Lists in Java
Table of contents:
- Creating a list
- Defining the Type of Values That a List Can Contain
- Adding to a List and Retrieving a Value from a Specific Place
Creating a list
Creating a new list is done with the command ArrayList
import java.util.ArrayList;
public class Program {
public static void main(String[] args) {
// create a list
ArrayList<String> list = new ArrayList<>();
// the list isn't used yet
}
}
The type of the ArrayList variable is ArrayList. When a list variable is initialized, the type of the values to be stored is also defined in addition to the variable type — all the variables stored in a given list are of the same type. As such, the type of an ArrayList that stores strings is ArrayList<String>
. A new list is created with the command new ArrayList<>();
.
Defining the Type of Values That a List Can Contain
ArrayList<Integer> list = new ArrayList<>();
list.add(1);
ArrayList<Boolean> list = new ArrayList<>();
list.add(true);
ArrayList<String> list = new ArrayList<>();
list.add("Some text");
Once a list has been created, ArrayList assumes that all the variables contained in it are reference types. Java automatically converts an int
variable into Integer
when one is added to a list, and the same occurs when a variable is retrieved from a list. The same conversion occurs for double
-type variables, which are converted to Double
. This means that even though a list is defined to contain Integer
-type variables, variables of type int
can also be added to it.
ArrayList<Integer> integers = new ArrayList<>();
int integer = 1;
integers.add(integer);
ArrayList<Double> doubles = new ArrayList<>();
double d = 3.3;
doubles.add(d)
Adding to a List and Retrieving a Value from a Specific Place
Addition is done with the list method add
, which takes the value to be added as a parameter. We then print the value at position zero. To retrieve a value from a certain position, you use the list method get
, which is given the place of retrieval as a parameter.
To call a list method you first write the name of the variable describing the list, followed by a dot and the name of the method.
...
public static void main(String[] args) {
// create the word list for storing strings
ArrayList<String> wordList = new ArrayList<>();
// add two values to the word list
wordList.add("First");
wordList.add("Second");
// retrieve the value from position 0 of the word list, and print it
System.out.println(wordList.get(0));
}
...
As can be seen, the get
method retrieves the first value from the list when it is given the parameter 0
. This is because list positions are counted starting from zero. The first value is found by wordList.get(0)
, the second by wordList.get(1)
, and so on.
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