input preparation for EAST India Region
Map<String, String> eastIndiaStateCapital =
new HashMap<String, String>();
eastIndiaStateCapital.put("Bihar", "Patna");
eastIndiaStateCapital.put("Jharkhand", "Ranchi");
eastIndiaStateCapital.put("West Bengal", "Kolkata");
eastIndiaStateCapital.put("Odisha", "Bhubaneswar");
// HashMap 4: input preparation for SOUTH India Region
Map<String, String> northIndiaStateCapital =
new HashMap<String, String>();
northIndiaStateCapital.put("Delhi", "New Delhi");
northIndiaStateCapital.put("Himachal Pradesh", "Shimla");
northIndiaStateCapital.put("Jammu n Kashmir", "Srinagar");
northIndiaStateCapital.put("Punjab", "Chandigarh");
// HashMap 5: input preparation for CENTRAL India Region
Map<String, String> centralIndiaStateCapital =
new HashMap<String, String>();
centralIndiaStateCapital.put("Madhya Pradesh", "Bhopal");
centralIndiaStateCapital.put("Uttar Pradesh", "Lucknow");
centralIndiaStateCapital.put("Chhattisgarh", "Raipur");
// create ArrayList and insert all 5 HashMap into it
List<Map<String, String>> listOfIndianRegions =
new ArrayList<Map<String, String>>();
// add all 5 regions to AL
listOfIndianRegions.add(westIndiaStateCapital);
listOfIndianRegions.add(southIndiaStateCapital);
listOfIndianRegions.add(eastIndiaStateCapital);
listOfIndianRegions.add(northIndiaStateCapital);
listOfIndianRegions.add(centralIndiaStateCapital);
// Way 3: Using Iterable.forEach() in Java 8 version
System.out.println("Way 3: Using "
+ "Iterable.forEach() in Java 8 version");
// Iterating using forEach() in Java 8
listOfIndianRegions.forEach(System.out::println);
}
}
Output:
Way 3: Using Iterable.forEach() in Java 8 version
{Gujarat=Gandhinagar, Goa=Panaji,
Maharashtra=Mumbai, Rajasthan=Jaipur}
{Karnataka=Bengaluru, Telangana=Hyderabad,
Andhra Pradesh=Amaravati, Tamil Nadu=Chennai,
Kerala=Thiruvananthapuram}
{Jharkhand=Ranchi, Bihar=Patna,
Odisha=Bhubaneswar, West Bengal=Kolkata}
{Delhi=New Delhi, Himachal Pradesh=Shimla,
Jammu n Kashmir=Srinagar, Punjab=Chandigarh}
{Madhya Pradesh=Bhopal, Uttar Pradesh=Lucknow,
Chhattisgarh=Raipur}
From above example, ArrayList
- allows duplicate records/elements to be stored
- null object is allowed (any number of times and all are counted)
- while iterating, elements/records are retrieved as per insertion-order
Related Articles:
- Java – Various ways to iterate through ArrayList
- Java – Various ways to iterate through Vector in 5 ways
- Java – Various ways to iterate through LinkedList in 5 ways
- Java – Various ways to iterate through HashSet in 3 ways
- Java – How to reverse LinkedHashSet contents
- Java – Various ways to iterate through TreeSet in 3 ways
- Java – Various ways to iterate over List of HashMap
- Java – Various ways to iterate over HashMap of ArrayList
- Java – Various ways to iterate Arrays in 5 ways
- Java 8 – Iterating List using forEach()
- Java 8 – Iterating Set using forEach()
- Java 8 – Iterating Map using forEach()
Happy Coding !!
Happy Learning !!
