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Java ArrayList
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Review the key moves
What is the main idea behind Java ArrayList?
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import java.util.ArrayList; // Import the ArrayList ___Put the learning moves in the order that makes the concept easiest to apply.
An ArrayList is like a resizable array .
It is part of the java.util package and implements the List interface.
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.
Create an ArrayList
To use an ArrayList , you must first import it from java.util :
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 objectNow you can use methods like add() , get() , set() , and remove() to manage your list of elements.
Add Elements
To add elements to an ArrayList , use the add() method:
Example
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);
}
}You can also add an element at a specified position by referring to the index number:
Example
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(0, "Mazda"); // Insert element at the beginning of the list (0)
System.out.println(cars);
}
}An ArrayList keeps elements in the same order you add them, so the first item you add will be at index 0, the next at index 1, and so on.
Access an Element
To access an element in the ArrayList , use the get() method and refer to the index number:
cars.get(0); // Get the first elementChange an Element
To modify an element, use the set() method and refer to the index number:
cars.set(0, "Opel");Remove an Element
To remove an element, use the remove() method and refer to the index number:
cars.remove(0);To remove all the elements in the ArrayList , use the clear() method:
cars.clear();ArrayList Size
To find out how many elements an ArrayList have, use the size method:
cars.size();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:
Example
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));
}
}
}You can also loop through an ArrayList with the for-each loop:
Example
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);
}
}
}Other Types
Elements in an ArrayList are actually objects. In the examples above, we created elements (objects) of type "String". Remember that a String in Java is an object (not a primitive type). To use other types, such as int , you must specify an equivalent wrapper class : Integer . For other primitive types, use: Boolean for boolean, Character for char, Double for double, etc:
ArrayListSort 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:
Example
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);
}
}
}Example
import java.util.ArrayList;
import java.util.Collections; // Import the Collections class
public class Main {
public static void main(String[] args) {
ArrayList<Integer> myNumbers = new ArrayList<Integer>();
myNumbers.add(33);
myNumbers.add(15);
myNumbers.add(20);
myNumbers.add(34);
myNumbers.add(8);
myNumbers.add(12);
Collections.sort(myNumbers); // Sort myNumbers
for (int i : myNumbers) {
System.out.println(i);
}
}
}The var Keyword
From Java 10, you can use the var keyword to declare an ArrayList variable without writing the type twice. The compiler figures out the type from the value you assign.
This makes code shorter, but many developers still use the full type for clarity . Since var is valid Java, you may see it in other code, so it's good to know that it exists:
// Without var ArrayList<String> cars = new ArrayList<String>();
// With var var cars = new ArrayList<String>();The List Interface
Note
Sometimes you will see both List and ArrayList in Java code, like this:
import java.util.List;
import java.util.ArrayList;
List<String> cars = new ArrayList<>();This means the variable (cars) is declared as a List (the interface), but it stores an ArrayList object (the actual list). Since ArrayList implements the List interface, this is possible.
It works the same way, but some developers prefer this style because it gives them more flexibility to change the type later.