Java ArrayList

ArrayList

An ArrayList is used to hold many items of the same type. The main difference between an Array and an ArrayList is that an ArrayList's size is not fixed. Items can be easily added or removed from an ArrayList.

List of Planets Example

Create the list

To create an empty list of Strings to hold our planets we can do the following:

ArrayList<String> planets = new ArrayList<String>();

If you wanted the list to hold numbers, you can change String to int.

Adding to the list

To add some planets to our list:

planets.add("Mercury");
planets.add("Venus");
planets.add("Earth");

Getting the number of items in the ArrayList

To find out how many items are in the ArrayList, use can use the size method:

System.out.print(planets.size());

Get a specific item

To get an item at a specific position, we use the get() method. The items start at zero so to get the first item:

System.out.print(planets.get(0));

Looping through the array

One of the main advantages of using arrays instead of using multiple variables is that you can use loops to go through arrays.

To print the items in the list using the size() and get() methods, we can do the following:

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

A much simpler way though the list is with a foreach loop, sometimes referred to as an 'enhanced' for loop.

// Each value is taken from the planet list and put into the 'p' variable.
// 'var' means we let the compiler figure out the data type (String)
for(var p:planets){
	System.out.println(num);
}

Removing items in a list

We can remove specific items using their position with the remove() method, to remove the first item in the list:

planets.remove(0);

To remove all items individually, we could use:

for(int c=0;c<planets.size();c++){
	planets.remove(c);
}

A much shorter method to remove all items is:

planets.clear();

Full Program

With the Java JDK installed, from a command line:

• Save the following as planets.java
• Compile with javac planets.java
• Run with java Planets

import java.util.ArrayList;

class Planets{
	public static void main(String[] args){	
		ArrayList<String> planets = new ArrayList<String>();
		
		// Add two planets
		planets.add("Venus");
		planets.add("Earth");
		planets.add("Ceres");
		
		// Add at the beginning
		planets.add(0,"Mercury");
		
		// Remove the last item, size() is 4 (0 to 3) so we subtract 1
		planets.remove(planets.size()-1);
		
		// Modify the 2nd item
		planets.set(2, "Earth 1");
		
		for(var p:planets){
			System.out.println(p);
		}
		
		System.out.println(planets.size()+" Planets");
	}
}

Updated (08/05/2025)