[java] Java: How to convert List to Map

Since Java 8, the answer by @ZouZou using the Collectors.toMap collector is certainly the idiomatic way to solve this problem.

And as this is such a common task, we can make it into a static utility.

That way the solution truly becomes a one-liner.

/**
 * Returns a map where each entry is an item of {@code list} mapped by the
 * key produced by applying {@code mapper} to the item.
 *
 * @param list the list to map
 * @param mapper the function to produce the key from a list item
 * @return the resulting map
 * @throws IllegalStateException on duplicate key
 */
public static <K, T> Map<K, T> toMapBy(List<T> list,
        Function<? super T, ? extends K> mapper) {
    return list.stream().collect(Collectors.toMap(mapper, Function.identity()));
}

And here's how you would use it on a List<Student>:

Map<Long, Student> studentsById = toMapBy(students, Student::getId);

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