collections.md

Collections

Previously you've seen how to declare variables of a particular type and assign them values:

int x = 1;

What if you want to deal with an arbitrary amount of objects, all of the same type/ You can't write something like this:

int firstNumber = 1;
int secondNumber = 2;
int thirdNumber = 3;
int fourthNumber = 4;

// This is getting out of hand!

Instead, Java provides some data types that represent a whole group of things all at once. Here are two of the most important, and how to use them.

List

In Java, the List stores a sequential list of data of a specified type.

Creating a List

Here's an example of creating a new (empty) list that will hold integer values:

List<Integer> scores = new ArrayList<>();

The type that goes in between the angle brackets (in this case Integer) is the type of object that will go in our list. Since we want a list of integers, we use the Integer type.

Adding items to a List

You can add items to a List using the add method:

List<Integer> luckyNumbers = new ArrayList<>();
luckyNumbers.add(4);
luckyNumbers.add(8);
luckyNumbers.add(15);
luckyNumbers.add(16);
luckyNumbers.add(23);
luckyNumbers.add(42);

// luckyNumbers now holds the data [4, 8, 15, 16, 23, 42]

Getting items out of a List

You can get items out of a list using the get method:

// Filling up the list just as above
List<Integer> luckyNumbers = new ArrayList<>();
luckyNumbers.add(4);
luckyNumbers.add(8);
luckyNumbers.add(15);
luckyNumbers.add(16);
luckyNumbers.add(23);
luckyNumbers.add(42);

// Getting numbers out of the list
int four = luckyNumbers.get(0);
int twentyThree = luckyNumbers.get(4);

Inserting items into the middle of a List

You can insert items into the middle of a List using a variant of the add method that also accepts an index to insert the item at:

List<Integer> numbers = new ArrayList<>();
numbers.add(1);
numbers.add(2);
numbers.add(3);

// numbers is now [1, 2, 3]

// Insert 4 at the beginning (index 0)
numbers.add(0, 4);
// numbers is now [4, 1, 2, 3]

// Insert 5 in the middle (index 2)
numbers.add(2, 5);
// numbers is now [4, 1, 5, 2, 3]

Iterating over a List

You can iterate over every item in a list in two ways.

Firstly using a plain for-loop:

List<String> words = new ArrayList<>();
words.add("Veni");
words.add("Vidi");
words.add("Vici");

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

This code loops over each index from 0 to the size of the list, and for each index, gets that item from the list and prints it out.

You can also iterate over the items of a list directly without needing to use the index:

List<String> words = new ArrayList<>();
words.add("Veni");
words.add("Vidi");
words.add("Vici");

for (String word : words) {
    System.out.println(word);
}

This automatically loops over every String kept in words and assgns it to a variable called word, which is then printed in the body of the loop.

Map

The Map structure in Java acts a bit like a dictionary. Whereas a dictionary contains a set of words and a definition for each word, a Map contains a set of keys and a value for each key.

When you look up a word in a dictionary, you can get the definition for that word. Similarly, when you look up a key in a Map, you can retrieve the value associated with that key.

Creating a Map

You can create a Map using the following code:

Map<String, Integer> agesOfPeople = new HashMap<>();

This creates a Map where the keys are of type String and the values are of type Integer.

Adding values to a Map

You can add values to a Map using the put method. The put method accepts the key you want to save, and the value that should be associated with it. For example, to store the ages of some people:

Map<String, Integer> agesOfPeople = new HashMap<>();
agesOfPeople.put("Sam", 24);
agesOfPeople.put("Tom", 22);

Getting values out of a Map

You can look up the value associated with a particular key using the get method on Map:

Map<String, Integer> agesOfPeople = new HashMap<>();
agesOfPeople.put("Sam", 24);
agesOfPeople.put("Tom", 22);

int samsAge = agesOfPeople.get("Sam");
int tomsAge = agesOfPeople.get("Tom");

Iterating over a Map

You can iterate over the keys of a map like this:

Map<String, Integer> myMap;

// ...

for (String key : myMap.keySet()) {
    System.out.println(key + " : " + myMap.get(key));
}

This code will loop over all the keys of myMap and print out each key along with its associated value.

If you don't care about keys and just want the values, you can do so using a loop like this:

Map<String, Integer> myMap;

// ...

for (Integer value : myMap.values()) {
    System.out.println(value);
}