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Java List Collection Tutorial and Examples

rial-and-examples[3/2/2019 12:18:25 PM]

Last Updated on 10 December 2018

| Print Email Master Microservices with Spring Boot and Spring Cloud List is a fundamental and widely-used collection type in the Java Collec tions Framework. In this tutorial, we'll see characteristics of list collections, how to use list implementation s for day-to-day work and look at examples of common programming practices when using lists.

1. Overview of List collection

2. Creating a new list

3. Basic operations

4. Iterating over a list

5. Searching for an element in a list

6. Sorting a list

7. Copying one list into another

8. Shuffling elements in a list

9. Reversing elements in a list

10. Extracting a portion of a list

11. Converting between Lists and arrays

12. List to Stream

13. Concurrent lists

Basically, a list collection stores elements by insertion order (either at the end or at a specific position in the list). A list maintains indices of its elements so it allows adding, r etrieving, modifying, removing elements by an integer index (zero-based index; the first element is at 0-index, t he second at 1-index, the third at 2- index, and so on). The following picture illustrates a list that stores some String elements:

Java List Collection Tutorial and Examples

rial-and-examples[3/2/2019 12:18:25 PM] A list can store objects of any types. Primitive types are automatically converted to corresponding wrapper types, e.g. integer numbers are converted to Integerobjects. It allows null and duplicate elements, and orders them by their insertion order (index). The following class diagram depicts the primary methods defined in the java.util.List interface: The List is the base interface for all list types, and the ArrayList and LinkedList classes are two common

List's implementations.

ArrayList: An implementation that stores elements in a backing array. The array' s size will be automatically expanded if there isn't enough room when adding new el ements into the list. It's possible to set the default size by specifying an initial capacity when creating a new

ArrayList.

Basically, an

ArrayList offers constant time for the following operations: size, isEmpty, get, set, iterator and listIterator; amortized constant time for the add operation; and linear time for other operations. Therefore, this implementation can be considered if w e want fast, random access of the elements. LinkedList: An implementation that stores elements in a doubly-linked list data st ructure. It offers constant time for adding and removing elements at the end of the list; and linear time for operations at other positions in the list. Therefore, we can consider using a

LinkedList if fast adding and

removing elements at the end of the list is required.

Besides

ArrayList and LinkedList,Vector class is a legacy collection and later was retrofitted to implement the List interface. Vector is thread-safe, but ArrayList and LinkedList are not. The following class diagram depicts the inheritance tree of the

List collections:

Java List Collection Tutorial and Examples

rial-and-examples[3/2/2019 12:18:25 PM]

The following is a quick example of creating a new ArrayList and LinkedList which hold String objects;

add some elements to them; and then print out the collections:

ArrayList quick example:

List listStrings = new ArrayList(); listStrings.add("One"); listStrings.add("Two"); listStrings.add("Three"); listStrings.add("Four");

System.out.println(listStrings);

LinkedList quick example:

List listStrings = new LinkedList(); listStrings.add("Five"); listStrings.add("Six"); listStrings.add("Seven"); listStrings.add("Eight");

System.out.println(listStrings);

The code examples in this tutorial revolve on the two common implementat ions:

ArrayList and LinkedList.

It's a good practice to declare a list instance with a generic type p arameter, for example:

Java List Collection Tutorial and Examples

rial-and-examples[3/2/2019 12:18:25 PM] List listAnything = new ArrayList();

List listWords = new ArrayList();

List listNumbers = new ArrayList(); List linkedWords = new LinkedList(); Since Java 7, we can remove the type parameter on the right side as foll ows:

List listNumbers = new ArrayList<>();

List linkedWords = new LinkedList<>();

The compiler is able to infer the actual type parameter from the declara tion on the left side.

Since Java 9, you can create a

List collection from a fixed set of elements by using the factory method

List.of(e1, e2, e3...). For example:

List listNumbers = List.of(1, 2, 3, 4, 5, 6);

Note that the List collection returned by the List.of() factory method is immutable - meaning that you

can't add more elements to it. Since Java 10, you can shorten the declaration of a List collection by u sing the var reserved word like this: var employees = new ArrayList(); The compiler can infer the type of the variable on the left based on the object type on the right side. And var can be used to declare local variables only.

When creating a new

ArrayList using the empty constructor, the list is constructed with an initial ca pacity of ten. If you are sure how many elements will be added to the list, it 's recommended to specify a capacity which is large enough. Let's say, if we know that a list contains ar ound 1000 elements, declare the list as follows: List listNumbers = new ArrayList<>(1000); It's also possible to construct a list that takes elements from an ex isting collection, for example:

List listNumberOne;

// existing collection List listNumberTwo = new ArrayList<>(listNumberOne); The listNumberTwo constructed with copies of all elements from the listNumberOne.

3. Basic operations: adding, retrieving, updating,

removing elements

Adding elements

Java List Collection Tutorial and Examples

rial-and-examples[3/2/2019 12:18:25 PM] The methods add(Object), add(index, Object) and addAll() are used to add elements to the list. It requires to add elements of the same type (or sub type) as the type p arameter declared by the list. For example: List listStrings = new ArrayList(); // OK to add Strings: listStrings.add("One"); listStrings.add("Two"); listStrings.add("Three"); // But this will cause compile error listStrings.add(123); Adding elements of sub types of the declared type:

List linkedNumbers = new LinkedList<>();

linkedNumbers.add(new Integer(123)); linkedNumbers.add(new Float(3.1415)); linkedNumbers.add(new Double(299.988)); linkedNumbers.add(new Long(67000)); We can insert an element into the list at a specified index, for example listStrings.add(1, "Four"); That inserts the String "Four" at the 2nd position in the list. We can also add all elements of an existing collection to the end of the list: listStrings.addAll(listWords); Or add the elements to the list at a specified position: listStrings.addAll(2, listWords); That inserts all elements of the listWordscollection at 3 position of the listStrings collection.

Retrieving elements

The get() method is used to retrieve an element from the list at a specified inde x. For example, the

following code gets an element at 2 position in the array list and an element at 4 position in the linked

list:

String element = listStrings.get(1);

Number number = linkedNumbers.get(3);

For a LinkedListimplementation, we can get the first and the last elements like this: LinkedList numbers = new LinkedList(); // add elements to the list... rd ndth

Java List Collection Tutorial and Examples

rial-and-examples[3/2/2019 12:18:25 PM] // get the first and the last elements:

Number first = numbers.getFirst();

Number last = numbers.getLast();

Note that the getFirst() and getLast() methods are specific to the LinkedListclass.

Updating elements

Use the set(index, element) method to replace the element at the specified index by the specified element. For example: listStrings.set(2, "Hi"); That replaces the 3 element in the list by the new String "Hi".

Removing elements

To remove an element from the list, use the remove(index) or remove(Object) method which removes the element at the specified index or by object reference. For example:

Remove the element at the 3 position in the list:

listStrings.remove(2); If the specified index is out of range (index < 0 or index >= list size ), a java.lang.IndexOutOfBoundsException is thrown.

Remove the String element "Two" in the list:

listStrings.remove("Two");

Notes about the remove(Object) method:

It compares the specified object with the elements in the list using the ir equals() method, so if you use your own defined object type, make sure it implements the equals() method correctly. It only removes the first occurrence of the specified element in the lis t (i.e. if a list contains duplicate elements, only the first element is removed).

It returns true if the list contained the specified element, or falseotherwise. Thus it's recommended

to check return value of this method, for example: if (listStrings.remove("Ten")) {

System.out.println("Removed");

} else {

System.out.println("There is no such element");

To remove all elements in the list, use the

clear() method: listStrings.clear(); rd rd

Java List Collection Tutorial and Examples

rial-and-examples[3/2/2019 12:18:25 PM] Basically, we can use the enhanced for loop to iterate through all eleme nts in the list, as follows: for (String element : listStrings) {

System.out.println(element);

Or use an iterator like this:

Iterator iterator = listStrings.iterator(); while (iterator.hasNext()) {

System.out.println(iterator.next());

For more list-specific, use a list iterator as shown below: Iterator iterator = linkedNumbers.listIterator(); while (iterator.hasNext()) {

System.out.println(iterator.next());

Since Java 8, we can use the forEach()method like this: listStrings.forEach(s -> System.out.println(s)); For more details and examples, see the tutorial: Java Collections Looping Example

For more about the forEach iteration method, see the tutorial: The 4 Methods for Iterating Collections in Java

To search for position of a specific element in the list or to know if t he list contains the specified element, the following methods can be used: boolean contains(Object): returns trueif the list contains the specified element. int indexOf(Object): returns the index of the first occurrence of the specified element in the list, or -1 if the element is not found. int lastIndexOf(Object): returns the index of the last occurrence of the specified element in t he list, or -1 if the element is not found.

Examples:

if (listStrings.contains("Hello")) {

System.out.println("Found the element");

} else {

System.out.println("There is no such element");

Java List Collection Tutorial and Examples

rial-and-examples[3/2/2019 12:18:25 PM] int firstIndex = linkedNumbers.indexOf(1234); int lastIndex = listStrings.indexOf("Hello");

Note that the above methods compare the elements using their equals() method, so if you define your own

type, make sure it implements the equals() method correctly.

6. Sorting a list

Before Java 8, the simplest way to sort out elements in a list is using the Collections.sort() static method which sorts the specified list into ascending order, based on th e natural ordering of its elements.

Here's an example:

List listStrings = new ArrayList(); listStrings.add("D"); listStrings.add("C"); listStrings.add("E"); listStrings.add("A"); listStrings.add("B"); System.out.println("listStrings before sorting: " + listStrings);

Collections.sort(listStrings);

System.out.println("listStrings after sorting: " + listStrings);

Output:

listStrings before sorting: [D, C, E, A, B] listStrings after sorting: [A, B, C, D, E]

Note that all elements in the list must implement the Comparableinterface, so if you define your own type,

make sure it implements that interface and its compareTo() method.

Since Java 8, the List interface introduces the

sort() method, so you can sort elements in an ArrayList or LinnkedList directly like this: listStrings.sort(null); // sort by natural ordering of the elements For more details and examples, see the article: Sorting List Collections Examples

The Collections.copyList(dest, src) static method allows us to copy all elements from the source list

into the destination one. Note that the destination list must be large enough to contain the entire source list.

Here's an example:

List sourceList = new ArrayList(); sourceList.add("A"); sourceList.add("B");

Java List Collection Tutorial and Examples

rial-and-examples[3/2/2019 12:18:25 PM] sourceList.add("C"); sourceList.add("D");

List destList = new ArrayList();

destList.add("V"); destList.add("W"); destList.add("X"); destList.add("Y"); destList.add("Z"); System.out.println("destList before copy: " + destList);

Collections.copy(destList, sourceList);

System.out.println("destList after copy: " + destList);quotesdbs_dbs5.pdfusesText_10