How to Convert List to Map in Java

This tutorial shows the different ways to convert a List to Map in Java.

Java 7 and Before

With Java 7 and older releases, the only way to convert a List  to Map  is to iterate over the list and populate the map manually.

In the following example, we implement a utility method that accepts a list of Student objects and populates out of it a  HashMap  with id  as a key and name  as a value.

public static Map<Integer,String> listToHashmapJava7Below(List<Student> students)
{
    Map<Integer, String> studentsMap = new HashMap<Integer, String>();

    for(Student student : students)
    {
        studentsMap.put(student.getId(), student.getName());
    }

    return studentsMap;
}

Java 8

With Java 8, you can convert a List  to Map  in one line using the  stream()  and  Collectors.toMap() utility methods.

public static Map<Integer,String> listToHashmapJava8(List<Student> students)
{
    Map<Integer, String> studentsMap = students.stream().collect(Collectors.toMap(Student :: getId, Student :: getName));
    return studentsMap;
}

 The Collectors.toMap() method collects a stream as a Mapand uses its arguments to decide what key/value to use.

Java 8: Handle Duplicate Keys

 The Collectors.toMap() fails when converting a list with duplicate items.

In order to handle duplicates, you can pass a third argument that informs the  toMap()  which value to consider when facing duplicate items.

In the following example, we decide to consider the old value or, in other words, to keep the existing value without updating each time the map faces a duplicate:

public static Map<Integer,String> listToHashmapJava8WithDuplicates(List<Student> students)
{
    Map<Integer, String> studentsMap = students.stream().collect(Collectors.toMap(Student :: getId, Student :: getName
                                           , (oldValue, newValue) -> oldValue));
    return studentsMap;
}

If you want to override the existing value on duplicates, use  (oldValue, newValue) -> newValue .

Java 8: Preserve List Order

In order to preserve the order of the list items inside the  Map , you can pass another parameter to  Collectors.toMap()  which decides what type of map to use.  LinkedHashMap  is well known in preserving the order of its entries.

public static Map<Integer,String> listToHashmapJava8PreserveOrder(List<Student> students)
{
    Map<Integer, String> studentsMap = students.stream().collect(Collectors.toMap(Student :: getId, Student :: getName
                                           , (oldValue, newValue) -> oldValue,LinkedHashMap::new));
    return studentsMap;
}

If you enjoyed this article and want to learn more about Java Collections, check out this collection of tutorials and articles on all things Java Collections.