How to get first and last elements from ArrayList in Java?
The get() method of the ArrayList class accepts an integer representing the index value and, returns the element of the current ArrayList object at the specified index.
Therefore, if you pass 0 to this method you can get the first element of the current ArrayList and, if you pass list.size()-1 you can get the last element.
Example
import java.util.ArrayList; public class FirstandLastElemets < public static void main(String[] args)< ArrayListlist = new ArrayList(); //Instantiating an ArrayList object list.add("JavaFX"); list.add("Java"); list.add("WebGL"); list.add("OpenCV"); list.add("OpenNLP"); list.add("JOGL"); list.add("Hadoop"); list.add("HBase"); list.add("Flume"); list.add("Mahout"); list.add("Impala"); System.out.println("Contents of the Array List: \n"+list); //Removing the sub list System.out.println("First element of the array list: "+list.get(0)); System.out.println("Last element of the array list: "+list.get(list.size()-1)); > > Output
Contents of the Array List: [JavaFX, Java, WebGL, OpenCV, OpenNLP, JOGL, Hadoop, HBase, Flume, Mahout, Impala] First element of the array list: JavaFX Last element of the array list: Impala
Example 2
To get minimum and maximum values of an ArrayList −
- Create an ArrayList object.
- Add elements to it.
- Sort it using the sort() method of the Collections class.
- Then, the first element of the ArrayList will be the minimum value and the last element of the ArrayList will be the maximum value.
import java.util.ArrayList; import java.util.Collections; public class MinandMax < public static void main(String[] args)< ArrayListlist = new ArrayList(); //Instantiating an ArrayList object list.add(1001); list.add(2015); list.add(4566); list.add(90012); list.add(100); list.add(21); list.add(43); list.add(2345); list.add(785); list.add(6665); list.add(6435); System.out.println("Contents of the Array List: \n"+list); //Sorting the array list Collections.sort(list); System.out.println("Minimum value: "+list.get(0)); System.out.println("Maximum value: "+list.get(list.size()-1)); > > Output
Contents of the Array List: [1001, 2015, 4566, 90012, 100, 21, 43, 2345, 785, 6665, 6435] Minimum value: 21 Maximum value: 90012
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.
Get first element in list
This example will show how to get the first element in a collection. While this may seem trivial, utilities exist to provide a more readable, eloquent approach vs the solution outlined in the straight up java section.
Straight up Java
This snippet will get the first element in a collection using java. It will check if the ArrayList is empty and if the size is greater than 0. It then will return the first element by getting the first element in the list.
@Test public void get_first_element_in_list_with_java () ListString> strings = new ArrayListString>(); strings.add("one"); strings.add("two"); strings.add("three"); String firstElement = null; if (!strings.isEmpty() && strings.size() > 0) firstElement = strings.get(0); > assertEquals("one", firstElement); >Java 8
This snippet will find the first element in arraylist using java 8. The Java 8 Streams API provides Streams.findFirst which will return the wrapper Optional object describing the first element of the stream. If the arraylist is empty, an empty Optional will be returned.
@Test public void get_first_element_in_list_with_java8 () ListString> strings = new ArrayListString>(); strings.add("one"); strings.add("two"); strings.add("three"); OptionalString> firstElement = strings.stream().findFirst(); assertEquals("one", firstElement.orElse("two")); >Google Guava
Guava Iterables.getFirst will return the first element in the list or the default value if the list is empty.
@Test public void get_first_element_in_list_with_guava () ListString> strings = Lists.newArrayList("one", "two", "three"); String firstElement = Iterables.getFirst(strings, null); assertEquals("one", firstElement); >Apache Commons
Apache commons CollectionUtils.get will return the element at the specified index.
@Test public void get_first_element_in_list_with_apachecommons () ListString> strings = Lists.newArrayList("one", "two", "three"); String firstElement = (String) CollectionUtils.get(strings, 0); assertEquals("one", firstElement); >Get first element in list posted by Justin Musgrove on 31 January 2014
Tagged: java and java-collections
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.