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.
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.
Length of ArrayList in Java
How do I find the size of an ArrayList in Java? I do not mean the number of elements, but the number of indexes.
public static void main(String[] args)
System.out.println(hash.toArray().length); Also prints out a «0.» I have looked in http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html but I do not see a method that will help me. Is my ArrayList reverting to a size of 0 if I do not add anything to it? EDIT The assignment is to create a hash table using ArrayList. I am supposed to create a hash function using the formula
double hashkey = Math.floor(hash.size()*(Math.E*key-Math.floor(Math.E*key))); Where key is an integer. hashkey then becomes the index where the value will be stored. I am using hash.size() as a placeholder at the moment, but that value should be the capacity of my ArrayList.
there is a difference between initial capacity and size. capacity defines initial memory allocation to array but don’t have any object.
What is the need for the information? Is it just for the sake of education, or is to solve a specific programming problem? If the latter, then smells of an XY problem.
The implementation hides the size of the underlying array. Also, a list is most definitely not a hash.
There’s no such method. Why would you need it? Note: You could get it by using reflection in an ArrayList , but seems for curiosity purposes only, so there’s no good to solve it.
It’s also a bit confusing to call an ArrayList hash . That’s a recipe for disaster. You may as well call your HashMap array .
3 Answers 3
ArrayList.size() will give the current size.That’s why hash.size() giving you the current size of your ArrayList hash. It will not give you the capacity.
You just initialized the list. Have not add any elements to your arraylist, that’s why its giving 0.
ensureCapacity() increases the capacity to at least the number passed to it; it could theoretically make it more than that. I don’t think a maxmimum is specified.
@iamnotmaynard You are right. I added the same because we can use the ensureCapacity() method to check that the internal data structure has enough capacity before adding elements.Although I removed the same.
There is no such method in the ArrayList API. The capacity of an ArrayList is hidden by design.
However, I think that your question is based on a misunderstanding.
How do I find the size of an ArrayList in Java? I do not mean the number of elements, but the number of indexes.
In fact, the size of a List , the number of elements in a List , and the number of indexes (i.e. indexable positions) for a List . are all the same thing.
The capacity of an ArrayList is something different. It is the number of elements that the object could contain, without reallocating the list’s backing array. However, the fact that the list has a capacity N does NOT mean that you can index up to N — 1 . In fact, you can only index up to size() — 1 , irrespective of the capacity.
Now to deal with your examples:
ArrayList list = new ArrayList(5); System.out.println(list.size()); This prints out zero because the list has zero elements. The ArrayList() and ArrayList(int) constructors both create and return lists that are empty. The list currently has space for 5 elements (because you gave it an initial capacity of 5) but you can’t index those slots.
System.out.println(list.toArray().length); This prints zero because when you copy the list’s contents to an array (using toArray() ), the array is the same size as the list. By definition.
This does not mean that the list’s backing array has changed. On the contrary, it is still big enough to hold 5 elements without reallocation . just like before.
But . I hear you say . the array’s length is zero!
Yes, but that is not the backing array! The toArray() method allocates a new array and copies the List contents into that array. It does NOT return the actual backing array.
How to get length of an ArrayList in Java
In this tutorial, we are going to learn about how to get the length/size of a given ArrayList in Java.
Consider, we have the following ArrayList:
ArrayListString> users = new ArrayList>(Arrays.asList("john", "kevin", "rose"));Now, we need to find out the length of an above users arraylist.
To get the length of an ArrayList, we can use the built-in size() method of java.util.ArrayList class.
The size() method returns the total number of elements in a arraylist in integer format.
Here is an example, that gets the length of a users list:
import java.util.ArrayList; import java.util.Arrays; public class Main public static void main(String[] args) ArrayListString> users = new ArrayList>(Arrays.asList("john", "kevin", "rose")); int length = users.size(); System.out.println(length); > >