Lists

Creating a List

A List is more versatile than an array because we can add and remove items. Its size can change. There are several types of List but the most common is the ArrayList.

java

import java.util.ArrayList;

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

    authors.add("Chinua Achebe");
    authors.add("Toni Morrison");
    authors.add("Gabriel García Márquez");
    authors.add("Arundhati Roy");
    authors.add("Marjane Satrapi");

    System.out.println(authors);
  }
}

output

[Chinua Achebe, Toni Morrison, Gabriel García Márquez, Arundhati Roy, Marjane Satrapi]

TIP

If you're comfortable with using var, it's possible to create an empty ArrayList a little more concisely:

var authors = new ArrayList<String>();

ArrayList methods

There are lots of methods which allow us to work with Lists.

Method	Description
add(e)	Appends the specified element to the end of the list.
add(index, element)	Inserts the element at the specified position in the list.
remove(index)	Removes the element at the specified position.
remove(o)	Removes the first occurrence of the specified element from the list.
get(index)	Returns the element at the specified position.
set(index, element)	Replaces the element at the specified position with the specified element.
size()	Returns the number of elements in the list.
contains(o)	Returns true if the list contains the specified element.

There are more which your editor can highlight for you.

add(index, element)

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> authors = new ArrayList<>();
    authors.add("Chinua Achebe");
    authors.add("Toni Morrison");
    authors.add("Gabriel García Márquez");

    authors.add(1, "Amy Tan");
    System.out.println(authors);
    // [Chinua Achebe, Amy Tan, Toni Morrison, Gabriel García Márquez]
  }
}

remove(index) and remove(element)

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> authors = new ArrayList<>();
    authors.add("Chinua Achebe");
    authors.add("Toni Morrison");
    authors.add("Gabriel García Márquez");

    authors.remove(1);
    System.out.println(authors);
    // [Chinua Achebe, Gabriel García Márquez]

    authors.remove("Chinua Achebe");
    System.out.println(authors);
    // [Gabriel García Márquez]
  }
}

get(index) and set(index, element)

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> authors = new ArrayList<>();
    authors.add("Chinua Achebe");
    authors.add("Toni Morrison");
    authors.add("Gabriel García Márquez");

    var author = authors.get(1);
    System.out.println(author); // "Toni Morrison"

    authors.set(2, "Sandra Cisneros");
    System.out.println(authors);
    // [Chinua Achebe, Toni Morrison, Sandra Cisneros]

  }
}

Methods on lists

We can create our own methods which accept lists and work with them.

INFO

To allow the method to work with lists of any type, we need to declare a generic type parameter. In the below example we use T to represent the type of data stored in the list.

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> authors = new ArrayList<>();
    authors.add("Chinua Achebe");
    authors.add("Toni Morrison");
    authors.add("Gabriel García Márquez");

    ArrayList<String> rotated = rotateArray(authors);
    System.out.println(rotated);
  }

  public static <T> ArrayList<T> rotateArray(ArrayList<T> list) {
    var lastEl = list.removeFirst();
    list.add(lastEl);
    return list;
  }
}