Java List Methods Tutorial - Util List API Eksempel

Lister er almindeligt anvendte datastrukturer på hvert programmeringssprog.

I denne vejledning skal vi undersøge Java's List API. Vi starter med grundlæggende operationer, og så kommer vi ind i mere avancerede ting (som en sammenligning af forskellige listetyper, såsom ArrayList og LinkedList).

Jeg vil også give dig nogle retningslinjer for at hjælpe dig med at vælge den listeimplementering, der passer bedst til din situation.

Selvom grundlæggende Java-viden er nok til at følge vejledningen, kræver det sidste afsnit grundlæggende datastrukturer (Array, LinkedList) og Big-O-viden. Hvis du ikke er fortrolig med dem, er du velkommen til at springe denne sektion over.

Definition af lister

Lister bestilles samlinger af objekter. De ligner sekvenser i matematik i den forstand. De er imidlertid i modsætning til sæt, som ikke har en bestemt rækkefølge.

Et par ting at huske på: lister har lov til at have duplikater og null-elementer. De er reference- eller objekttyper, og som alle objekter i Java er de gemt i bunken.

En liste i Java er en grænseflade, og der er mange listetyper, der implementerer denne grænseflade.

Jeg vil bruge ArrayList i de første par eksempler, fordi det er den mest anvendte type liste.

ArrayList er grundlæggende et resizable array. Næsten altid vil du bruge ArrayList over regelmæssige arrays, da de giver mange nyttige metoder.

En matrixs eneste fordel plejede at være deres faste størrelse (ved ikke at tildele mere plads, end du har brug for). Men lister understøtter også faste størrelser nu.

Sådan oprettes en liste i Java

Nok chatter, lad os starte med at oprette vores liste.

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(); } }

I vinkelparenteserne () specificerer vi den type objekter, vi skal gemme.

Husk, at typen i parentes skal være en objekttype og ikke en primitiv type . Derfor er vi nødt til at bruge objektindpakninger, heltaleklasse i stedet for int, dobbelt i stedet for dobbelt osv.

Der er mange måder at oprette en ArrayList på, men jeg præsenterede tre almindelige måder i uddraget ovenfor.

Den første måde er ved at oprette objektet fra den konkrete ArrayList-klasse ved at specificere ArrayList på venstre side af opgaven.

Det andet kodestykke gør brug af polymorfisme ved hjælp af listen på venstre side. Dette gør opgaven løst kombineret med ArrayList-klassen og giver os mulighed for let at tildele andre typer lister og nemt skifte til en anden listeimplementering.

Den tredje måde er Java 10-metoden til at oprette lokale variabler ved at bruge var-nøgleordet. Compileren fortolker typen af ​​variabel ved at kontrollere højre side.

Vi kan se, at alle opgaver resulterer i samme type:

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

Produktion:

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

Vi kan også specificere den oprindelige kapacitet på listen.

List list = new ArrayList(20);

Dette er nyttigt, fordi når listen bliver fuld, og du prøver at tilføje et andet element, bliver den aktuelle liste kopieret til en ny liste med dobbelt så stor kapacitet som den forrige liste. Alt dette sker bag kulisserne.

Denne operation gør imidlertid vores kompleksitet O (n) , så vi vil undgå det. Standardkapaciteten er 10, så hvis du ved, at du vil gemme flere elementer, skal du angive den oprindelige kapacitet.

Sådan tilføjes og opdateres listeelementer i Java

For at tilføje elementer til listen kan vi bruge tilføjelsesmetoden . Vi kan også specificere indekset for det nye element, men vær forsigtig, når du gør det, da det kan resultere i en 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); } }

Produktion:

[hello, world]

Vi kan bruge den indstillede metode til at opdatere et element.

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

Produktion:

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

Sådan hentes og slettes listeelementer i Java

For at hente et element fra listen kan du bruge get- metoden og angive indekset for det element, du vil hente.

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)); } }

Produktion:

freeCodeCamp 

Kompleksiteten af ​​denne operation på ArrayList er O (1), da den bruger et almindeligt tilfældigt adgangsarray i baggrunden.

Fjern metoden til at fjerne et element fra ArrayList .

list.remove(0);

Dette fjerner elementet ved indeks 0, som er "hej" i dette eksempel.

We can also call the remove method with an element to find and remove it. Keep in mind that it only removes the first occurrence of the element if it is present.

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); }

Output:

[hello, freeCodeCamp]

To remove all occurrences, we can use the removeAll method in the same way.

These methods are inside the List interface, so every List implementations has them (whether it is ArrayList, LinkedList or Vector).

How to Get the Length of a List in Java

To get the length of a list, or the number of elements,we can use the size() method.

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()); } } 

Output:

2

Two-Dimensional Lists in Java

It is possible to create two-dimensional lists, similar to 2D arrays.

ArrayList
    
      listOfLists = new ArrayList();
    

We use this syntax to create a list of lists, and each inner list stores integers. But we have not initialized the inner lists yet. We need to create and put them on this list ourselves:

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.

Vi lavede også en sammenligning af ArrayList, LinkedList og Vector, som er et ofte stillet emne i interviews.

Tak fordi du tog dig tid til at læse hele artiklen, og jeg håber, det var nyttigt.

Du kan få adgang til hele koden fra dette lager.

Hvis du er interesseret i at læse flere artikler som denne, kan du abonnere på min blogs mailingliste for at få besked, når jeg offentliggør en ny artikel.