It’s a fairly common task as a Java developer to convert from a List to an Array or from an Array to a list.<\/p>\n
In one of my previous post, I discussed about converting Map to List<\/a>.<\/p>\n Like many things in Java, there is often more than one way to accomplish a task. In this post, I’ll discuss various approaches of converting data between List objects and arrays.<\/p>\n The A I have written a JUnit test case to test the If you are new to JUnit, I would suggest going through my series of posts on JUnit<\/a>.<\/p>\n The test class, The test case calls the With the introduction of Streams in Java 8, you can convert a The code to convert a The JUnit test code is this.<\/p>\n When you call the One approach is to do the conversion yourself, like this.<\/p>\n The second approach is to use Java 8 Stream, like this.<\/p>\n Another approach to convert a Apache Commons Lang provides extra methods for common functionalities and methods for manipulation of the core Java classes that is otherwise not available in the standard Java libraries.<\/p>\n To use Commons Lang, add the following dependency to your Maven POM.<\/p>\n Commons Lang comes with an The code to convert a The test code is this.<\/p>\n Guava<\/a> is a project developed and maintained by Google that comprise of several libraries extensively used by Java developer. To use The The code to convert a The test code is this.<\/p>\n To convert an array to a An The JUnit test class is this.<\/p>\n Commons Collection<\/a>, similar to Commons Lang is part of the larger Apache Commons project. Commons Collection builds upon the Java collection framework by providing new interfaces, implementations, and utilities.<\/p>\n Before using the Commons Collection in your code, you need this dependency in your Maven POM.<\/p>\n The code to convert an array to a List using the The test code is this.<\/p>\n Guava comes with a The code to convert an array to a The test code is this.<\/p>\n It’s a fairly common task as a Java developer to convert from a List to an Array or from an Array to a list. In one of my previous post, I discussed about converting Map to List. Like many things in Java, there is often more than one way to accomplish a task. In this […]Continue reading<\/a><\/p>\n","protected":false},"author":1,"featured_media":4592,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_lmt_disableupdate":"","_lmt_disable":"","_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"_uf_show_specific_survey":0,"_uf_disable_surveys":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","enabled":false},"version":2}},"categories":[20],"tags":[202,27,201],"class_list":["post-4185","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-java","tag-array","tag-java","tag-list"],"jetpack_publicize_connections":[],"aioseo_notices":[],"modified_by":"Simanta","jetpack_sharing_enabled":true,"jetpack_featured_media_url":"https:\/\/springframework.guru\/wp-content\/uploads\/2015\/03\/Banner560x292_07web.jpg","jetpack_shortlink":"https:\/\/wp.me\/p5BZrZ-15v","_links":{"self":[{"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/posts\/4185"}],"collection":[{"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/comments?post=4185"}],"version-history":[{"count":10,"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/posts\/4185\/revisions"}],"predecessor-version":[{"id":5768,"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/posts\/4185\/revisions\/5768"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/media\/4592"}],"wp:attachment":[{"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/media?parent=4185"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/categories?post=4185"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/springframework.guru\/wp-json\/wp\/v2\/tags?post=4185"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Converting List to Array<\/h2>\n
List<\/code> interface comes with a toArray()<\/code> method that returns an array containing all of the elements in this list in proper sequence (from first to last element). The type of the returned array is that of the array that you pass as parameter.<\/p>\n ListToArrayConvertor<\/code> class that converts a List<\/code> to an array is this.<\/p>\nListToArrayConvertor.java<\/h5>\n
import java.util.List;\r\npublic class ListToArrayConvertor {\r\n public String[] convert(List<String> list){\r\n return list.toArray(new String[list.size()]);\r\n }\r\n}<\/pre>\n convert()<\/code> method.<\/p>\nListToArrayConvertorTest<\/code> is this.<\/p>\nListToArrayConvertorTest.java<\/h5>\n
package springframework.guru;\r\n\r\nimport org.junit.After;\r\nimport org.junit.Before;\r\nimport org.junit.Test;\r\nimport java.util.ArrayList;\r\nimport java.util.List;\r\nimport static org.junit.Assert.*;\r\n\r\npublic class ListToArrayConvertorTest {\r\n\r\n ListToArrayConvertor listToArrayConvertor;\r\n List<String> stringList;\r\n List<Integer> integerList;\r\n @Before\r\n public void setUp(){\r\n listToArrayConvertor=new ListToArrayConvertor();\r\n\r\n \/*List of String*\/\r\n stringList = new ArrayList<>();\r\n stringList.add(\"Java\");\r\n stringList.add(\"C++\");\r\n stringList.add(\"JavaScript\");\r\n stringList.add(\"TypeScript\");\r\n\r\n \/*List of Integer*\/\r\n integerList = new ArrayList<>();\r\n integerList.add(21);\r\n integerList.add(12);\r\n integerList.add(3);\r\n }\r\n\r\n @After\r\n public void tearDown(){\r\n listToArrayConvertor=null;\r\n stringList=null;\r\n integerList=null;\r\n }\r\n\r\n @Test\r\n public void convert() {\r\n String[] languages = listToArrayConvertor.convert(stringList);\r\n assertNotNull(languages);\r\n assertEquals(stringList.size(),languages.length);\r\n }\r\n}<\/pre>\n convert()<\/code> method of the ListToArrayConvertor<\/code> class and asserts two things. First it asserts that the array returned by the convert()<\/code> method is not null<\/code>. Second it asserts that the array contains the same number of elements as that of the list passed to the convert()<\/code> method.<\/p>\nConverting List to Array using Java 8 Stream<\/h3>\n
List<\/code> into a sequential stream of elements. Once you obtain a stream as a Stream<\/code> object from a collection, you can call the Stream.toArray()<\/code> method that returns an array containing the elements of this stream.<\/p>\n List<\/code> to an array using stream is this.<\/p>\npublic String[] convertWithStream(List<String> list){\r\n return list.stream().toArray(String[]::new);\r\n}<\/pre>\n@Test\r\npublic void convertWithStream() {\r\n String[] languages = listToArrayConvertor.convertWithStream(stringList);\r\n assertNotNull(languages);\r\n assertEquals(stringList.size(),languages.length);\r\n}\r\n<\/pre>\nConverting List to Primitive Array<\/h3>\n
List.toArray()<\/code> method, you get wrapper arrays, such as Integer[]<\/code>, Double[]<\/code>, and Boolean[]<\/code>. What if you need primitive arrays, such as int[]<\/code>, double[]<\/code>, and boolean []<\/code> instead?<\/p>\npublic int[] convertAsPrimitiveArray(List<Integer> list){\r\n int[] intArray = new int[list.size()];\r\n for(int i = 0; i < list.size(); i++) intArray[i] = list.get(i);\r\n return intArray;\r\n}<\/pre>\nint[] array = list.stream().mapToInt(i->i).toArray();<\/pre>\n
List<\/code> to a primitive array is by using Apache Commons Lang<\/a>.<\/p>\nConverting List to Primitive Array with Apache Commons Lang<\/h3>\n
<dependency>\r\n <groupId>commons-lang<\/groupId>\r\n <artifactId>commons-lang<\/artifactId>\r\n <version>2.6<\/version>\r\n<\/dependency><\/pre>\n
ArrayUtils<\/code> class meant specifically to perform operations on arrays. Out of the several methods that ArrayUtils<\/code> provide, the methods of our interest are the overloaded toPrimitives()<\/code> methods. Each of these overloaded methods accepts a wrapper array and returns the corresponding primitive array.<\/p>\n List<\/code> to a primitive array using Commons Lang is this.<\/p>\npublic int[] convertWithApacheCommons(List<Integer> list){\r\n return ArrayUtils.toPrimitive(list.toArray(new Integer[list.size()]));\r\n}<\/pre>\n@Test\r\npublic void convertWithApacheCommons() {\r\n int[] numbers = listToArrayConvertor.convertWithApacheCommons(integerList);\r\n assertNotNull(numbers);\r\n assertEquals(integerList.size(),numbers.length);\r\n}<\/pre>\nConverting List to Primitive Array with Guava<\/h3>\n
Guava<\/code>, add the following dependency to your Maven POM.<\/p>\n<dependency>\r\n <groupId>com.google.guava<\/groupId>\r\n <artifactId>guava<\/artifactId>\r\n <version>25.0-jre<\/version>\r\n<\/dependency><\/pre>\n
Guava<\/code> contains several classes, such as Ints<\/code>, Longs<\/code>, Floats<\/code>, Doubles<\/code>, and Booleans<\/code> with utility methods that are not already present in the standard Java wrapper classes. For example, the Ints<\/code> class contains static utility methods pertaining to int<\/code> primitives.<\/p>\nGuava<\/code> wrapper classes come with a toArray()<\/code> method that returns a primitive array containing each value of the collection passed to it.<\/p>\n List<\/code> to a primitive array using Guava<\/code> is this.<\/p>\npublic int[] convertWithGuava(List<Integer> list){\r\n return Ints.toArray(list);\r\n}<\/pre>\n@Test\r\npublic void convertWithGuava() {\r\n int[] languages = listToArrayConvertor.convertWithGuava(integerList);\r\n assertNotNull(languages);\r\n assertEquals(integerList.size(),languages.length);\r\n}<\/pre>\nConverting Array to List<\/h2>\n
List<\/code>, you can use the Arrays.asList()<\/code> method. This method returns a fixed-size list from the elements of the array passed to it.<\/p>\n ArrayToListConverter<\/code> class that converts an array to a List<\/code> is this.<\/p>\nArrayToListConverter.java<\/h5>\n
package springframework.guru;\r\n\r\nimport java.util.ArrayList;\r\nimport java.util.Arrays;\r\nimport java.util.List;\r\n\r\npublic class ArrayToListConverter {\r\n public List<String> convert(String[] strArray){\r\n return Arrays.asList(strArray);\r\n }\r\n}\r\n<\/pre>\nArrayToListConverterTest.java<\/h5>\n
package springframework.guru;\r\n\r\nimport org.junit.After;\r\nimport org.junit.Before;\r\nimport org.junit.Test;\r\nimport java.util.ArrayList;\r\nimport java.util.List;\r\nimport static org.junit.Assert.*;\r\n\r\npublic class ArrayToListConverterTest {\r\n\r\n ArrayToListConverter arrayToListConverter;\r\n String[] stringArray;\r\n int[] intArray;\r\n @Before\r\n public void setUp(){\r\n arrayToListConverter = new ArrayToListConverter();\r\n \/*initialize array of String*\/\r\n stringArray = new String[]{\"Java\",\"C++\",\"JavaScript\",\"TypeScript\"};\r\n \/*initialize array of int*\/\r\n intArray = new int[]{21,12,3};\r\n }\r\n\r\n @After\r\n public void tearDown(){\r\n arrayToListConverter=null;\r\n stringArray=null;\r\n intArray=null;\r\n }\r\n @Test\r\n public void convert() {\r\n List<String> languageList = arrayToListConverter.convert(stringArray);\r\n assertNotNull(languageList);\r\n assertEquals(stringArray.length,languageList.size());\r\n }\r\n}<\/pre>\nConverting Array to List using Commons Collection<\/h3>\n
<dependency>\r\n <groupId>commons-collections<\/groupId>\r\n <artifactId>commons-collections<\/artifactId>\r\n <version>3.2.2<\/version>\r\n<\/dependency><\/pre>\n
CollectionUtils<\/code> is one class in Commons Collection that provides utility methods and decorators for Collection<\/code> instances. You can use the addAll()<\/code> method of CollectionUtils<\/code> to convert an array to a List<\/code>. This method takes as parameters a Collection<\/code> and an array. This method adds all the elements present in the array to the collection.<\/p>\nCollectionUtils<\/code> class of Commons Collection is this.<\/p>\npublic List<String> convertWithApacheCommons(String[] strArray){\r\n List<String> strList = new ArrayList<>(strArray.length);\r\n CollectionUtils.addAll(strList, strArray);\r\n return strList;\r\n}<\/pre>\n@Test\r\npublic void convertWithApacheCommons() {\r\n List<String> languageList = arrayToListConverter.convertWithApacheCommons(stringArray);\r\n assertNotNull(languageList);\r\n assertEquals(stringArray.length,languageList.size());\r\n}<\/pre>\nConverting Array to List using Guava<\/h3>\n
Lists<\/code> class with static utility methods to work with List<\/code> objects. You can use the newArrayList()<\/code> method of the Lists<\/code> class to convert an array to a List<\/code>. The newArrayList()<\/code> method takes an array and returns an ArrayList<\/code> initialized with the elements of the array.<\/p>\nList<\/code> using the Lists<\/code> class of Guava is this.<\/p>\npublic List<String> convertWithGuava(String[] strArray){\r\n return Lists.newArrayList(strArray);\r\n}<\/pre>\n@Test\r\npublic void convertWithGuava() {\r\n List<String> languageList = arrayToListConverter.convertWithGuava(stringArray);\r\n assertNotNull(languageList);\r\n assertEquals(stringArray.length,languageList.size());\r\n}<\/pre>\n","protected":false},"excerpt":{"rendered":"