Creating a List

Giving your list a type

To create a list you need a type (any class, e.g. String). This is the type of your List. The List will only store objects of the specified type. For example:

List<String> strings;

Can store "string1", "hello world!", "goodbye", etc, but it can’t store 9.2, however:

List<Double> doubles;

Can store 9.2, but not "hello world!".

Initialising your list

If you try to add something to the lists above you will get a NullPointerException, because strings and doubles both equal null!

There are two ways to initialise a list:

Option 1: Use a class that implements List

List is an interface, which means that does not have a constructor, rather methods that a class must override. ArrayList is the most commonly used List, though LinkedList is also common. So we initialise our list like this:

List<String> strings = new ArrayList<String>();

or

List<String> strings = new LinkedList<String>();

Starting from Java SE 7, you can use a diamond operator:

List<String> strings = new ArrayList<>();

or

List<String> strings = new LinkedList<>();

Option 2: Use the Collections class

The Collections class provides two useful methods for creating Lists without a List variable:

• emptyList(): returns an empty list.
• singletonList(T): creates a list of type T and adds the element specified.

And a method which uses an existing List to fill data in:

• addAll(L, T...): adds all the specified elements to the list passed as the first parameter.

Examples:

import java.util.List;
import java.util.Collections;

List<Integer> l = Collections.emptyList();
List<Integer> l1 = Collections.singletonList(42);
Collections.addAll(l1, 1, 2, 3);