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 a most common task related to the
arrayList in java
which comes in our day-to-day programming practices. For example: a list of
books (ArrayList<Book>) can be convert into a HashMap which maps BookId
with the object of the Book (HashMap<BookId, Book>). It can be done in
many ways. We will learn a few of them in this article.
Content of the Article
How to convert ArrayList to HashMap in Java - Using ArrayList Iteration?
We can convert an ArrayList to HashMap by just
iterating the ArrayList. We just need to iterate on each of the element of the ArrayList and the
element can converted into the 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 into a HashMap but HashMap does not
maintain the order of the ArrayList. To maintain the order, we can use
LinkedHashMap which is an implementation of HashMap and helps us to maintain
the order of the elements. So that, we can use LinkedHashMap to convert
ArrayList to HashMap. Let see how.
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));
The keyMapper takes the element of the ArrayList one by one as input and
generates the key, similarly, the valueMapper takes the element of the
ArrayList as input one-by-one and generates the value. Please note that, both
the keyMapper and valueMapper are the instance of the Function<Input,
Output> class of java.util.function package.
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 is keyMapper as s -> s which can be replaced with Function.identity() static method of java.util.function package, Function.identity() method generates output same as input i.e., without any change in input.
- And another for valueMapper as s -> s.length() can be replaced with method refernce String::length.
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,
(oldValue, newValue) -> newValue, LinkedHashMap::new
));
return linkedHashMap;
}
Output
-------------ArrayList--------------- Java C++ Python PHP NodeJS --------------HashMap---------------- Java : 4 C++ : 3 Python : 6 PHP : 3 NodeJS : 6
The 3rd lambda expression is used to make choice between the old and new
value, if any more than one value is generated for the same key. And, the
fourth lambda is used to generates the object of LinkedHashMap.
Conclusion
In this article, we have learned how to convert ArrayList to HashMap by
iterating ArrayList in java. Also, we have seen the stream APIs method of Java
8 to convert ArrayList to HashMap.
Thanks for reading this article.
If, we missed something please let us know in the comment section below.
Happy Learning
Thanks for reading this article.
If, we missed something please let us know in the comment section below.
Happy Learning
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