Finding All Duplicates in a List in Java- BAEL-6075 (#13291)
* Deep & Shallow Copy * Revert "Deep & Shallow Copy" This reverts commit 62f75b002a21feb90c0b6be02072356b64284283. * duplicates in a list in java * Unit test changes * unit test changes * Tests change after review * Code formatted * code formatted * new module core-java-lang-6 * entry in parent pom and test setup method name change * module renamed * parent pom entry added * pom.xml changes reverted
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package com.baeldung.listduplicate;
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import java.util.ArrayList;
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import java.util.Collections;
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import java.util.HashMap;
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import java.util.HashSet;
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import java.util.List;
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import java.util.Map;
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import java.util.Set;
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import java.util.function.Function;
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import java.util.stream.Collectors;
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public class ListDuplicate {
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public List<Integer> listDuplicateUsingSet(List<Integer> list) {
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List<Integer> duplicates = new ArrayList<>();
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Set<Integer> set = new HashSet<>();
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for (Integer i : list) {
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if (set.contains(i)) {
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duplicates.add(i);
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} else {
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set.add(i);
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}
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}
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return duplicates;
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}
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public List<Integer> listDuplicateUsingMap(List<Integer> list) {
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List<Integer> duplicates = new ArrayList<>();
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Map<Integer, Integer> frequencyMap = new HashMap<>();
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for (Integer number : list) {
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frequencyMap.put(number, frequencyMap.getOrDefault(number, 0) + 1);
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}
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for (int number : frequencyMap.keySet()) {
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if (frequencyMap.get(number) != 1) {
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duplicates.add(number);
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}
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}
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return duplicates;
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}
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public List<Integer> listDuplicateUsingFilterAndSetAdd(List<Integer> list) {
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Set<Integer> elements = new HashSet<Integer>();
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return list.stream()
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.filter(n -> !elements.add(n))
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.collect(Collectors.toList());
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}
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public List<Integer> listDuplicateUsingCollectionsFrequency(List<Integer> list) {
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List<Integer> duplicates = new ArrayList<>();
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Set<Integer> set = list.stream()
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.filter(i -> Collections.frequency(list, i) > 1)
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.collect(Collectors.toSet());
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duplicates.addAll(set);
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return duplicates;
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}
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public List<Integer> listDuplicateUsingMapAndCollectorsGroupingBy(List<Integer> list) {
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List<Integer> duplicates = new ArrayList<>();
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Set<Integer> set = list.stream()
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.collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
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.entrySet()
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.stream()
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.filter(m -> m.getValue() > 1)
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.map(Map.Entry::getKey)
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.collect(Collectors.toSet());
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duplicates.addAll(set);
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return duplicates;
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}
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}
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package com.baeldung.listduplicate;
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import java.util.Arrays;
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import java.util.List;
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import org.junit.Assert;
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import org.junit.BeforeClass;
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import org.junit.Test;
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public class ListDuplicateUnitTest {
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private static ListDuplicate listDuplicate;
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@BeforeClass
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public static void setup() {
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listDuplicate = new ListDuplicate();
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}
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@Test
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public void givenList_whenUsingSet_thenReturnDuplicateElements() {
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List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
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List<Integer> duplicates = listDuplicate.listDuplicateUsingSet(list);
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Assert.assertEquals(duplicates.size(), 2);
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Assert.assertEquals(duplicates.contains(3), true);
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Assert.assertEquals(duplicates.contains(4), true);
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Assert.assertEquals(duplicates.contains(1), false);
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}
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@Test
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public void givenList_whenUsingFrequencyMap_thenReturnDuplicateElements() {
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List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
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List<Integer> duplicates = listDuplicate.listDuplicateUsingMap(list);
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Assert.assertEquals(duplicates.size(), 2);
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Assert.assertEquals(duplicates.contains(3), true);
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Assert.assertEquals(duplicates.contains(4), true);
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Assert.assertEquals(duplicates.contains(1), false);
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}
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@Test
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public void givenList_whenUsingFilterAndSetAdd_thenReturnDuplicateElements() {
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List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
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List<Integer> duplicates = listDuplicate.listDuplicateUsingFilterAndSetAdd(list);
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Assert.assertEquals(duplicates.size(), 2);
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Assert.assertEquals(duplicates.contains(3), true);
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Assert.assertEquals(duplicates.contains(4), true);
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Assert.assertEquals(duplicates.contains(1), false);
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}
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@Test
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public void givenList_whenUsingCollectionsFrequency_thenReturnDuplicateElements() {
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List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
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List<Integer> duplicates = listDuplicate.listDuplicateUsingCollectionsFrequency(list);
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Assert.assertEquals(duplicates.size(), 2);
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Assert.assertEquals(duplicates.contains(3), true);
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Assert.assertEquals(duplicates.contains(4), true);
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Assert.assertEquals(duplicates.contains(1), false);
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}
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@Test
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public void givenList_whenUsingMapAndCollectorsGroupingBy_thenReturnDuplicateElements() {
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List<Integer> list = Arrays.asList(1, 2, 3, 3, 4, 4, 5);
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List<Integer> duplicates = listDuplicate.listDuplicateUsingCollectionsFrequency(list);
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Assert.assertEquals(duplicates.size(), 2);
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Assert.assertEquals(duplicates.contains(3), true);
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Assert.assertEquals(duplicates.contains(4), true);
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Assert.assertEquals(duplicates.contains(1), false);
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}
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}
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