Java adding items to list

Interface List

An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.

Unlike sets, lists typically allow duplicate elements. More formally, lists typically allow pairs of elements e1 and e2 such that e1.equals(e2) , and they typically allow multiple null elements if they allow null elements at all. It is not inconceivable that someone might wish to implement a list that prohibits duplicates, by throwing runtime exceptions when the user attempts to insert them, but we expect this usage to be rare.

The List interface places additional stipulations, beyond those specified in the Collection interface, on the contracts of the iterator , add , remove , equals , and hashCode methods. Declarations for other inherited methods are also included here for convenience.

The List interface provides four methods for positional (indexed) access to list elements. Lists (like Java arrays) are zero based. Note that these operations may execute in time proportional to the index value for some implementations (the LinkedList class, for example). Thus, iterating over the elements in a list is typically preferable to indexing through it if the caller does not know the implementation.

The List interface provides a special iterator, called a ListIterator , that allows element insertion and replacement, and bidirectional access in addition to the normal operations that the Iterator interface provides. A method is provided to obtain a list iterator that starts at a specified position in the list.

The List interface provides two methods to search for a specified object. From a performance standpoint, these methods should be used with caution. In many implementations they will perform costly linear searches.

The List interface provides two methods to efficiently insert and remove multiple elements at an arbitrary point in the list.

Note: While it is permissible for lists to contain themselves as elements, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a list.

Some list implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException . Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the list may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as «optional» in the specification for this interface.

Unmodifiable Lists

• They are unmodifiable. Elements cannot be added, removed, or replaced. Calling any mutator method on the List will always cause UnsupportedOperationException to be thrown. However, if the contained elements are themselves mutable, this may cause the List’s contents to appear to change.
• They disallow null elements. Attempts to create them with null elements result in NullPointerException .
• They are serializable if all elements are serializable.
• The order of elements in the list is the same as the order of the provided arguments, or of the elements in the provided array.
• The lists and their subList views implement the RandomAccess interface.
• They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
• They are serialized as specified on the Serialized Form page.

This interface is a member of the Java Collections Framework.

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How To Use add() and addAll() Methods for Java List

In this article, you will learn about Java’s List methods add() and addAll() .

Java List add()

This method is used to add elements to the list. There are two methods to add elements to the list.

1. add(E e ) : appends the element at the end of the list. Since List supports Generics , the type of elements that can be added is determined when the list is created.
2. add(int index , E element ) : inserts the element at the given index . The elements from the given index are shifted toward the right of the list. The method throws IndexOutOfBoundsException if the given index is out of range.

Here are some examples of List add() methods:

package com.journaldev.examples; import java.util.ArrayList; import java.util.List; public class ListAddExamples  public static void main(String[] args)  ListString> vowels = new ArrayList>(); vowels.add("A"); // [A] vowels.add("E"); // [A, E] vowels.add("U"); // [A, E, U] System.out.println(vowels); // [A, E, U] vowels.add(2, "I"); // [A, E, I, U] vowels.add(3, "O"); // [A, E, I, O, U] System.out.println(vowels); // [A, E, I, O, U] > > 

First, this code will add A , E , and U to a list and output [A, E, U] .

Next, this code will add I to the index of 2 to produce [A, E, I, U] . Then, it will add O to the index of 3 to produce [A, E, I, O, U] . Finally, this list will be output to the screen.

Java List addAll()

This method is used to add the elements from a collection to the list. There are two overloaded addAll() methods.

1. addAll(Collection c ) : This method appends all the elements from the given collection to the end of the list. The order of insertion depends on the order in which the collection iterator returns them.
2. addAll(int index , Collection c ) : we can use this method to insert elements from a collection at the given index . All the elements in the list are shifted toward the right to make space for the elements from the collection.

Here are some examples of List addAll() methods:

package com.journaldev.examples; import java.util.ArrayList; import java.util.Arrays; import java.util.List; public class ListAddAllExamples  public static void main(String[] args)  ListInteger> primeNumbers = new ArrayList>(); primeNumbers.addAll(Arrays.asList(2, 7, 11)); System.out.println(primeNumbers); // [2, 7, 11] primeNumbers.addAll(1, Arrays.asList(3, 5)); System.out.println(primeNumbers); // [2, 3, 5, 7, 11] > > 

First, this code creates a new list with the value of [2, 7, 11] . Then, this list is output to the screen.

Next, a second list is created with the value of [3, 5] . Then, this second list is added to the first list with addAll() with an index of 1 . This results in a list of [2, 3, 5, 7, 11] that is output to the screen.

UnsupportedOperationException with List add() Methods

The List add() and addAll() method documentation states that the operation is mentioned as optional. It means that the list implementation may not support it. In this case, the list add() methods throw UnsupportedOperationException . There are two common scenarios where you will find this exception when adding elements to the list:

1. Arrays.asList() : This method returns a fixed-size list because it’s backed by the specified array. Any operation where the list size is changed throws UnsupportedOperationException .
2. Collections.unmodifiableList(List l ) : This method returns a unmodifiable view of the given list. So the add() operations throw UnsupportedOperationException .

Here is an example of UnsupportedOperationException with add operation on a fixed-size list:

jshell> List ints = Arrays.asList(1,2,3); ints ==> [1, 2, 3] jshell> ints.add(4); | Exception java.lang.UnsupportedOperationException | at AbstractList.add (AbstractList.java:153) | at AbstractList.add (AbstractList.java:111) | at (#4:1) 

First, this code creates a fixed-size list of [1, 2, 3] . Then, this code attempts to add 4 to the list. This results in throwing a UnsupportedOperationException .

Here is an example of UnsupportedOperationException with add operation on an unmodifiable view of the given list:

jshell> List strs = new ArrayList<>(); strs ==> [] jshell> strs.add("A"); $6 ==> true jshell> List strs1 = Collections.unmodifiableList(strs); strs1 ==> [A] jshell> strs1.add("B"); | Exception java.lang.UnsupportedOperationException | at Collections$UnmodifiableCollection.add (Collections.java:1058) | at (#8:1) 

First, this code adds A to a list. Next, this code attempts to add B to an unmodifiable view of the previous list. This results in throwing a UnsupportedOperationException .

Conclusion

In this article, you learned about Java’s List methods add() and addAll() .

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ArrayList в Java

ArrayList — реализация изменяемого массива интерфейса List, часть Collection Framework, который отвечает за список (или динамический массив), расположенный в пакете java.utils. Этот класс реализует все необязательные операции со списком и предоставляет методы управления размером массива, который используется для хранения списка. В основе ArrayList лежит идея динамического массива. А именно, возможность добавлять и удалять элементы, при этом будет увеличиваться или уменьшаться по мере необходимости.

Что хранит ArrayList?

Только ссылочные типы, любые объекты, включая сторонние классы. Строки, потоки вывода, другие коллекции. Для хранения примитивных типов данных используются классы-обертки.

Конструкторы ArrayList

 ArrayList list = new ArrayList<>(); 

 ArrayList list2 = new ArrayList<>(list); 

 ArrayList list2 = new ArrayList<>(10000); 

Методы ArrayList

Ниже представлены основные методы ArrayList.

 ArrayList list = new ArrayList<>(); list.add("Hello"); 

 ArrayList secondList = new ArrayList<>(); secondList.addAll(list); System.out.println("Первое добавление: " + secondList); secondList.addAll(1, list); System.out.println("Второе добавление в середину: " + secondList); 

 Первое добавление: [Amigo, Hello] Второе добавление в середину: [Amigo, Amigo, Hello, Hello] 

 ArrayList copyOfSecondList = (ArrayList) secondList.clone(); secondList.clear(); System.out.println(copyOfSecondList); 

 System.out.println(copyOfSecondList.contains("Hello")); System.out.println(copyOfSecondList.contains("Check")); 

 // Первый способ for(int i = 0; i < secondList.size(); i++) < System.out.println(secondList.get(i)); >И цикл for-each: // Второй способ for(String s : secondList)

В классе ArrayList есть метод для обработки каждого элемента, который называется также, forEach. В качестве аргумента передается реализация интерфейса Consumer, в котором нужно переопределить метод accept():

 secondList.forEach(new Consumer() < @Override public void accept(String s) < System.out.println(s); >>); 

Метод accept принимает в качестве аргумента очередной элемент того типа, который хранит в себе ArrayList. Пример для Integer:

 ArrayList integerList = new ArrayList<>(); integerList.forEach(new Consumer() < @Override public void accept(Integer integer) < System.out.println(integer); >>); 

 String[] array = new String[secondList.size()]; secondList.toArray(array); for(int i = 0; i

Ссылки на дополнительное чтение

1. Подробная статья о динамических массивах, а точнее — об ArrayList и LinkedList , которые выполняют их роль в языке Java.
2. Статья об удалении элементов из списка ArrayList.
3. Лекция о работе с ArrayList в схемах и картинках.

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