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
Various ways to iterate through List (ArrayList/Vector)
- regular 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
Various ways to iterate through Map (HashMap/TreeMap)
- 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
Various 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
Way 1: Iterate using Iterator interface and looping through Map using entrySet(); method
- 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
Way 2: Iterate using enhanced for loop and looping through Map using keySet(); method
- 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
Way 3: Using Iterable.forEach() method in Java 8 version
- 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
Happy Coding !!
Happy Learning !!