ArrayList in Java
The ArrayList class is a resizable array, which can be found in the java.util package.
The difference between a built-in array and an ArrayList in Java, is that the size of an array cannot be modified (if you want to add or remove elements to/from an array, you have to create a new one). While elements can be added and removed from an ArrayList whenever you want. The syntax is also slightly different:
Example
Create an ArrayList object called cars that will store strings:
import java.util.ArrayList; // import the ArrayList class ArrayList<String> cars = new ArrayList<String>(); // Create an ArrayList object
Add Items:
The ArrayList class has many useful methods. For example, to add elements to the ArrayList, use the add() method:
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<String> cars = new ArrayList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("Mazda");
System.out.println(cars);
}
}
Output:
Access an Item
To access an element in the ArrayList, use the get() method and refer to the index number:
cars.get(2);
Change an Item To modify an element, use the set() method and refer to the index number: cars.set(0, "Opel"); Remove an Item To remove an element, use the remove() method and refer to the index number: cars.remove(0);
Remove all Array list:
To remove all the elements in the ArrayList, use the clear() method:
cars.clear();
Loop Through an ArrayList
Loop through the elements of an ArrayList with a for loop, and use the size() method to specify how many times the loop should run:
public class Main {
public static void main(String[] args) {
ArrayList<String> cars = new ArrayList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("Mazda");
for (int i = 0; i < cars.size(); i++) {
System.out.println(cars.get(i));
}
}
}
Output:
for each loop
You can also loop through an ArrayList with the for-each loop:
public class Main {
public static void main(String[] args) {
ArrayList<String> cars = new ArrayList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("Mazda");
for (String i : cars) {
System.out.println(i);
}
}
}
Output:
Sort an ArrayList
Another useful class in the java.util package is the Collections class, which include the sort() method for sorting lists alphabetically or numerically:
import java.util.ArrayList;
import java.util.Collections; // Import the Collections class
public class Main {
public static void main(String[] args) {
ArrayList<String> cars = new ArrayList<String>();
cars.add("Volvo");
cars.add("BMW");
cars.add("Ford");
cars.add("Mazda");
Collections.sort(cars); // Sort cars
for (String i : cars) {
System.out.println(i);
}
}
}
Output:
