Tutorial de métodos de lista de Java - Ejemplo de API de lista de utilidades

Las listas son estructuras de datos de uso común en todos los lenguajes de programación.

En este tutorial vamos a investigar la API List de Java. Comenzaremos con operaciones básicas y luego pasaremos a cosas más avanzadas (como una comparación de diferentes tipos de listas, como ArrayList y LinkedList).

También le daré algunas pautas para ayudarlo a elegir la implementación de la lista que sea mejor para su situación.

Aunque el conocimiento básico de Java es suficiente para seguir el tutorial, la última sección requiere estructuras de datos básicas (Array, LinkedList) y conocimiento de Big-O. Si no está familiarizado con ellos, no dude en omitir esa sección.

Definición de listas

Las listas son colecciones ordenadas de objetos. Son similares a las secuencias en matemáticas en ese sentido. Sin embargo, son diferentes a los conjuntos que no tienen un orden determinado.

Un par de cosas a tener en cuenta: las listas pueden tener elementos duplicados y nulos. Son tipos de objetos o de referencia y, como todos los objetos en Java, se almacenan en el montón.

Una lista en Java es una interfaz y hay muchos tipos de listas que implementan esta interfaz.

Usaré ArrayList en los primeros ejemplos, porque es el tipo de lista más utilizado.

ArrayList es básicamente una matriz de tamaño variable. Casi siempre, desea utilizar ArrayList sobre matrices regulares, ya que proporcionan muchos métodos útiles.

La única ventaja de una matriz solía ser su tamaño fijo (al no asignar más espacio del necesario). Pero las listas también admiten tamaños fijos ahora.

Cómo crear una lista en Java

Basta de charlar, comencemos por crear nuestra lista.

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(); } }

En los corchetes angulares () especificamos el tipo de objetos que vamos a almacenar.

Tenga en cuenta que el tipo entre paréntesis debe ser un tipo de objeto y no un tipo primitivo . Por lo tanto, tenemos que usar envoltorios de objetos, clase Integer en lugar de int, Double en lugar de double, y así sucesivamente.

Hay muchas formas de crear una ArrayList, pero presenté tres formas comunes en el fragmento anterior.

La primera forma es creando el objeto a partir de la clase ArrayList concreta especificando ArrayList en el lado izquierdo de la asignación.

El segundo fragmento de código hace uso de polimorfismo al usar la lista en el lado izquierdo. Esto hace que la asignación esté vagamente acoplada con la clase ArrayList y nos permite asignar otros tipos de listas y cambiar fácilmente a una implementación de Lista diferente.

La tercera forma es la forma de Java 10 de crear variables locales haciendo uso de la palabra clave var. El compilador interpreta el tipo de variable comprobando el lado derecho.

Podemos ver que todas las asignaciones dan como resultado el mismo tipo:

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

Salida:

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

También podemos especificar la capacidad inicial de la lista.

List list = new ArrayList(20);

Esto es útil porque siempre que la lista se llena e intenta agregar otro elemento, la lista actual se copia a una nueva lista con el doble de capacidad que la lista anterior. Todo esto sucede detrás de escena.

Sin embargo, esta operación hace que nuestra complejidad sea O (n) , por lo que queremos evitarla. La capacidad predeterminada es 10, por lo que si sabe que almacenará más elementos, debe especificar la capacidad inicial.

Cómo agregar y actualizar elementos de lista en Java

Para agregar elementos a la lista, podemos usar el método agregar . También podemos especificar el índice del nuevo elemento, pero tenga cuidado al hacerlo, ya que puede resultar en una 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); } }

Salida:

[hello, world]

Podemos usar el método set para actualizar un elemento.

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

Salida:

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

Cómo recuperar y eliminar elementos de lista en Java

Para recuperar un elemento de la lista, puede usar el método get y proporcionar el índice del elemento que desea obtener.

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)); } }

Salida:

freeCodeCamp

La complejidad de esta operación en ArrayList es O (1) ya que utiliza una matriz de acceso aleatorio regular en segundo plano.

Para eliminar un elemento de ArrayList, se utiliza el método remove .

list.remove(0);

Esto elimina el elemento en el índice 0, que es "hola" en este ejemplo.

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:

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.

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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