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ArrayList in Java

Array lists are an alternative to arrays in Java. The main difference is that array lists are not fixed in size and do not receive a number of elements to hold. Therefore, elements can be added to an array lists and deleted from it.

Declaring and Initializing an Array List​

Important is to import ArrayList from the package java.util, which is available by default.

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> numbers = new ArrayList<Integer>();

        numbers.add(1);
        numbers.add(2);
        numbers.add(3);

    }
}

Instead of adding the elements to the empty list, you can initialize the list right away: (You need to import java.util.Arrays in order for this to work)

ArrayList<Integer> numbers = new ArrayList<Integer>(
    Arrays.asList(1, 2, 3)
);

There is even a third way:

ArrayList<Integer> numbers = new ArrayList<Integer>() {
    {
        add(1);
        add(2);
        add(3);
    }
};

Initializing an Array List from an Array​

The above shown method Arrays.asList() can be used to get an array list based on an array: (Don't forget to import java.util.Arrays)

Integer[] numbers = {1, 2, 3};

ArrayList<Integer> numbersList = new ArrayList<Integer>(
    Arrays.asList(numbers)
);

Accessing, Altering and Deleting Elements from the Array List​

Accessing an element, for example in order to print it, can be done with get():

numbersList.get(0)

0 is the index of the first element in the list.

Deleting an element from the list:

numbersList.reomve(index)

When deleting from an array list, the size is reduced. Therefore, there is no empty space in the list left.

In a similar way, we can overwrite values at a given position:

numbersLits.set(index, newValue)

Looping over the Array List​

Iterating can be done with a class for-loop:

for (int i = 0; i < numbersList.size(); i++) {
    System.out.println(numbersList.get(i));
}

or with the cleaner for-each loop:

 for (int num : numbersList) {
    System.out.println(num);
}