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ArrayList to HashMap in Java

How to convert ArrayList to HashMap in Java?

In this article, we are going to learn how we can convert ArrayList to HashMap in java. This is the most common task related to an ArrayList in Java which comes in our day-to-day programming practices. For example: an ArrayList<Book> (list of books) can be convert into HashMap<BookId, Book> (a HashMap which map BookId with actual Book Object). To achieve this, we have many alternatives. Let's learn a few of them.

Content of the Article

How to convert ArrayList to HashMap in Java - Using ArrayList Iteration?

We can convert an ArrayList to HashMap by iterating ArrayList in java. We just need to iterate each element of ArrayList and by using the element create a key-value pair and store into the HashMap.

Example 1: ArrayList to HashMap in Java - Using ArrayList Iteration

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class ArrayListExample {
        public static void main(String[] args) {
                ArrayList<String> languageList = new ArrayList<>(
                                Arrays.asList("Java", "C++", "Python", "PHP" , "NodeJS"));
                System.out.println("-------------ArrayList---------------");
                for (String language : languageList) {
                        System.out.println(language);
                }
                System.out.println("--------------HashMap----------------");
                HashMap<String, Integer> languageMap = convertArrayListToHashMap(languageList);
                for(Map.Entry<String, Integer> entry : languageMap.entrySet()) {
                        System.out.println(entry.getKey() + " : " + entry.getValue());
                }
        }

        private static HashMap<String, Integer> convertArrayListToHashMap(ArrayList<String> arrayList) {
                HashMap<String, Integer> hashMap = new HashMap<>();
                for (String str : arrayList) {
                        hashMap.put(str, str.length());
                }
                return hashMap;
        }
}
Output
-------------ArrayList---------------
Java
C++
Python
PHP
NodeJS
--------------HashMap----------------
Java : 4
C++ : 3
PHP : 3
NodeJS : 6
Python : 6

How about the order of the ArrayList?

As we can see, the ArrayList is converted to a HashMap but HashMap does not maintain the order of the ArrayList. We can use LinkedHashMap which is an implementation of HashMap and maintain the order so that we can use LinkedHashMap to convert ArrayList to HashMap.

Example 2: ArrayList to HashMap in Java - Using ArrayList Iteration with LinkedHashMap

private static HashMap<String, Integer> convertArrayListToHashMap(ArrayList<String> arrayList) {
        LinkedHashMap<String, Integer> linkedHashMap = new LinkedHashMap<>();
        for (String str : arrayList) {
                linkedHashMap.put(str, str.length());
        }
        return linkedHashMap;
}
Output
-------------ArrayList---------------
Java
C++
Python
PHP
NodeJS
--------------HashMap----------------
Java : 4
C++ : 3
Python : 6
PHP : 3
NodeJS : 6

How to convert ArrayList to HashMap in Java - Using Java 8 Stream APIs?

Java 8 introduced stream APIs and lambda expression which provides a very easy way to convert an ArrayList to HashMap.
  • Through stream API methods, we can start the stream of the ArrayList and with the help of collect() method and the java.util.stream.Collectors.toMap() static method, we can collect each element of ArrayList to HashMap.
  • And By using lambda expression, we can write keyMapper and valueMapper.
Syntax
Map<K, V> map = arrayList.stream().collect(Collectors.toMap(keyMapper, valueMapper));

Example 3: ArrayList to HashMap in Java - Using Java 8 Stream APIs

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class ArrayListExample {
        public static void main(String[] args) {
                ArrayList<String> languageList = new ArrayList<>(
                                Arrays.asList("Java", "C++", "Python", "PHP", "NodeJS"));
                System.out.println("-------------ArrayList---------------");
                for (String language : languageList) {
                        System.out.println(language);
                }
                System.out.println("--------------HashMap----------------");
                HashMap<String, Integer> languageMap = convertArrayListToHashMap(languageList);
                for (Map.Entry<String, Integer> entry : languageMap.entrySet()) {
                        System.out.println(entry.getKey() + " : " + entry.getValue());
                }
        }

        private static HashMap<String, Integer> convertArrayListToHashMap(ArrayList<String> arrayList) {
                HashMap<String, Integer> hashMap = new HashMap<>(
                        arrayList.stream().collect(Collectors.toMap(s -> s, s -> s.length()))
                );
                return hashMap;
        }
}
Output
-------------ArrayList---------------
Java
C++
Python
PHP
NodeJS
--------------HashMap----------------
Java : 4
PHP : 3
C++ : 3
NodeJS : 6
Python : 6
We have used two lambda expression here,
  • One for keyMapper as s -> s which can be replaced with Function.identity() static method of java.util.function package
  • And another for valueMapper as s -> s.length() can be replaced with method refernce String::length.
So we can rewrite the method as: 
private static HashMap<String, Integer> convertArrayListToHashMap(ArrayList<String> arrayList) {
        HashMap<String, Integer> hashMap = new HashMap<>(
                arrayList.stream().collect(Collectors.toMap(Function.identity(), String::length))
        );
        return hashMap;
}

Using Java 8 Stream APIs with LinkedHashMap to maintain order

Example 4: ArrayList to HashMap in Java - Using Java 8 Stream APIs with LinkedHashMap to maintain order

private static HashMap<String, Integer> convertArrayListToHashMap(ArrayList<String> arrayList) {
        LinkedHashMap<String, Integer> linkedHashMap = arrayList.stream().collect(
                Collectors.toMap(
                        Function.identity(), String::length, (v1, v2) -> v2, LinkedHashMap::new
                ));
        return linkedHashMap;
}
Output
-------------ArrayList---------------
Java
C++
Python
PHP
NodeJS
--------------HashMap----------------
Java : 4
C++ : 3
Python : 6
PHP : 3
NodeJS : 6

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Conclusion

In this article, we have learned how to convert ArrayList to HashMap using ArrayList Iteration in Java. Also, we have seen the stream APIs method of Java 8 to convert ArrayList to HashMap.

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