Vodič za metode Java liste - Primjer API-ja za popis komunalnih usluga

Popisi su uobičajene strukture podataka u svakom programskom jeziku.

U ovom uputstvu istražit ćemo Java-ov API popisa. Krenut ćemo s osnovnim operacijama, a zatim ćemo ući u naprednije stvari (poput usporedbe različitih vrsta popisa, poput ArrayList i LinkedList).

Također ću vam dati neke smjernice koje će vam pomoći da odaberete primjenu popisa koja je najbolja za vašu situaciju.

Iako je osnovno Java znanje dovoljno za praćenje vodiča, posljednji odjeljak zahtijeva osnovne podatkovne strukture (Array, LinkedList) i Big-O znanje. Ako vam nisu poznati, slobodno preskočite taj odjeljak.

Definicija popisa

Popisi su poredane zbirke predmeta. U tom su smislu slični sljedovima iz matematike. Međutim, razlikuju se od setova koji nemaju određeni redoslijed.

Nekoliko stvari koje morate imati na umu: popisi smiju imati duplikate i null elemente. Oni su referentni ili tipovi objekata, i kao i svi objekti u Javi, pohranjeni su u hrpu.

Popis u Javi je sučelje i postoji mnogo vrsta popisa koji implementiraju ovo sučelje.

U prvih nekoliko primjera koristit ću ArrayList jer je to najčešće korištena vrsta popisa.

ArrayList je u osnovi promjenjivi niz. Gotovo uvijek želite koristiti ArrayList preko redovnih nizova jer oni nude mnoge korisne metode.

Jedina prednost niza nekad je bila njihova fiksna veličina (ne dodijelivši više prostora nego što vam treba). Ali popisi sada podržavaju i fiksne veličine.

Kako stvoriti popis na Javi

Dosta je bilo čavrljanja, krenimo od stvaranja našeg popisa.

import java.util.ArrayList; import java.util.List; public class CreateArrayList { public static void main(String[] args) { ArrayList list0 = new ArrayList(); // Makes use of polymorphism List list = new ArrayList(); // Local variable with "var" keyword, Java 10 var list2 = new ArrayList(); } }

U kutnim zagradama () odredimo vrstu predmeta koje ćemo pohraniti.

Imajte na umu da tip u zagradama mora biti vrsta objekta, a ne primitivni tip . Stoga moramo koristiti omote predmeta, klasu Integer umjesto int, Double umjesto double i tako dalje.

Postoji mnogo načina za stvaranje ArrayList-a, ali u gornjem sam isječku predstavio tri uobičajena načina.

Prvi je način stvaranjem objekta iz konkretne klase ArrayList specificiranjem ArrayList na lijevoj strani dodjele.

Drugi isječak koda koristi polimorfizam pomoću popisa s lijeve strane. To dodjelu dodjeljuje labavo povezanu s razredom ArrayList i omogućuje nam dodjeljivanje drugih vrsta popisa i lako prebacivanje na drugu implementaciju Popisa.

Treći način je Java 10 način stvaranja lokalnih varijabli korištenjem ključne riječi var. Kompajler interpretira vrstu varijable provjeravanjem desne strane.

Vidimo da svi zadaci rezultiraju istim tipom:

System.out.println(list0.getClass()); System.out.println(list.getClass()); System.out.println(list2.getClass());

Izlaz:

class java.util.ArrayList class java.util.ArrayList class java.util.ArrayList 

Također možemo odrediti početni kapacitet popisa.

List list = new ArrayList(20);

To je korisno jer kad god se popis popuni i pokušate dodati još jedan element, trenutni se popis kopira na novi popis dvostrukog kapaciteta prethodnog popisa. Sve se to događa iza kulisa.

Ova operacija , međutim, čini našu složenost O (n) , pa je želimo izbjeći. Zadani kapacitet je 10, pa ako znate da ćete pohraniti više elemenata, trebali biste odrediti početni kapacitet.

Kako dodati i ažurirati elemente popisa na Javi

Za dodavanje elemenata na popis možemo koristiti metodu add . Također možemo odrediti indeks novog elementa, ali budite oprezni kada to radite jer to može rezultirati IndexOutOfBoundsException .

import java.util.ArrayList; public class AddElement { public static void main(String[] args) { ArrayList list = new ArrayList(); list.add("hello"); list.add(1, "world"); System.out.println(list); } }

Izlaz:

[hello, world]

Metodu skupa možemo koristiti za ažuriranje elementa.

list.set(1, "from the otherside"); System.out.println(list);

Izlaz:

[hello, world] [hello, from the otherside]

Kako dohvatiti i izbrisati elemente popisa na Javi

Da biste element preuzeli s popisa, možete koristiti metodu get i pružiti indeks elementa koji želite dobiti.

import java.util.ArrayList; import java.util.List; public class GetElement { public static void main(String[] args) { List list = new ArrayList(); list.add("hello"); list.add("freeCodeCamp"); System.out.println(list.get(1)); } }

Izlaz:

freeCodeCamp 

Složenost ove operacije na ArrayListu je O (1) jer koristi redoviti niz slučajnog pristupa u pozadini.

Za uklanjanje elementa s ArrayList koristi se metoda uklanjanja .

list.remove(0);

Ovo uklanja element s indeksom 0, što je "zdravo" u ovom primjeru.

Također možemo pozvati metodu uklanjanja s elementom kako bismo ga pronašli i uklonili. Imajte na umu da uklanja prvu pojavu elementa samo ako je prisutan.

public static void main(String[] args) { List list = new ArrayList(); list.add("hello"); list.add("freeCodeCamp"); list.add("freeCodeCamp"); list.remove("freeCodeCamp"); System.out.println(list); }

Izlaz:

[hello, freeCodeCamp]

Da bismo uklonili sve pojave, na isti način možemo koristiti metodu removeAll .

Te su metode unutar sučelja Popisa, pa ih ima svaka implementacija Popisa (bilo da se radi o ArrayList, LinkedList ili Vector).

Kako dobiti dužinu popisa na Javi

Da biste dobili duljinu popisa ili broj elemenata,možemo koristiti metodu size () .

import java.util.ArrayList; import java.util.List; public class GetSize { public static void main(String[] args) { List list = new ArrayList(); list.add("Welcome"); list.add("to my post"); System.out.println(list.size()); } } 

Izlaz:

2

Dvodimenzionalni popisi na Javi

Moguće je stvoriti dvodimenzionalne popise, slične 2D nizovima.

ArrayList
    
      listOfLists = new ArrayList();
    

Ovu sintaksu koristimo za stvaranje popisa popisa, a svaki unutarnji popis pohranjuje cijele brojeve. Ali još nismo inicijalizirali unutarnje popise. Moramo ih sami stvoriti i staviti na ovaj popis:

int numberOfLists = 3; for (int i = 0; i < numberOfLists; i++) { listOfLists.add(new ArrayList()); }

I am initializing my inner lists, and I am adding 3 lists in this case. I can also add lists later if I need to.

Now we can add elements to our inner lists. To add an element, we need to get the reference to the inner list first.

For example, let's say we want to add an element to the first list. We need to get the first list, then add to it.

listOfLists.get(0).add(1);

Here is an example for you. Try to guess the output of the below code segment:

public static void main(String[] args) { ArrayList
    
      listOfLists = new ArrayList(); System.out.println(listOfLists); int numberOfLists = 3; for (int i = 0; i < numberOfLists; i++) { listOfLists.add(new ArrayList()); } System.out.println(listOfLists); listOfLists.get(0).add(1); listOfLists.get(1).add(2); listOfLists.get(2).add(0,3); System.out.println(listOfLists); }
    

Output:

[] [[], [], []] [[1], [2], [3]]

Notice that it is possible to print the lists directly (unlike with regular arrays) because they override the toString() method.

Useful Methods in Java

There are some other useful methods and shortcuts that are used frequently. In this section I want to familiarize you with some of them so you will have an easier time working with lists.

How to Create a List with Elements in Java

It is possible to create and populate the list with some elements in a single line. There are two ways to do this.

The following is the old school way:

public static void main(String[] args) { List list = Arrays.asList( "freeCodeCamp", "let's", "create"); }

You need to be cautious about one thing when using this method: Arrays.asList returns an immutable list. So if you try to add or remove elements after creating the object, you will get an UnsupportedOperationException.

You might be tempted to use final keyword to make the list immutable but it won't work as expected.

It just makes sure that the reference to the object does not change – it does not care about what is happening inside the object. So it permits inserting and removing.

final List list2 = new ArrayList(); list2.add("erinc.io is the best blog ever!"); System.out.println(list2);

Output:

[erinc.io is the best blog ever!] 

Now let's look at the modern way of doing it:

ArrayList friends = new ArrayList(List.of("Gulbike", "Sinem", "Mete")); 

The List.of method was shipped with Java 9. This method also returns an immutable list but we can pass it to the  ArrayList constructor to create a mutable list with those elements. We can add and remove elements to this list without any problems.

How to Create a List with N Copies of Some Element in Java

Java provides a method called NCopies that is especially useful for benchmarking. You can fill an array with any number of elements in a single line.

public class NCopies { public static void main(String[] args) { List list = Collections.nCopies(10, "HELLO"); System.out.println(list); } }

Output:

[HELLO, HELLO, HELLO, HELLO, HELLO, HELLO, HELLO, HELLO, HELLO, HELLO] 

How to Clone a List in Java

As previously mentioned, Lists are reference types, so the rules of passing by reference apply to them.

public static void main(String[] args) { List list1 = new ArrayList(); list1.add("Hello"); List list2 = list1; list2.add(" World"); System.out.println(list1); System.out.println(list2); }

Output:

[Hello, World] [Hello, World]

The list1 variable holds a reference to the list. When we assign it to list2 it also points to the same object. If we do not want the original list to change, we can clone the list.

ArrayList list3 = (ArrayList) list1.clone(); list3.add(" Of Java"); System.out.println(list1); System.out.println(list3);

Output:

[Hello, World] [Hello, World, Of Java]

Since we cloned list1, list3 holds a reference to its clone in this case. Therefore list1 remains unchanged.

How to Copy a List to an Array in Java

Sometimes you need to convert your list to an array to pass it into a method that accepts an array. You can use the following code to achieve that:

List list = new ArrayList(List.of(1, 2)); Integer[] toArray = list.toArray(new Integer[0]);

You need to pass an array and the toArray method returns that array after filling it with the elements of the list.

How to Sort a List in Java

To sort a list we can use Collections.sort. It sorts in ascending order by default but you can also pass a comparator to sort with custom logic.

List toBeSorted = new ArrayList(List.of(3,2,4,1,-2)); Collections.sort(toBeSorted); System.out.println(toBeSorted);

Output:

[-2, 1, 2, 3, 4]

How do I choose which list type to use?

Before finishing this article, I want to give you a brief performance comparison of different list implementations so you can choose which one is better for your use case.

We will compare ArrayList, LinkedList and Vector. All of them have their ups and downs so make sure you consider the specific context before you decide.

Java ArrayList vs LinkedList

Here is a comparison of runtimes in terms of algorithmic complexity.

| | ArrayList | LinkedList | |-----------------------|----------------------------|------------| | GET(index) | O(1) | O(n) | | GET from Start or End | O(1) | O(1) | | ADD | O(1), if list is full O(n) | O(1) | | ADD(index) | O(n) | O(1) | | Remove(index) | O(n) | O(1) | | Search and Remove | O(n) | O(n) |

Generally, the get operation is much faster on ArrayList but add and remove are faster on LinkedList.

ArrayList uses an array behind the scenes, and whenever an element is removed, array elements need to be shifted (which is an O(n) operation).

Choosing data structures is a complex task and there is no recipe that applies to every situation. Still, I will try to provide some guidelines to help you make that decision easier:

  • If you plan to do more get and add operations other than remove, use ArrayList since the get operation is too costly on LinkedList. Keep in mind that insertion is O(1) only if you call it without specifying the index and add to the end of the list.
  • If you are going to remove elements and/or insert in the middle (not at the end) frequently, you can consider switching to a LinkedList because these operations are costly on ArrayList.
  • Keep in mind that if you access the elements sequentially (with an iterator), you will not experience a performance loss with LinkedList while getting elements.

Java ArrayList vs Vector

Vector is very similar to ArrayList. If you are coming from a C++ background, you might be tempted to use a Vector, but its use case is a bit different than C++.

Vector's methods have the synchronized keyword, so Vector guarantees thread safety whereas ArrayList does not.

You might prefer Vector over ArrayList in multithreaded programming or you can use ArrayList and handle the synchronization yourself.

In a single-threaded program, it is better to stick with ArrayList because thread-safety comes with a performance cost.

Conclusion

In this post, I have tried to provide an overview of Java's List API. We have learned to use basic methods, and we've also looked at some more advanced tricks to make our lives easier.

We also made a comparison of ArrayList, LinkedList and Vector which is a commonly asked topic in interviews.

Thank you for taking the time to read the whole article and I hope it was helpful.

You can access the whole code from this repository.

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