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Java List Collection Tutorial and ExamplesDetailsWritten by Nam Ha MinhLast Updated on 14 June 2019 | Print Email 1. Overview of List collection Show
2. Creating a new list 3. Basic List operations 4. Iterating over elements in a list 5. Searching for an element in a list 6. Sorting a list 7. Copying elements from 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 1. Overview of List collectionList is a fundamental and widely-used collection type in the Java Collections Framework. 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, retrieving, modifying, removing elements by an integer index (zero-based index; the first element is at 0-index, the second at 1-index, the third at 2-index, and so on). The following picture illustrates a list that stores some String elements: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 Lists implementations.
2. Creating a new listIts a good practice to declare a list instance with a generic type parameter, for example:List<Object> listAnything = new ArrayList<Object>(); List<String> listWords = new ArrayList<String>(); List<Integer> listNumbers = new ArrayList<Integer>(); List<String> linkedWords = new LinkedList<String>(); Since Java 7, we can remove the type parameter on the right side as follows:List<Integer> listNumbers = new ArrayList<>(); List<String> linkedWords = new LinkedList<>(); The compiler is able to infer the actual type parameter from the declaration 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<Integer> listNumbers = List.of(1, 2, 3, 4, 5, 6);Note that the Listcollection returned by the List.of() factory method is immutable - meaning that you cant add more elements to it.Since Java 10, you can shorten the declaration of a List collection by using the var reserved word like this:var employees = new ArrayList<Employee>();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 capacity of ten. If you are sure how many elements will be added to the list, its recommended to specify a capacity which is large enough. Lets say, if we know that a list contains around 1000 elements, declare the list as follows:List<Integer> listNumbers = new ArrayList<>(1000); Its also possible to construct a list that takes elements from an existing collection, for example:List<Integer> listNumberOne; // existing collection List<Integer> listNumberTwo = new ArrayList<>(listNumberOne); The listNumberTwo constructed with copies of all elements from the listNumberOne.3. Basic List operations: adding, retrieving, updating, removing elementsAdding elements to a List: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 parameter declared by the list. For example:List<String> listStrings = new ArrayList<String>(); // 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<Number> 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 3rd position of the listStrings collection.Retrieving elements from a ListThe get() method is used to retrieve an element from the list at a specified index. For example, the following code gets an element at 2nd position in the array list and an element at 4th 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<Number> numbers = new LinkedList<Number>(); // add elements to the list... // 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 in a ListUse 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 3rd element in the list by the new String Hi.Removing elements from a ListTo 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:
4. Iterating over elements in a listBasically, we can use the enhanced for loop to iterate through all elements in the list, as follows:for (String element : listStrings) { System.out.println(element); } Or use an iterator like this:Iterator<String> iterator = listStrings.iterator(); while (iterator.hasNext()) { System.out.println(iterator.next()); } For more list-specific, use a list iterator as shown below:Iterator<Number> 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 ExampleFor more about the forEach iteration method, see the tutorial: The 4 Methods for Iterating Collections in Java5. Searching for an element in a listTo search for position of a specific element in the list or to know if the list contains the specified element, the following methods can be used:
6. Sorting a listBefore 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 the natural ordering of its elements. Heres an example:List<String> listStrings = new ArrayList<String>(); 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 elementsFor more details and examples, see the article: Sorting List Collections Examples7. Copying elements from one list into anotherThe 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. Heres an example:List<String> sourceList = new ArrayList<String>(); sourceList.add("A"); sourceList.add("B"); sourceList.add("C"); sourceList.add("D"); List<String> destList = new ArrayList<String>(); 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); The output would be:destList before copy: [V, W, X, Y, Z] destList after copy: [A, B, C, D, Z]8. Shuffling elements in a listTo randomly permute elements in a list, use the Collections.shuffle() static method. Heres a quick example:List<Integer> numbers = new ArrayList<Integer>(); for (int i = 0; i <= 10; i++) numbers.add(i); System.out.println("List before shuffling: " + numbers); Collections.shuffle(numbers); System.out.println("List after shuffling: " + numbers); The output would be:List before shuffling: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] List after shuffling: [6, 4, 5, 0, 1, 3, 9, 7, 2, 10, 8]9. Reversing elements in a listTo reverse order of elements in a list, use the Collections.reverse() static method. Heres a quick example:List<Integer> numbers = new ArrayList<Integer>(); for (int i = 0; i <= 10; i++) numbers.add(i); System.out.println("List before reversing: " + numbers); Collections.reverse(numbers); System.out.println("List after reversing: " + numbers); The output would be:List before reversing: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] List after reversing: [10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]10. Extracting a portion of a listThe subList(fromIndex, toIndex) allows us to get a portion of the list between the specified fromIndex(inclusive) and toIndex(exclusive). Heres an example:List<String> listNames = Arrays.asList("Tom", "John", "Mary", "Peter", "David", "Alice"); System.out.println("Original list: " + listNames); List<String> subList = listNames.subList(2, 5); System.out.println("Sub list: " + subList);Output:Original list: [Tom, John, Mary, Peter, David, Alice] Sub list: [Mary, Peter, David]Note that the sub list is just a view of the original list, so any modifications made on the original list will reflect in the sub list.11. Converting between Lists and arraysThe Java Collection Framework allows us to easily convert between lists and arrays.The Arrays.asList(T a) method converts an array of type T to a list of type T. Heres an example:List<String> listNames = Arrays.asList("John", "Peter", "Tom", "Mary", "David", "Sam"); List<Integer> listNumbers = Arrays.asList(1, 3, 5, 7, 9, 2, 4, 6, 8); System.out.println(listNames); System.out.println(listNumbers);Output:[John, Peter, Tom, Mary, David, Sam] [1, 3, 5, 7, 9, 2, 4, 6, 8]And the Listinterface provides the toArray() method that returns an array of Objects containing all of the elements in the list in proper sequence (from first to last element). Heres an example:List<String> listWords = new ArrayList<String>(); // add elements to the list Object[] arrayWords = listWords.toArray();And the toArray(T[] a) method returns an array of type T, for example:String[] words = listWords.toArray(new String[0]); Integer[] numbers = listNumbers.toArray(new Integer[0]);Note that the returned array contains copies of elements in the list, which that means we can safely modify the array without affecting the list.12. List to StreamFrom Java 8, you can convert a List to a stream to take advantages of the Streams API: List.stream(): returns a sequential stream. List.parallelStream(): returns a possibly parallel stream.For example, the following code converts a List numbers to a stream and uses the Stream API to calculate the sum of all numbers:int sum = numbers.stream().reduce(0, (x, y) -> x + y);For more information about Java stream, read Understand Java Stream API.13. Concurrent listsBy default, ArrayListand LinkedList are not thread-safe, so if you want to use them in concurrent context, you have to synchronize them externally using the Collections.synchronizedList() static method which returns a synchronized list that wraps the specified list. For example:List<Object> unsafeList = new ArrayList<Object>(); List<Object> safeList = Collections.synchronizedList(unsafeList); Note that you must manually synchronize the returned list when iterating over it, for example:synchronized (safeList) { Iterator<Object> it = safeList.iterator(); while (it.hasNext()) { System.out.println(it.next()); } }Conclusion:So far you have learned almost everything you need to know about Java list collection for beginners. Now, you're not only able to use List in your Java program, you also grasp a comprehensive understand about it. For more in-depth learning about Java collections framework, I recommend you to read the book Java Generics and Collections byMaurice Naftalin and Philip Wadler. And if you want to learn a full video course about Java programming, I recommend this Java programming masterclass.For hands-on practice on Java list collection, I recommend you to watch this video tutorial:Related Java List tutorials:
Other Java Collections Tutorials:
API References for Java List:
About the Author:Nam Ha Minh is certified Java programmer (SCJP and SCWCD). He started programming with Java in the time of Java 1.4 and has been falling in love with Java since then. Make friend with him on Facebook and watch his Java videos you YouTube.Add comment
Notify me of follow-up comments Send Cancel Comments1234 #16Chinnu2021-03-25 02:51 Wonderful and thanks for the info Quote#15Pepe2021-02-24 21:21 Nice post, it was very useful. QuoteThanks and keep doing this, you're awesome. #14Harmeet Singh2020-04-19 02:42 int firstIndex = linkedNumbers.indexOf(1234); Quoteint lastIndex = listStrings.indexOf("Hello"); Please check the lastindex method, i think it should use lastIndexOf(Object) method #13manu2020-03-26 02:00 bcz Quote1) as we know we can not create an object for interface 2) we can use super class referance to teh sub class objects #12Rajababu2019-07-01 07:50 Beacause list is interface we can use multiple classes with list interface. QuoteWith ArrayList: ArrayList al = new ArrayList(); With List: Listaa=new ArrayList(); Lista1=new LinkedList(); Lista2=new Vector(); List a3=new Stack(); we can use all this with list interface 1234 Refresh comments list |