Java ArrayList Class

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 ArrayListwhenever you want. The syntax is also slightly different.

Other advantages and disadvantages of ArrayList are:

• Java ArrayList class can contain duplicate elements.

• Java ArrayList class maintains insertion order.

• Java ArrayList class is nonsynchronized.

• Java ArrayList allows random access because the array works on an index basis.

• In ArrayList, manipulation is a little bit slower than the LinkedList in Java because a lot of shifting needs to occur if any element is removed from the array list.

ArrayList Methods

• add()  to add a new element, e.g. cars.add("Volvo");

• get()  to access an element, e.g. cars.get(0);

• set() to modify an element, e.g. cars.set(0, "Opel");

• remove()  to remove an element, e.g. cars.remove(0);

• clear() to remove all the elements,  cars.clear();

• size  to find out how many elements, e.g. cars.size();

Add Items

Example: CarList

import java.util.ArrayList;

public class CarList {

  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);

  }

}

The ouput:

[Volvo, BMW, Ford, Mazda]

Access an Item

To access an element in the ArrayList, use the get() method and refer to the index number.

Example: 

ArrayList<Integer> num = new ArrayList<Integer>();

num.add(23);

num.add(1);

num.add(15);

num.add(30);

System.out.println(num.get(1));

output:

1

Change an Item

To modify an element, use the set() method and refer to the index number.

Example:

ArrayList<Integer> num = new ArrayList<Integer>();

num.add(23);

num.add(1);

num.add(15);

num.add(30);

num.set(1, 4);

System.out.println(num.get(1));

Output:

4

Remove an Item

To remove an element, use the remove() method and refer to the index number.

Example:

ArrayList<Integer> num = new ArrayList<Integer>();

num.add(23);

num.add(1);

num.add(15);

num.add(30);

num.remove(2);

System.out.println(num);

Output:

[23, 1, 30]

To remove all the elements in the ArrayList, use the clear() method: 

Example:

ArrayList<Integer> num = new ArrayList<Integer>();

num.add(23);

num.add(1);

num.add(15);

num.add(30);

num.clear();;

System.out.println(num);

Output:

[]

ArrayList Size

To find out how many elements an ArrayList have, use the size method.

Example:

ArrayList<Integer> num = new ArrayList<Integer>();

num.add(23);

num.add(1);

num.add(15);

num.add(30);

System.out.println(num.size());

Output:

4

Sort an ArrayList

Another useful class in the java.util package is the Collections class, which includes the sort() method for sorting lists alphabetically or numerically:

Example: CarList2

import java.util.ArrayList;

import java.util.Collections; 

public class CarList2 {

  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);

    }

   }

}

The output:

BMW

Ford

Mazda

Volvo

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: IntList

import java.util.ArrayList;

public class IntList {

  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);

    }

  }

}

The output:

10

15

20

25