1. Introduction

Acollection is a group of objects and treated as a single unit. It is used to store, retrieve, and manipulate aggregate data. Collections are part of the core java and all java developers must have familiarity with them. Java has provided acollection framework which contains interfaces, classes, and algorithms since version 1.2 and enhanced it over time. The most important interfaces are Java Set, List, and Map. Here are some enhancements:

• Major enhancement atversion 5, including generic type.
• Added more interfaces and classes atversion 6, includingDeque,BlockingDeque,NavigableSet,NavigableMap,ConcurrentNavigableMap, etc.
• Added a newTransferQueue interface atversion 7.
• Collection added theforeach(Consumer<? super T>> action) method atversion 8.
• Added factory methods for creating immutable lists atversion 9, such asstaticof() method.
• List addedstaticcopyOf() method atversion 10.
• Collection added a new default method toArray(IntFunction<T[]> generator) atversion 11.

You can also check this tutorial in the following video:

In this example, I will create several test classes to demonstrate how to use the most commonly used interfaces:List,Set,Queue and classes:ArrayList,LinkedList,Vector,Stack,HashSet,TreeSet.

2. Technologies Used

The example code in this article was built and run using:

• Java 11
• Maven 3.3.9
• Eclipse Oxygen
• Logback 1.2.3

3. Maven Project

3.1 Pom.xml

Pom.xml manages the project dependencies.

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>jcg.zheng.demo</groupId><artifactId>java-collections-demo</artifactId><version>0.0.1-SNAPSHOT</version><build><sourceDirectory>src</sourceDirectory><plugins><plugin><artifactId>maven-compiler-plugin</artifactId><version>3.8.0</version><configuration><release>11</release></configuration></plugin></plugins></build><dependencies><dependency><groupId>junit</groupId><artifactId>junit</artifactId><version>4.12</version></dependency><dependency><groupId>ch.qos.logback</groupId><artifactId>logback-access</artifactId><version>1.2.3</version></dependency><dependency><groupId>ch.qos.logback</groupId><artifactId>logback-classic</artifactId><version>1.2.3</version></dependency><dependency><groupId>ch.qos.logback</groupId><artifactId>logback-core</artifactId><version>1.2.3</version></dependency></dependencies></project>

3.2 SomeData

Java collection classes work withObject, not the primitive data type. In this step, I will create aSomeData class which implements theComparable interface.

SomeData.java

package jcg.zheng.demo.data;import java.io.Serializable;public class SomeData implements Comparable , Serializable {private static final long serialVersionUID = -5243901184267064976L;private int id;private String name;@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;SomeData other = (SomeData) obj;if (id != other.id)return false;if (name == null) {if (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}public int getId() {return id;}public String getName() {return name;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + id;result = prime * result + ((name == null) ? 0 : name.hashCode());return result;}public void setId(int id) {this.id = id;}public void setName(String name) {this.name = name;}@Overridepublic String toString() {return "SomeData [name=" + name + ",]";}@Overridepublic int compareTo(SomeData other) {int byName = this.name.compareTo(other.getName());return byName;}}

4. Abstract Test Classes

In this step, I will create three abstract test classes:

• CollectionBase_POJOTest – It demonstrates how toadd,remove,clear,contains,getSize,foreach for aCollection ofSomeData objects.
• CollectionBase_StringTest – It does the same as above class but forString objects.
• ListBase_StringTest – It extends fromCollectionBase_StringTest with additional test methods for theList interface withString objects.

4.1 CollectionBase_StringTest

TheCollection interface includes methods to add, remove, and retrieve elements from a collection. It checks the element’s existence with thecontains method. In this example, I will create a test class which demonstrates how to use these methods forString objects.

CollectionBase_StringTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertFalse;import static org.junit.Assert.assertTrue;import java.util.Arrays;import java.util.Collection;import java.util.Iterator;import org.junit.After;import org.junit.FixMethodOrder;import org.junit.Test;import org.junit.runners.MethodSorters;import org.slf4j.Logger;@FixMethodOrder(MethodSorters.NAME_ASCENDING)public abstract class CollectionBase_StringTest {protected Collection<String> colItems;protected Logger logger;@Testpublic void Collection_add() {colItems.add("new Item");assertEquals(5, colItems.size());logger.info("The arraycolItems contains the following elements: " + colItems);}@Afterpublic void Collection_clear() {colItems.clear();assertTrue(colItems.isEmpty());}@Testpublic void Collection_contains() {assertTrue(colItems.contains("Item1"));assertFalse(colItems.contains("Item5"));}@Testpublic void Collection_getSize() {assertEquals(4, colItems.size());}@Testpublic void Collection_isEmpty() {assertFalse(colItems.isEmpty());}@Testpublic void Collection_loop_via_for() {logger.info("Retrieving items using for loop");for (String str : colItems) {logger.info("Item is: " + str);}}@Testpublic void Collection_loop_via_foreach_java8() {logger.info("Retrieving items using Java 8 Stream");colItems.forEach((item) -> {logger.info(item);});}@Testpublic void Collection_loop_via_iterator_next() {logger.info("Retrieving items using iterator");for (Iterator<String> it = colItems.iterator(); it.hasNext();) {logger.info("Item is: " + it.next());}}@Testpublic void Collection_loop_via_iterator_while() {Iterator<String> it = colItems.iterator();logger.info("Retrieving items using iterator");while (it.hasNext()) {logger.info("Item is: " + it.next());}}@Testpublic void Collection_remove() {boolean removed = colItems.remove("Item1");assertTrue(removed);assertEquals(3, colItems.size());colItems.remove("Item3");assertEquals(2, colItems.size());}@Testpublic void Collection_remove_via_iterator_remove() {logger.info("Removing items using iterator");for (Iterator<String> it = colItems.iterator(); it.hasNext();) {String item = it.next();logger.info("Item is: " + item);it.remove();assertFalse(colItems.contains(item));}assertTrue(colItems.isEmpty());}@Testpublic void Collection_toArray_default_java11() {String[] simpleArray = colItems.toArray(new String[colItems.size()]);logger.info("The array created after the conversion of our arraycolItems is: " + Arrays.toString(simpleArray));String[] testArr = colItems.toArray(size -> new String[size]);logger.info("default toArray(intfunction):" + Arrays.toString(testArr));}}

Note: thecolItems variable is a type ofCollection<String>

4.2 CollectionBase_POJOTest

In this step, I will create aCollectionBase_POJOTest class which has the same methods as theCollectionBase_StringTest class but with theSomeData type.

CollectionBase_POJOTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertFalse;import static org.junit.Assert.assertTrue;import java.util.Arrays;import java.util.Collection;import java.util.Iterator;import org.junit.After;import org.junit.FixMethodOrder;import org.junit.Test;import org.junit.runners.MethodSorters;import org.slf4j.Logger;import jcg.zheng.demo.data.SomeData;@FixMethodOrder(MethodSorters.NAME_ASCENDING)public abstract class CollectionBase_POJOTest {Collection<SomeData> list;protected Logger logger;protected SomeData buildSomeData(int id, String name) {SomeData sd = new SomeData();sd.setId(id);sd.setName(name);return sd;}@Testpublic void Collection_add() {list.add(buildSomeData(1, "test"));assertEquals(5, list.size());logger.info("The arraylist contains the following elements: " + list);}@Afterpublic void Collection_clear() {list.clear();assertTrue(list.isEmpty());}@Testpublic void Collection_contains() {assertTrue(list.contains(buildSomeData(1, "Item1")));assertFalse(list.contains(buildSomeData(5, "Item5")));}@Testpublic void Collection_getSize() {assertEquals(4, list.size());}@Testpublic void Collection_isEmpty() {assertFalse(list.isEmpty());}@Testpublic void Collection_loop_via_for() {logger.info("Retrieving items using for loop");for (SomeData str : list) {logger.info("Item is: " + str);}}@Testpublic void Collection_loop_via_foreach_lambda() {logger.info("Retrieving items using Java 8 Stream");list.forEach((item) -> {logger.info(item.toString());});}@Testpublic void Collection_loop_via_iterator_for() {logger.info("Retrieving items using iterator");for (Iterator<SomeData> it = list.iterator(); it.hasNext();) {logger.info("Item is: " + it.next().toString());}}@Testpublic void Collection_loop_via_iterator_while() {Iterator<SomeData> it = list.iterator();logger.info("Retrieving items using iterator");while (it.hasNext()) {logger.info("Item is: " + it.next().toString());}}@Testpublic void Collection_remove() {list.remove(buildSomeData(3, "Item3"));assertEquals(3, list.size());}@Testpublic void Collection_toArray() {SomeData[] simpleArray = list.toArray(new SomeData[list.size()]);logger.info("The array created after the conversion of our arraylist is: " + Arrays.toString(simpleArray));}}

Note: thelist variable is a type ofCollection<SomeData>

4.3 ListBase_StringTest

TheList interface extends from theCollection interface with additional methods. In this step, I will demonstrate how to add, remove, and retrieve at a given index position for aList ofString. I will also demonstrate two Collections algorithms with thesort andbinarySearch methods.

ListBase_StringTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertTrue;import java.util.Collections;import java.util.List;import org.junit.Test;public abstract class ListBase_StringTest extends CollectionBase_StringTest {protected List<String> listItems;@Testpublic void Collections_sort_binarySearch() {listItems.add("Tom");listItems.add("Mary");listItems.add("Shan");listItems.add("Zheng");Collections.sort(listItems);assertEquals("Mary", listItems.get(0));assertEquals("Shan", listItems.get(1));assertEquals("Tom", listItems.get(2));assertEquals("Zheng", listItems.get(3));logger.info("Sorted List");for (int i = 0; i < listItems.size(); i++) {logger.info("Index: " + i + " - Item: " + listItems.get(i));}int foundTom = Collections.binarySearch(listItems, "Tom");assertEquals(2, foundTom);}@Testpublic void List_can_add_get_remove_via_index() {listItems.add(0, "Mary");assertEquals(1, listItems.size());int pos = listItems.indexOf("Mary");assertEquals(0, pos);String item0 = listItems.get(0);assertEquals("Mary", item0);listItems.set(0, "Zheng");assertEquals("Zheng", listItems.get(0));listItems.remove(0);assertTrue(listItems.isEmpty());}@Testpublic void List_copyOf_java10() {listItems.add("JCG");listItems.add("Demo");List<String> copiedValues = List.copyOf(listItems);assertEquals(2, copiedValues.size());}@Testpublic void List_loop_via_for_get_index() {listItems.add("Tom");listItems.add("Mary");listItems.add("Shan");listItems.add("Zheng");logger.info("Retrieving items with loop using index and size list");for (int i = 0; i < listItems.size(); i++) {logger.info("Index: " + i + " - Item: " + listItems.get(i));}}@Testpublic void List_of_java9() {List<String> emptyList = List.of();assertTrue(emptyList.isEmpty());List<String> itemList = List.of("Mary", "Zheng", "Developer");assertEquals(3, itemList.size());}}

Note: thelistItems variable is a type ofList<String>

5. Test Classes

In this step, I will create test classes for the commonly usedCollection classes. Each test class is extends from one of the three abstract test classes with its unique operations. The test method is named as{$InterfaceName}_{$MethodName}.

5.1 ArrayList_StringTest

ArrayList implementsList,Serializable,Cloneable,Iterable,Collection, andRandomAccess interfaces. It is used to store dynamically sized collection of elements.

In this step, I will create anArrayList_StringTest class which extends fromListBase_StringTest.

In thesetup_list_with_4_items method, it will create anArrayList instance and add four items. This method is also annotated with@Before annotation so it will be called for each test method.

In theArrayList_clone method, it will clone an array list.

ArrayList_StringTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertTrue;import java.util.ArrayList;import org.junit.Before;import org.junit.Test;import org.slf4j.LoggerFactory;public class ArrayList_StringTest extends ListBase_StringTest { @SuppressWarnings("unchecked") @Test public void Object_clone() { ArrayList<String> arrList = (ArrayList<String>) listItems; arrList.add("Mary"); arrList.add("JCG"); ArrayList<String> cloneArr = (ArrayList<String>) arrList.clone(); assertEquals(2, cloneArr.size()); } @Before public void setup_list_with_4_items() { logger = LoggerFactory.getLogger(this.getClass()); colItems = new ArrayList<String>(); assertTrue(colItems.isEmpty()); colItems.add("Item1"); assertEquals(1, colItems.size()); colItems.add("Item2"); assertEquals(2, colItems.size()); colItems.add("Item3"); assertEquals(3, colItems.size()); colItems.add("Item4"); assertEquals(4, colItems.size()); listItems = new ArrayList<String>(); listItems.addAll(colItems); assertEquals(4, listItems.size()); listItems.clear(); }}

Executemvn test -Dtest=ArrayList_StringTest

Output

------------------------------------------------------- T E S T S-------------------------------------------------------Running jcg.zheng.demo.ArrayList_StringTest05:19:33.334 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Retrieving items with loop using index and size list05:19:33.377 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Index: 0 - Item: Tom05:19:33.377 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Index: 1 - Item: Mary05:19:33.377 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Index: 2 - Item: Shan05:19:33.377 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Index: 3 - Item: Zheng05:19:33.381 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Sorted List05:19:33.383 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Index: 0 - Item: Mary05:19:33.384 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Index: 1 - Item: Shan05:19:33.384 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Index: 2 - Item: Tom05:19:33.390 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Index: 3 - Item: Zheng05:19:33.417 [main] INFO jcg.zheng.demo.ArrayList_StringTest - The arraycolItems contains the following elements: [Item1, Item2, Item3, Item4, new Item]05:19:33.425 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Retrieving items using for loop05:19:33.426 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item105:19:33.427 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item205:19:33.427 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item305:19:33.427 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item405:19:33.428 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Retrieving items using Java 8 Stream05:19:33.431 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item105:19:33.431 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item205:19:33.431 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item305:19:33.431 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item405:19:33.432 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Retrieving items using iterator05:19:33.432 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item105:19:33.433 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item205:19:33.433 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item305:19:33.433 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item405:19:33.434 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Retrieving items using iterator05:19:33.435 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item105:19:33.436 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item205:19:33.437 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item305:19:33.437 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item405:19:33.445 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Removing items using iterator05:19:33.446 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item105:19:33.446 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item205:19:33.447 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item305:19:33.448 [main] INFO jcg.zheng.demo.ArrayList_StringTest - Item is: Item405:19:33.455 [main] INFO jcg.zheng.demo.ArrayList_StringTest - The array created after the conversion of our arraycolItems is: [Item1, Item2, Item3, Item4]05:19:33.458 [main] INFO jcg.zheng.demo.ArrayList_StringTest - default toArray(intfunction):[Item1, Item2, Item3, Item4]Tests run: 17, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.526 secResults :Tests run: 17, Failures: 0, Errors: 0, Skipped: 0[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------[INFO] Total time:  11.850 s[INFO] Finished at: 2019-08-04T05:19:33-05:00[INFO] ------------------------------------------------------------------------C:\MaryZheng\Workspaces\jdk12\java-collections-demo>

5.2 LinkedList_StringTest

LinkedList implementsSerializable,Cloneable,Iterable,Collection,List andQueue interfaces. It models thelinked-list data structure.

In this step, I will create aLinkedList_StringTest class which extends fromListBase_StringTest.

In thesetup_colItem_with_4_items method, it will create aLinkedList instance and add four items. This method is also annotated with@Before annotation so it will be called for each test method.

In theDeQue_peekFirst_peekLast method, it will find the first element and last element from theDeQue interface’s methods.

In theDeQue_peek method, it will find the head element fromDeque‘s method.

LinkedList_StringTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertTrue;import java.util.LinkedList;import org.junit.Before;import org.junit.Test;import org.slf4j.LoggerFactory;public class LinkedList_StringTest extends ListBase_StringTest { @Test public void DeQue_peekFirst_peekLast() { LinkedList<String> linkedLst = (LinkedList<String>) listItems; linkedLst.add("Mary"); linkedLst.add("Zheng"); assertEquals("Mary", linkedLst.peekFirst()); assertEquals("Zheng", linkedLst.peekLast()); assertEquals("Mary", linkedLst.peek()); } @Test public void DeQue_peek() { LinkedList<String> linkedLst = (LinkedList<String>) listItems; linkedLst.add("Mary"); linkedLst.add("Zheng"); assertEquals("Mary", linkedLst.peek()); } @Before public void setup_colItems_with_4_items() { logger = LoggerFactory.getLogger(this.getClass()); colItems = new LinkedList<String>(); assertTrue(colItems.isEmpty()); colItems.add("Item1"); assertEquals(1, colItems.size()); colItems.add("Item2"); assertEquals(2, colItems.size()); colItems.add("Item3"); assertEquals(3, colItems.size()); colItems.add("Item4"); assertEquals(4, colItems.size()); listItems = new LinkedList<String>(); listItems.addAll(colItems); assertEquals(4, listItems.size()); listItems.clear(); }}

Executemvn test -Dtest=LinkedList_StringTest

Output

------------------------------------------------------- T E S T S-------------------------------------------------------Running jcg.zheng.demo.LinkedList_StringTest05:22:22.802 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Retrieving items with loop using index and size list05:22:22.865 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Index: 0 - Item: Tom05:22:22.866 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Index: 1 - Item: Mary05:22:22.867 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Index: 2 - Item: Shan05:22:22.869 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Index: 3 - Item: Zheng05:22:22.873 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Sorted List05:22:22.875 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Index: 0 - Item: Mary05:22:22.876 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Index: 1 - Item: Shan05:22:22.877 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Index: 2 - Item: Tom05:22:22.878 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Index: 3 - Item: Zheng05:22:22.908 [main] INFO jcg.zheng.demo.LinkedList_StringTest - The arraycolItems contains the following elements: [Item1, Item2, Item3, Item4, new Item]05:22:22.914 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Retrieving items using for loop05:22:22.915 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item105:22:22.917 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item205:22:22.918 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item305:22:22.918 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item405:22:22.920 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Retrieving items using Java 8 Stream05:22:22.921 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item105:22:22.922 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item205:22:22.922 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item305:22:22.922 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item405:22:22.922 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Retrieving items using iterator05:22:22.923 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item105:22:22.923 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item205:22:22.924 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item305:22:22.925 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item405:22:22.925 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Retrieving items using iterator05:22:22.926 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item105:22:22.926 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item205:22:22.926 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item305:22:22.926 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item405:22:22.930 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Removing items using iterator05:22:22.930 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item105:22:22.931 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item205:22:22.931 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item305:22:22.932 [main] INFO jcg.zheng.demo.LinkedList_StringTest - Item is: Item405:22:22.934 [main] INFO jcg.zheng.demo.LinkedList_StringTest - The array created after the conversion of our arraycolItems is: [Item1, Item2, Item3, Item4]05:22:22.936 [main] INFO jcg.zheng.demo.LinkedList_StringTest - default toArray(intfunction):[Item1, Item2, Item3, Item4]Tests run: 18, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.726 secResults :Tests run: 18, Failures: 0, Errors: 0, Skipped: 0[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------[INFO] Total time:  13.643 s

5.3 Vector_StringTestBase

Vector implementsSerializable,Cloneable,Iterable,Collection,List, andRandomAccess interfaces. It implements a grow-able array of objects.

In this step, I will create aVector_StringTest class which extends fromListBase_StringTest.

In thesetup_colItems_with_4_items method, it will create aVector instance and add four items. This method is also annotated with@Before annotation so it will be called for each test method.

In theVector_firstElement_lastElement method, it will find the first and last elements.

Vector_StringTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertTrue;import java.util.Vector;import org.junit.Before;import org.junit.Test;import org.slf4j.LoggerFactory;public class Vector_StringTest extends ListBase_StringTest {@Beforepublic void setup_colItems_with_4_items() {logger = LoggerFactory.getLogger(this.getClass());colItems = new Vector<String>();assertTrue(colItems.isEmpty());colItems.add("Item1");assertEquals(1, colItems.size());colItems.add("Item2");assertEquals(2, colItems.size());colItems.add("Item3");assertEquals(3, colItems.size());colItems.add("Item4");assertEquals(4, colItems.size());listItems = new Vector<String>();listItems.addAll(colItems);assertEquals(4, listItems.size());listItems.clear();}@Testpublic void Vector_firstElement_lastElement() {Vector<String> vectorObj = (Vector<String>) listItems;vectorObj.add("Mary");vectorObj.add("Zheng");assertEquals("Mary", vectorObj.firstElement());assertEquals("Zheng", vectorObj.lastElement());}}

Executemvn test -Dtest=Vector_StringTest

Output

------------------------------------------------------- T E S T S-------------------------------------------------------Running jcg.zheng.demo.Vector_StringTest05:25:12.397 [main] INFO jcg.zheng.demo.Vector_StringTest - Retrieving items with loop using index and size list05:25:12.443 [main] INFO jcg.zheng.demo.Vector_StringTest - Index: 0 - Item: Tom05:25:12.443 [main] INFO jcg.zheng.demo.Vector_StringTest - Index: 1 - Item: Mary05:25:12.443 [main] INFO jcg.zheng.demo.Vector_StringTest - Index: 2 - Item: Shan05:25:12.443 [main] INFO jcg.zheng.demo.Vector_StringTest - Index: 3 - Item: Zheng05:25:12.447 [main] INFO jcg.zheng.demo.Vector_StringTest - Sorted List05:25:12.448 [main] INFO jcg.zheng.demo.Vector_StringTest - Index: 0 - Item: Mary05:25:12.448 [main] INFO jcg.zheng.demo.Vector_StringTest - Index: 1 - Item: Shan05:25:12.449 [main] INFO jcg.zheng.demo.Vector_StringTest - Index: 2 - Item: Tom05:25:12.449 [main] INFO jcg.zheng.demo.Vector_StringTest - Index: 3 - Item: Zheng05:25:12.488 [main] INFO jcg.zheng.demo.Vector_StringTest - The arraycolItems contains the following elements: [Item1, Item2, Item3, Item4, new Item]05:25:12.493 [main] INFO jcg.zheng.demo.Vector_StringTest - Retrieving items using for loop05:25:12.498 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item105:25:12.498 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item205:25:12.498 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item305:25:12.498 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item405:25:12.500 [main] INFO jcg.zheng.demo.Vector_StringTest - Retrieving items using Java 8 Stream05:25:12.501 [main] INFO jcg.zheng.demo.Vector_StringTest - Item105:25:12.502 [main] INFO jcg.zheng.demo.Vector_StringTest - Item205:25:12.502 [main] INFO jcg.zheng.demo.Vector_StringTest - Item305:25:12.502 [main] INFO jcg.zheng.demo.Vector_StringTest - Item405:25:12.503 [main] INFO jcg.zheng.demo.Vector_StringTest - Retrieving items using iterator05:25:12.504 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item105:25:12.504 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item205:25:12.504 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item305:25:12.504 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item405:25:12.505 [main] INFO jcg.zheng.demo.Vector_StringTest - Retrieving items using iterator05:25:12.507 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item105:25:12.508 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item205:25:12.508 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item305:25:12.508 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item405:25:12.516 [main] INFO jcg.zheng.demo.Vector_StringTest - Removing items using iterator05:25:12.517 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item105:25:12.517 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item205:25:12.518 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item305:25:12.518 [main] INFO jcg.zheng.demo.Vector_StringTest - Item is: Item405:25:12.522 [main] INFO jcg.zheng.demo.Vector_StringTest - The array created after the conversion of our arraycolItems is: [Item1, Item2, Item3, Item4]05:25:12.524 [main] INFO jcg.zheng.demo.Vector_StringTest - default toArray(intfunction):[Item1, Item2, Item3, Item4]Tests run: 17, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.518 secResults :Tests run: 17, Failures: 0, Errors: 0, Skipped: 0[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------[INFO] Total time:  01:09 min[INFO] Finished at: 2019-08-04T05:25:12-05:00[INFO] ------------------------------------------------------------------------C:\MaryZheng\Workspaces\jdk12\java-collections-demo>

5.4 Stack_StringTest

Stack extends from Vector and implementsList interface. It models theStack data structure.

In this step, I will create aStack_StringTest class which extends fromListBase_StringTest.

In thesetup_colItems_with_4_items method, it will create aStack instance and add four items. This method is also annotated with@Before annotation so it will be called for each test method.

In theStack_push_pop method, it will push an element and pop an element based on first-in-last-out order.

Stack_StringTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertTrue;import java.util.Stack;import org.junit.Before;import org.junit.Test;import org.slf4j.LoggerFactory;public class Stack_StringTest extends ListBase_StringTest { @Before public void setup_colItems_with_4_items() { logger = LoggerFactory.getLogger(this.getClass()); colItems = new Stack<String>(); assertTrue(colItems.isEmpty()); colItems.add("Item1"); assertEquals(1, colItems.size()); colItems.add("Item2"); assertEquals(2, colItems.size()); colItems.add("Item3"); assertEquals(3, colItems.size()); colItems.add("Item4"); assertEquals(4, colItems.size()); listItems = new Stack<String>(); listItems.addAll(colItems); assertEquals(4, listItems.size()); listItems.clear(); } @Test public void Stack_push_pop() { ((Stack<String>) listItems).push("Mary"); String item = ((Stack<String>) listItems).pop(); assertEquals("Mary", item); }}

Executemvn test -Dtest=Stack_StringTest

Output

------------------------------------------------------- T E S T S-------------------------------------------------------Running jcg.zheng.demo.Stack_StringTest05:26:53.868 [main] INFO jcg.zheng.demo.Stack_StringTest - Retrieving items with loop using index and size list05:26:53.933 [main] INFO jcg.zheng.demo.Stack_StringTest - Index: 0 - Item: Tom05:26:53.934 [main] INFO jcg.zheng.demo.Stack_StringTest - Index: 1 - Item: Mary05:26:53.935 [main] INFO jcg.zheng.demo.Stack_StringTest - Index: 2 - Item: Shan05:26:53.936 [main] INFO jcg.zheng.demo.Stack_StringTest - Index: 3 - Item: Zheng05:26:53.939 [main] INFO jcg.zheng.demo.Stack_StringTest - Sorted List05:26:53.940 [main] INFO jcg.zheng.demo.Stack_StringTest - Index: 0 - Item: Mary05:26:53.942 [main] INFO jcg.zheng.demo.Stack_StringTest - Index: 1 - Item: Shan05:26:53.943 [main] INFO jcg.zheng.demo.Stack_StringTest - Index: 2 - Item: Tom05:26:53.943 [main] INFO jcg.zheng.demo.Stack_StringTest - Index: 3 - Item: Zheng05:26:53.973 [main] INFO jcg.zheng.demo.Stack_StringTest - The arraycolItems contains the following elements: [Item1, Item2, Item3, Item4, new Item]05:26:53.975 [main] INFO jcg.zheng.demo.Stack_StringTest - Retrieving items using for loop05:26:53.977 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item105:26:53.978 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item205:26:53.978 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item305:26:53.978 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item405:26:53.981 [main] INFO jcg.zheng.demo.Stack_StringTest - Retrieving items using Java 8 Stream05:26:53.984 [main] INFO jcg.zheng.demo.Stack_StringTest - Item105:26:53.984 [main] INFO jcg.zheng.demo.Stack_StringTest - Item205:26:53.984 [main] INFO jcg.zheng.demo.Stack_StringTest - Item305:26:53.984 [main] INFO jcg.zheng.demo.Stack_StringTest - Item405:26:53.985 [main] INFO jcg.zheng.demo.Stack_StringTest - Retrieving items using iterator05:26:53.986 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item105:26:53.986 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item205:26:53.986 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item305:26:53.986 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item405:26:53.987 [main] INFO jcg.zheng.demo.Stack_StringTest - Retrieving items using iterator05:26:53.988 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item105:26:53.988 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item205:26:53.988 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item305:26:53.988 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item405:26:53.994 [main] INFO jcg.zheng.demo.Stack_StringTest - Removing items using iterator05:26:53.995 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item105:26:53.996 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item205:26:53.996 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item305:26:53.996 [main] INFO jcg.zheng.demo.Stack_StringTest - Item is: Item405:26:53.997 [main] INFO jcg.zheng.demo.Stack_StringTest - The array created after the conversion of our arraycolItems is: [Item1, Item2, Item3, Item4]05:26:53.999 [main] INFO jcg.zheng.demo.Stack_StringTest - default toArray(intfunction):[Item1, Item2, Item3, Item4]Tests run: 18, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.616 secResults :Tests run: 18, Failures: 0, Errors: 0, Skipped: 0[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------[INFO] Total time:  10.761 s[INFO] Finished at: 2019-08-04T05:26:54-05:00[INFO] ------------------------------------------------------------------------C:\MaryZheng\Workspaces\jdk12\java-collections-demo>

5.5 ArrayList_POJOTest

In this step, I will create anArrayList_POJOTest class which extends fromCollectionBase_POJOTest.

In thesetup_list_with_4_items method, it will create anArrayList instance and add four items. This method is also annotated with@Before annotation so it will be called for each test method.

ArrayList_POJOTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertTrue;import java.util.ArrayList;import org.junit.Before;import org.slf4j.LoggerFactory;import jcg.zheng.demo.data.SomeData;public class ArrayList_POJOTest extends CollectionBase_POJOTest {@Beforepublic void setup_list_with_4_items() {logger = LoggerFactory.getLogger(this.getClass());list = new ArrayList<SomeData>();assertTrue(list.isEmpty());list.add(buildSomeData(1, "Item1"));assertEquals(1, list.size());list.add(buildSomeData(2, "Item2"));assertEquals(2, list.size());list.add(buildSomeData(3, "Item3"));assertEquals(3, list.size());list.add(buildSomeData(4, "Item4"));assertEquals(4, list.size());}}

Executemvn test -Dtest=ArrayList_POJOTest

Output

------------------------------------------------------- T E S T S-------------------------------------------------------Running jcg.zheng.demo.ArrayList_POJOTest05:28:57.110 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - The arraylist contains the following elements: [SomeData [name=Item1, id=1], SomeData [name=Item2, id=2], SomeData [name=Item3, id=3], SomeData [name=Item4, id=4], SomeData [name=test, id=1]]05:28:57.127 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Retrieving items using for loop05:28:57.129 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item1, id=1]05:28:57.130 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item2, id=2]05:28:57.130 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item3, id=3]05:28:57.131 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item4, id=4]05:28:57.133 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Retrieving items using Java 8 Stream05:28:57.138 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - SomeData [name=Item1, id=1]05:28:57.139 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - SomeData [name=Item2, id=2]05:28:57.140 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - SomeData [name=Item3, id=3]05:28:57.140 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - SomeData [name=Item4, id=4]05:28:57.143 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Retrieving items using iterator05:28:57.145 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item1, id=1]05:28:57.145 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item2, id=2]05:28:57.145 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item3, id=3]05:28:57.145 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item4, id=4]05:28:57.147 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Retrieving items using iterator05:28:57.148 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item1, id=1]05:28:57.148 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item2, id=2]05:28:57.149 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item3, id=3]05:28:57.149 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - Item is: SomeData [name=Item4, id=4]05:28:57.157 [main] INFO jcg.zheng.demo.ArrayList_POJOTest - The array created after the conversion of our arraylist is: [SomeData [name=Item1, id=1], SomeData [name=Item2, id=2], SomeData [name=Item3, id=3], SomeData [name=Item4, id=4]]Tests run: 10, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.473 secResults :Tests run: 10, Failures: 0, Errors: 0, Skipped: 0[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------[INFO] Total time:  10.397 s[INFO] Finished at: 2019-08-04T05:28:57-05:00[INFO] ------------------------------------------------------------------------C:\MaryZheng\Workspaces\jdk12\java-collections-demo>

5.6 HashSet_POJOTest

HashSet implements the Java Set interface. Java Set interface is implemented by HashSet, Treeset and other classes. It is used to store a collection of unique elements.

In this step, I will create aHashSet_POJOTest class which extends fromCollectionBase_POJOTest.

In thesetup_with_4_items method, it will create aHashSet instance and add four items. This method is also annotated with@Before annotation so it will be called for each test method.

In theallow_null method, it will add anull object.

In theSet_no_duplicate method, it will show thatSet does not allow any duplicate elements.

HashSet_POJOTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertTrue;import java.util.HashSet;import java.util.Set;import org.junit.Before;import org.junit.Test;import org.slf4j.LoggerFactory;import jcg.zheng.demo.data.SomeData;public class HashSet_POJOTest extends CollectionBase_POJOTest { @Test public void allow_null() { Set<SomeData> setData = new HashSet<>(); setData.add(null); assertEquals(1, setData.size()); } @Test public void Set_no_duplicate() { Set<SomeData> setData = new HashSet<>(); setData.add(buildSomeData(1, "Tom")); setData.add(buildSomeData(3, "Mary")); setData.add(buildSomeData(2, "Shan")); setData.add(buildSomeData(2, "Shan")); setData.add(buildSomeData(4, "Zheng")); assertEquals(4, setData.size()); } @Before public void setup_with_4_items() { logger = LoggerFactory.getLogger(this.getClass()); list = new HashSet<SomeData>(); assertTrue(list.isEmpty()); list.add(buildSomeData(1, "Item1")); assertEquals(1, list.size()); list.add(buildSomeData(2, "Item2")); assertEquals(2, list.size()); list.add(buildSomeData(3, "Item3")); assertEquals(3, list.size()); list.add(buildSomeData(4, "Item4")); assertEquals(4, list.size()); }}

Executemvn test -Dtest=HashSet_POJOTest

Output

------------------------------------------------------- T E S T S-------------------------------------------------------Running jcg.zheng.demo.HashSet_POJOTest05:30:28.475 [main] INFO jcg.zheng.demo.HashSet_POJOTest - The arraylist contains the following elements: [SomeData [name=Item1, id=1], SomeData [name=Item2, id=2], SomeData [name=Item3, id=3], SomeData [name=Item4, id=4], SomeData [name=test, id=1]]05:30:28.488 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Retrieving items using for loop05:30:28.489 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item1, id=1]05:30:28.489 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item2, id=2]05:30:28.489 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item3, id=3]05:30:28.490 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item4, id=4]05:30:28.491 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Retrieving items using Java 8 Stream05:30:28.494 [main] INFO jcg.zheng.demo.HashSet_POJOTest - SomeData [name=Item1, id=1]05:30:28.496 [main] INFO jcg.zheng.demo.HashSet_POJOTest - SomeData [name=Item2, id=2]05:30:28.497 [main] INFO jcg.zheng.demo.HashSet_POJOTest - SomeData [name=Item3, id=3]05:30:28.497 [main] INFO jcg.zheng.demo.HashSet_POJOTest - SomeData [name=Item4, id=4]05:30:28.498 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Retrieving items using iterator05:30:28.500 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item1, id=1]05:30:28.500 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item2, id=2]05:30:28.500 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item3, id=3]05:30:28.500 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item4, id=4]05:30:28.502 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Retrieving items using iterator05:30:28.503 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item1, id=1]05:30:28.504 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item2, id=2]05:30:28.504 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item3, id=3]05:30:28.505 [main] INFO jcg.zheng.demo.HashSet_POJOTest - Item is: SomeData [name=Item4, id=4]05:30:28.508 [main] INFO jcg.zheng.demo.HashSet_POJOTest - The array created after the conversion of our arraylist is: [SomeData [name=Item1, id=1], SomeData [name=Item2, id=2], SomeData [name=Item3, id=3], SomeData [name=Item4, id=4]]Tests run: 12, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.483 secResults :Tests run: 12, Failures: 0, Errors: 0, Skipped: 0[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------[INFO] Total time:  10.608 s[INFO] Finished at: 2019-08-04T05:30:28-05:00[INFO] ------------------------------------------------------------------------C:\MaryZheng\Workspaces\jdk12\java-collections-demo>

5.7 TreeSet_POJOTest

TreeSet extends fromAbstractSet which implements the Java Set interface. Objects in aTreeSet are stored in a sorted and ascending order.

In this step, I will create aTreeSet_POJOTest class which extends fromCollectionBase_POJOTest.

In thesetup_with_4_items method, it will create aTreeSet instance and add four items. This method is also annotated with@Before annotation so it will be called for each test method.

In theSet_no_duplicate_ordered method, it will show thatTreeSet orders its unique elements.

TreeSet_POJOTest.java

package jcg.zheng.demo;import static org.junit.Assert.assertEquals;import static org.junit.Assert.assertTrue;import java.util.Set;import java.util.TreeSet;import org.junit.Before;import org.junit.Test;import org.slf4j.LoggerFactory;import jcg.zheng.demo.data.SomeData;public class TreeSet_POJOTest extends CollectionBase_POJOTest {@Testpublic void Set_no_duplicate_ordered() {Set<SomeData> testSet = new TreeSet<>();testSet.add(buildSomeData(1, "Tom"));testSet.add(buildSomeData(3, "Mary"));testSet.add(buildSomeData(2, "Shan"));testSet.add(buildSomeData(2, "Shan"));testSet.add(buildSomeData(4, "Zheng"));assertEquals(4, testSet.size());logger.info("TreeSet is ordered");testSet.forEach(item -> {logger.info(item.toString());});}@Beforepublic void setup_with_4_items() {logger = LoggerFactory.getLogger(this.getClass());list = new TreeSet<SomeData>();assertTrue(list.isEmpty());list.add(buildSomeData(1, "Item1"));assertEquals(1, list.size());list.add(buildSomeData(2, "Item2"));assertEquals(2, list.size());list.add(buildSomeData(3, "Item3"));assertEquals(3, list.size());list.add(buildSomeData(4, "Item4"));assertEquals(4, list.size());}}

Executemvn test -Dtest=TreeSet_POJOTest

Output

------------------------------------------------------- T E S T S-------------------------------------------------------Running jcg.zheng.demo.TreeSet_POJOTest05:31:47.416 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - TreeSet is ordered05:31:47.472 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - SomeData [name=Mary, id=3]05:31:47.472 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - SomeData [name=Shan, id=2]05:31:47.472 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - SomeData [name=Tom, id=1]05:31:47.473 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - SomeData [name=Zheng, id=4]05:31:47.499 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - The arraylist contains the following elements: [SomeData [name=Item1, id=1], SomeData [name=Item2, id=2], SomeData [name=Item3, id=3], SomeData [name=Item4, id=4], SomeData [name=test, id=1]]05:31:47.504 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Retrieving items using for loop05:31:47.507 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item1, id=1]05:31:47.508 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item2, id=2]05:31:47.508 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item3, id=3]05:31:47.508 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item4, id=4]05:31:47.511 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Retrieving items using Java 8 Stream05:31:47.514 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - SomeData [name=Item1, id=1]05:31:47.515 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - SomeData [name=Item2, id=2]05:31:47.515 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - SomeData [name=Item3, id=3]05:31:47.516 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - SomeData [name=Item4, id=4]05:31:47.517 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Retrieving items using iterator05:31:47.518 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item1, id=1]05:31:47.518 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item2, id=2]05:31:47.518 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item3, id=3]05:31:47.519 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item4, id=4]05:31:47.520 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Retrieving items using iterator05:31:47.522 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item1, id=1]05:31:47.522 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item2, id=2]05:31:47.523 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item3, id=3]05:31:47.523 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - Item is: SomeData [name=Item4, id=4]05:31:47.528 [main] INFO jcg.zheng.demo.TreeSet_POJOTest - The array created after the conversion of our arraylist is: [SomeData [name=Item1, id=1], SomeData [name=Item2, id=2], SomeData [name=Item3, id=3], SomeData [name=Item4, id=4]]Tests run: 11, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.489 secResults :Tests run: 11, Failures: 0, Errors: 0, Skipped: 0[INFO] ------------------------------------------------------------------------[INFO] BUILD SUCCESS[INFO] ------------------------------------------------------------------------[INFO] Total time:  11.171 s[INFO] Finished at: 2019-08-04T05:31:47-05:00[INFO] ------------------------------------------------------------------------C:\MaryZheng\Workspaces\jdk12\java-collections-demo>

6. Summary

In this article, I covered the fundamental of Java Collections framework, which is part of the core java skillset. I created several test classes to demonstrate how to add, remove, retrieve, clear, sort, and search items from a collection of objects.

Please checkherefor more details about the Big-O differences among these interfaces and classes in the Java collection framework.

7. Related articles

• ArrayList Java Example – How to use arraylist

8. Download the Source Code

This example consists of Maven project to demonstrate the Collections framework, which is part of the core java skillset.

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