Skip to content

Latest commit

 

History

History
235 lines (196 loc) · 6.77 KB

chapter09-list_and_map.md

File metadata and controls

235 lines (196 loc) · 6.77 KB
Raw

List and Map

In Java, the most used data structures are List (an indexed list) and Map (a dictionary)

List

To create a simple list

var numbers = List.of(1, 2, 3);
System.out.println(numbers);

a list is an indexed data structure that stores object in the order of insertions get() access to an element given an index

var firstElement = numbers.get(0);
var lastElement = numbers.get(numbers.size() - 1);

contains return true if a value is contained in the list

System.out.println(numbers.contains(4));

indexOf returns the first index of the element in the list

System.out.println(numbers.indexOf(2));

a list also defines the method equals()/hashCode() and toString(), so you can print a list or test if two list are equals

var friends = List.of("cesar", "rosalie", "david");
System.out.println(friends);
System.out.println(friends.hashCode());
System.out.println(friends.equals(numbers));

Unmodifiable/modifiable list

in Java, depending on how you create a data structure it can be changed after creation or not. Implementation that allow mutation after creation are called modifiable

by example, the list above (created with the static method of()) is not modifiable

var countries = List.of("UK", "US", "France");
countries.set(0, "Poland"); // throws an UnsupportedOperationException

To create a modifiable list, we use an ArrayList, created using the operator 'new' and because because there is no element in the list, the compiler has no way to know the type of the elements so we have to provide it in between angle brackets ('<' and '>')

var modifiableCountries = new ArrayList<String>();
System.out.println(modifiableCountries);

To add elements in a list, we have the method add()

modifiableCountries.add("UK");
modifiableCountries.add("US");
modifiableCountries.add("France");
modifiableCountries.add("Poland");
System.out.println(modifiableCountries);

to remove an element, we have the method remove()

modifiableCountries.remove("UK");
System.out.println(modifiableCountries);

Iterating

an unmodifiable list or a modifiable list have the same set of methods. to loop over the elements of a list, we have a special syntax using the keyword 'for'

var countries = List.of("UK", "US", "France");
for(var country: countries) {
  System.out.println(country);
}

you can also loop over the elements using a method forEach if you don't understand this one, don't panic, we will see it later

countries.forEach(country -> System.out.println(country));

Conversions

To can create an unmodifiable list from a modifiable one with List.copyOf()

var unmodifiableList = List.copyOf(modifiableCountries);
System.out.println(unmodifiableList);

To create a modifiable list from an unmodifiable one using new ArrayList(List) In that case you don't have to specify the type of the elements the compiler already knows the type of list hence the <> (diamond)

var modifiableList = new ArrayList<>(unmodifiableList);
System.out.println(modifiableList);

Useful patterns

To remove some elements depending on a predicate (if something is true)

var elements = new ArrayList<>(List.of("table", "chair", "stool"));
elements.removeIf(element -> element.charAt(0) == 'c');
System.out.println(elements);

Map

A Map associate a value to a key To create a simple Map

var petCost = Map.of("cat", 200, "dog", 350, "lion", 5000);
System.out.println(petCost);

to get the value from a key

var costOfADog = petCost.get("dog");
System.out.println(costOfADog);

Warning! warning! asking for a key which is not in the map will return null

var costOfAGirafe = petCost.get("girafe");
System.out.println(costOfAGirafe);

to avoid null, use the method getOrDefault() that let you specify a default value

var costOfAGirafe = petCost.getOrDefault("girafe", 0);
System.out.println(costOfAGirafe);

And like a list, a map defines the method equals()/hashCode() and toString()

var lighter = Map.of("blue", "lightblue", "gray", "white");
var darker = Map.of("blue", "darkblue", "gray", "black");
System.out.println(lighter);
System.out.println(darker.hashCode());
System.out.println(lighter.equals(darker));

Unmodifiable/modifiable

The Map create by Map.of() are non modifiable To create a modifiable map, we use new HashMap to create the map and map.put to put key/value in it

var modifiableMap = new HashMap<String, String>();
modifiableMap.put("blue", "lightblue");
modifiableMap.put("gray", "white");
System.out.println(modifiableMap);

Removing the key, remove the couple key/value

modifiableMap.remove("blue");
System.out.println(modifiableMap);

Iterating on keySet() or entrySet()

you can not use a for loop directly on a Map but you can do it on the set of keys

var petCost = Map.of("cat", 200, "dog", 350, "lion", 5000);
for(var pet: petCost.keySet()) {
  System.out.println(pet);
}

or on the set of couples key/value (a Map.Entry)

for(var entry: petCost.entrySet()) {
  var pet = entry.getKey();
  var cost = entry.getValue();
  System.out.println(pet + " " + cost);
}

You can also loop over the entries using the method forEach()

petCost.forEach((pet, cost) -> {
  System.out.println(pet + " " + cost);
});

Conversions

To create a unmodifiableMap from a modifiable map, use Map.copyOf()

var modifiableMap = new HashMap<String, String>();
modifiableMap.put("jack sparrow", "pirate");
modifiableMap.put("hector barbossa", "pirate");
var unmodifiableMap = Map.copyOf(modifiableMap);
System.out.println(unmodifiableMap);

To create a modifiableMap from an unmodifiable map, use new HashMap<>()

var unmodifiableMap = Map.of("jack sparrow", "pirate", "hector barbossa", "pirate");
var modifiableMap = new HashMap<>(unmodifiableMap);
System.out.println(modifiableMap);

Useful patterns

To make the Map acts as a cache, use computeIfAbsent()

record Person(String name, int age) { }
var persons = List.of(new Person("Bob", 23), new Person("Anna", 32), new Person("Bob", 12));
var group = new HashMap<String, List<Person>>();
persons.forEach(person -> group.computeIfAbsent(person.name(), name -> new ArrayList<>())
                                .add(person));
System.out.println(group);

to count the number of occurrence, use merge() that takes a key, a value and the function to call if there is already an existing value to combine them

var letters = List.of("a", "b", "e", "b");
var occurrenceMap = new HashMap<String, Integer>();
letters.forEach(letter -> occurrenceMap.merge(letter, 1, Integer::sum));
System.out.println(occurrenceMap);

More on transformations

To do more transformations of lists and maps, use the Stream API.