Java init empty list

How to Initialize an ArrayList in Java

The Java ArrayList can be initialized in a number of ways depending on the requirement. In this tutorial, we will learn to initialize ArrayList based on some frequently seen usecases.

1. Initialize ArrayList in One Line

Creating and initializing the ArrayList in different statements sometimes seems to generate unnecessary boilerplate code. We can optimize the ArrayList creation using the following ways.

1.1. Using Arrays.asList() – Initialize ArrayList from Array

To initialize an ArrayList in a single line statement, get all elements in form of array using Arrays.asList method and pass the array argument to ArrayList constructor.

ArrayList names = new ArrayList( Arrays.asList("alex", "brian", "charles") );

1.2. Unmodifiable Lists using List.of() – Java 9

We can use List.of() static factory methods to create unmodifiable lists. The only drawback is that add() operation is not supported in these lists.

List names = List.of("alex", "brian", "charles");

2. Using ArrayList Constructor

Using ArrayList constructor is the traditional approach. The new ArrayList() constructor takes an optional parameter initialCapacity. It must be a positive integer and denotes the initial capacity of the list.

new ArrayList(initialCapacity);

We create a blank ArrayList using the constructor and add elements to the list using add() method.

ArrayList names = new ArrayList<>(); names.add("alex"); //Adding a single element at a time names.add("brian"); names.add("charles");

We can add elements one by one or pass another collection to addAll() elements in one step.

HashMap details = new HashMap<>(); details.put("keanu", 23); details.put("max", 24); details.put("john", 53); names.addAll(details.keySet()); //Adding multiple elements in ArrayList 

That’s all about creating an ArrayList in Java. Drop me your questions in the comments.

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Class ArrayList

Type Parameters: E — the type of elements in this list All Implemented Interfaces: Serializable , Cloneable , Iterable , Collection , List , RandomAccess Direct Known Subclasses: AttributeList , RoleList , RoleUnresolvedList

Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null . In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. (This class is roughly equivalent to Vector , except that it is unsynchronized.)

The size , isEmpty , get , set , iterator , and listIterator operations run in constant time. The add operation runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.

Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.

An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.

Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be «wrapped» using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

List list = Collections.synchronizedList(new ArrayList(. ));

The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

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Java init empty list

Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null. In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. (This class is roughly equivalent to Vector, except that it is unsynchronized.) The size, isEmpty, get, set, iterator, and listIterator operations run in constant time. The add operation runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation. Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost. An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation. Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be «wrapped» using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

List list = Collections.synchronizedList(new ArrayList(. ));

The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs. This class is a member of the Java Collections Framework.

Field Summary

Fields inherited from class java.util.AbstractList

Constructor Summary

Constructs a list containing the elements of the specified collection, in the order they are returned by the collection’s iterator.

Method Summary

Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection’s Iterator.

Inserts all of the elements in the specified collection into this list, starting at the specified position.

Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.

Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.

Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.

Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element.

Returns a list iterator over the elements in this list (in proper sequence), starting at the specified position in the list.

Removes from this list all of the elements whose index is between fromIndex , inclusive, and toIndex , exclusive.

Returns a view of the portion of this list between the specified fromIndex , inclusive, and toIndex , exclusive.

Returns an array containing all of the elements in this list in proper sequence (from first to last element).

Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.

Methods inherited from class java.util.AbstractList

Methods inherited from class java.util.AbstractCollection

Methods inherited from class java.lang.Object

Methods inherited from interface java.util.List

Methods inherited from interface java.util.Collection

Constructor Detail

ArrayList

public ArrayList(int initialCapacity)

ArrayList

ArrayList

Constructs a list containing the elements of the specified collection, in the order they are returned by the collection’s iterator.

Method Detail

trimToSize

Trims the capacity of this ArrayList instance to be the list’s current size. An application can use this operation to minimize the storage of an ArrayList instance.

ensureCapacity

public void ensureCapacity(int minCapacity)

Increases the capacity of this ArrayList instance, if necessary, to ensure that it can hold at least the number of elements specified by the minimum capacity argument.

size

isEmpty

contains

Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

indexOf

Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

lastIndexOf

Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the highest index i such that (o==null ? get(i)==null : o.equals(get(i))), or -1 if there is no such index.

clone

toArray

Returns an array containing all of the elements in this list in proper sequence (from first to last element). The returned array will be «safe» in that no references to it are maintained by this list. (In other words, this method must allocate a new array). The caller is thus free to modify the returned array. This method acts as bridge between array-based and collection-based APIs.

toArray

Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the list fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this list. If the list fits in the specified array with room to spare (i.e., the array has more elements than the list), the element in the array immediately following the end of the collection is set to null. (This is useful in determining the length of the list only if the caller knows that the list does not contain any null elements.)

get

set

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