Java ArrayList size() Method
The Java ArrayList size() method returns the number of elements in this list i.e the size of the list. It is updated everytime a change is made to the ArrayList.
Declaration
Following is the declaration for java.util.ArrayList.size() method
Parameters
Return Value
This method returns the number of elements in this list.
Exception
Example 1
The following example shows the usage of Java ArrayList size() method. We’re adding couple of Integers to the ArrayList object using add() method calls per element. Size of the arraylist is printed using size() method. And using remove(index) method, we’re removing one element and size of the arraylist is again printed.
package com.tutorialspoint; import java.util.ArrayList; public class ArrayListDemo < public static void main(String[] args) < // create an empty array list ArrayListarrayList = new ArrayList<>(); // use add() method to add elements in the arrayList arrayList.add(20); arrayList.add(30); arrayList.add(20); arrayList.add(30); arrayList.add(15); arrayList.add(22); arrayList.add(11); // let us print the size of the arrayList again System.out.println("Arraylist Size = " + arrayList.size()); // remove an element at index 2 arrayList.remove(2); // let us print the size of the arrayList again System.out.println("Arraylist Size result notranslate"> Arraylist Size = 7 Arraylist Size = 6 Example 2
The following example shows the usage of Java ArrayList size() method. We’re adding couple of Strings to the ArrayList object using add() method calls per element. Size of the arraylist is printed using size() method. And using remove(index) method, we’re removing one element and size of the arraylist is again printed.
package com.tutorialspoint; import java.util.ArrayList; public class ArrayListDemo < public static void main(String[] args) < // create an empty array list ArrayListarrayList = new ArrayList<>(); // use add() method to add elements in the arrayList arrayList.add("Welcome"); arrayList.add("To"); arrayList.add("Tutorialspoint"); // let us print the size of the arrayList again System.out.println("Arraylist Size = " + arrayList.size()); // remove an element at index 2 arrayList.remove(2); // let us print the size of the arrayList again System.out.println("Arraylist Size result notranslate"> Arraylist Size = 3 Arraylist Size = 2 Example 3
The following example shows the usage of Java ArrayList size() method. We’re adding couple of Student objects to the ArrayList object using add() method calls per element. Size of the arraylist is printed using size() method. And using remove(index) method, we’re removing one element and size of the arraylist is again printed.
package com.tutorialspoint; import java.util.ArrayList; public class ArrayListDemo < public static void main(String[] args) < // create an empty arrayList ArrayListarrayList = new ArrayList<>(); // use add() method to add elements in the arrayList arrayList.add(new Student(1, "Julie")); arrayList.add(new Student(2, "Robert")); arrayList.add(new Student(3, "Adam")); // let us print the size of the arrayList again System.out.println("Arraylist Size = " + arrayList.size()); // remove an element at index 2 arrayList.remove(2); // let us print the size of the arrayList again System.out.println("Arraylist Size = " + arrayList.size()); > > class Student < int rollNo; String name; Student(int rollNo, String name)< this.rollNo = rollNo; this.name = name; >@Override public String toString() < return "[ " + this.rollNo + ", " + this.name + " ]"; >> Output
Let us compile and run the above program, this will produce the following result −
Arraylist Size = 3 Arraylist Size = 2
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.
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.