List vs Map in Java

Question

I didnt get the sense of Maps in Java. When is it recommended to use a Map instead of a List?

thanks in advance,

nohereman

Answer 1

Say you have a bunch of students with names and student IDs. If you put them in a List, the only way to find the student with student_id = 300 is to look at each element of the list, one at a time, until you find the right student.

With a Map, you associate each student's ID and the student instance. Now you can say, "get me student 300" and get that student back instantly.

Use a Map when you need to pick specific members from a collection. Use a List when it makes no sense to do so.

Say you had exactly the same student instances but your task was to produce a report of all students' names. You'd put them in a List since there would be no need to pick and choose individual students and thus no need for a Map.

Answer 2

A map is used as an association of a key and a value. With a list you have basically only values.
The indexes in List are always int, whereas in Map you can have another Object as a key.

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Resources :

• sun.com - Introduction to the Collections Framework, Map

Answer 3

Depends on your performance concerns. A Map more explicitly a HashMap will guarantee O(1) on inserts and removes. A List has at worst O(n) to find an item. So if you would be so kind as to elaborate on what your scenario is we may help more.

Answer 4

Map and List serve different purpose.

List holds collection of items. Ordered (you can get item by index).

Map holds mapping key -> value. E.g. map person to position: "JBeg" -> "programmer". And it is unordered. You can get value by key, but not by index.

Answer 5

Its probably a good idea to revise Random Access Vs Sequential Access Data Structures. They both have different run time complexities and suitable for different type of contexts.

Answer 6

Java map: An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

Java list: An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.

The difference is that they are different. Map is a mapping of key/values, a list of a list of items.