Java's Built-In Data Structures

ArrayLists

An ArrayList is a built-in data structure that uses a dynamic array to store its elements.

In order to use this data structure, you must import java.util.ArrayList at the top of your program.

// Create an ArrayList that stores Strings:
ArrayList<String> months = new ArrayList<String>();

// Add values to the end of the ArrayList:
months.add("January");
months.add("March");

// Add values to a specified index in the ArrayList:
months.add(1, "February");

// Access a value from the ArrayList:
System.out.println(months.get(0)); // Prints: January

// Remove a value from the ArrayList:
months.remove("February");
months.remove(0); // Removes the zeroeth element

LinkedLists

A LinkedList is a built-in data structure that uses a doubly linked list to store its elements.

In order to use this data structure, you must import java.util.LinkedList at the top of your program.

// Create a LinkedList that stores Strings:
LinkedList<String> days = new LinkedList<String>();

// Add values to the end of the LinkedList:
days.add("Monday");
days.add("Tuesday");

// Add values to a specified index in the LinkedList:
days.add(0, "Sunday");

// Access a value from the LinkedList:
System.out.println(days.get(0)); // Prints: Sunday

// Remove a value from the LinkedList:
days.remove("March");
days.remove(1); // Removes the element at index 1
days.remove(); // Removes the first element in the LinkedList

Sets

A Set is a built-in data structure that stores an unordered collection of unique values. A Set can only store reference type values.

There are multiple implementations of a Set: the HashSet, the TreeSet, and the LinkedHashSet. While these classes share the similarities of storing unique objects and having access to the same methods, there are a few differences between the three that impact the order items appear within the structure as well as the runtime of some of the methods.

In order to utilize a Set and its associated operations, import its class at the top of the program.

// Create a HashSet of Strings:
Set<String> shapes = new HashSet<String>();

// Add items to a Set:
shapes.add("square");
shapes.add("triangle");
shapes.add("circle"); 

// Remove a value:
shapes.remove("square");

// Check for a value:
System.out.println(shapes.contains("circle")); // Prints: true

// Finding the size of a Set:
System.out.println(shapes.size()); // Prints: 2

// Iterate through a Set:
for (String item: shapes) {
  System.out.println(item);
}
/* Prints:
triangle
circle
*/

HashMaps

A HashMap is a built-in data structure that stores a collection of key-value pairs. Each key acts as a unique identifier for its associated value.

In order to use this data structure, import the HashMap class at the top of the program.

// Create a HashMap with String keys and Integer values:
HashMap<String, Integer> fruitInventory = new HashMap<>();

// Add key-value pairs to HashMap:
fruitInventory.put("apples", 5);
fruitInventory.put("strawberries", 7);
fruitInventory.put("oranges", 4);

// Remove a key-value pair from HashMap:
fruitInventory.remove("apples");

// Access a value:
System.out.println(fruitInventory.get("oranges")); // Prints: 4

// Find HashMap size:
System.out.println(fruitInventory.size()); // Prints: 2

// Iterate over a HashMap:
for (String key : fruitInventory.keySet()) {
  System.out.println("Key: " + key + ", Value: " + fruitInventory.get(key));
}
/* Prints:
Key: strawberries, Value: 7
Key: oranges, Value: 4
*/