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Java ArrayList
Java ArrayList
The ArrayList class is a regenerative program, which can be found in the java.util package.
The difference between a built-in array and ArrayList in Java, is that the array size cannot be changed (if you want to add or remove elements to / from the list, you must create a new one). Although features can be added and removed from 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 section has many useful features. For example, to add elements to 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);
}
}
Access an Item
To access an item in ArrayList, use the get() method and refer to the reference number:
Example
cars.get(0);
Note: Index lists start with 0: [0] first item. [1] The second element, etc.
Change an Item
To edit an element, use the set() method and check the reference number:
Example
cars.set(0, "Opel");
Remove an Item
To delete an element, use the remove() method and refer to the reference number:
Example
cars.remove(0);
To delete all elements in ArrayList, use the method clear():
Example
cars.clear();
ArrayList Size
To find out how many elements ArrayList has, use the size method:
Example
cars.size();
Loop Through an ArrayList
Rotate ArrayList objects with a for loop, and use the size() method to specify how often the loop should work:
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 log into ArrayList 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
The elements in ArrayList are actually objects. In the examples above, we created elements (objects) of the "String" type. Remember that String in Java is an object (not an old version). To use other types, such as int, you must specify an equal wrapper category: Integer. For other classic types, use: Boolean boolean, Character for Character, Double for double, etc:
Example
Create an ArrayList to store numbers (add elements of type Integer):
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<Integer> myNumbers = new ArrayList<Integer>();
myNumbers.add(10);
myNumbers.add(15);
myNumbers.add(20);
myNumbers.add(25);
for (int i : myNumbers) {
System.out.println(i);
}
}
}
Sort an ArrayList
Another useful class in the java.util package is a collections class, which includes a sort() sorting list alphabetically or by numbers:
Example
Sort an ArrayList of Strings:
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
Sort an ArrayList of Integers:
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);
}
}
}