BAEL-5646: String permutation improvements (#12613)

algorithms-modules/algorithms-miscellaneous-4/src/main/java/com/baeldung/algorithms/stringpermutation/Helper.java

@@ -1,14 +1,20 @@
 package com.baeldung.algorithms.stringpermutation;
 
+import java.util.Collection;
 import java.util.List;
 import java.util.stream.Collectors;
 
-public class ArrayHelper {
+public class Helper {
 
-    private ArrayHelper() {
+    private Helper() {
     }
 
     static List<Character> toCharacterList(final String string) {
         return string.chars().mapToObj(s -> ((char) s)).collect(Collectors.toList());
     }
+
+    static String toString(Collection<Character> collection) {
+        return collection.stream().map(Object::toString).collect(Collectors.joining());
+    }
+
 }

algorithms-modules/algorithms-miscellaneous-4/src/main/java/com/baeldung/algorithms/stringpermutation/StringPermutationsApache.java

@@ -1,20 +1,29 @@
 package com.baeldung.algorithms.stringpermutation;
 
-import java.util.Collection;
+import java.util.ArrayList;
 import java.util.List;
+import java.util.stream.Collectors;
 import org.apache.commons.collections4.CollectionUtils;
 import org.apache.commons.collections4.iterators.PermutationIterator;
 
 public class StringPermutationsApache {
 
-    public Collection<List<Character>> eagerPermutationWithRepetitions(final String string) {
+    public List<String> eagerPermutationWithRepetitions(final String string) {
-        final List<Character> characters = ArrayHelper.toCharacterList(string);
+        final List<Character> characters = Helper.toCharacterList(string);
-        return CollectionUtils.permutations(characters);
+        return CollectionUtils.permutations(characters)
+            .stream()
+            .map(Helper::toString)
+            .collect(Collectors.toList());
     }
 
-    public PermutationIterator<Character> lazyPermutationWithoutRepetitions(final String string) {
+    public List<String> lazyPermutationWithoutRepetitions(final String string) {
-        final List<Character> characters = ArrayHelper.toCharacterList(string);
+        final List<Character> characters = Helper.toCharacterList(string);
-        return new PermutationIterator<>(characters);
+        final PermutationIterator<Character> permutationIterator = new PermutationIterator<>(characters);
+        final List<String> result = new ArrayList<>();
+        while (permutationIterator.hasNext()) {
+            result.add(Helper.toString(permutationIterator.next()));
+        }
+        return result;
     }
 
 }

algorithms-modules/algorithms-miscellaneous-4/src/main/java/com/baeldung/algorithms/stringpermutation/StringPermutationsCombinatoricsLib.java

@@ -7,19 +7,21 @@ import org.paukov.combinatorics3.PermutationGenerator.TreatDuplicatesAs;
 
 public class StringPermutationsCombinatoricsLib {
 
-    public List<List<Character>> permutationWithoutRepetitions(final String string) {
+public List<String> permutationWithoutRepetitions(final String string) {
-        List<Character> chars = ArrayHelper.toCharacterList(string);
+    List<Character> chars = Helper.toCharacterList(string);
     return Generator.permutation(chars)
         .simple()
         .stream()
+        .map(Helper::toString)
         .collect(Collectors.toList());
-    }
+}
 
-    public List<List<Character>> permutationWithRepetitions(final String string) {
+    public List<String> permutationWithRepetitions(final String string) {
-        List<Character> chars = ArrayHelper.toCharacterList(string);
+        List<Character> chars = Helper.toCharacterList(string);
         return Generator.permutation(chars)
             .simple(TreatDuplicatesAs.IDENTICAL)
             .stream()
+            .map(Helper::toString)
             .collect(Collectors.toList());
     }
 

algorithms-modules/algorithms-miscellaneous-4/src/main/java/com/baeldung/algorithms/stringpermutation/StringPermutationsGuava.java

@@ -1,18 +1,22 @@
 package com.baeldung.algorithms.stringpermutation;
 
 import com.google.common.collect.Collections2;
-import java.util.Collection;
 import java.util.List;
+import java.util.stream.Collectors;
 
 public class StringPermutationsGuava {
 
-    public Collection<List<Character>> permutationWithRepetitions(final String string) {
+    public List<String> permutationWithRepetitions(final String string) {
-        final List<Character> characters = ArrayHelper.toCharacterList(string);
+        final List<Character> characters = Helper.toCharacterList(string);
-        return Collections2.permutations(characters);
+        return Collections2.permutations(characters).stream()
+            .map(Helper::toString)
+            .collect(Collectors.toList());
+    }
+    public List<String> permutationWithoutRepetitions(final String string) {
+        final List<Character> characters = Helper.toCharacterList(string);
+        return Collections2.orderedPermutations(characters).stream()
+            .map(Helper::toString)
+            .collect(Collectors.toList());
     }
-public Collection<List<Character>> permutationWithoutRepetitions(final String string) {
-    final List<Character> characters = ArrayHelper.toCharacterList(string);
-    return Collections2.orderedPermutations(characters);
-}
 
 }

algorithms-modules/algorithms-miscellaneous-4/src/test/java/com/baeldung/algorithms/stringpermutation/HelperUnitTest.java

@@ -0,0 +1,36 @@
+package com.baeldung.algorithms.stringpermutation;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class HelperUnitTest {
+
+    @ParameterizedTest
+    @MethodSource("stringProvider")
+    void toListTest(String value, List<Character> expected) {
+        final List<Character> actual = Helper.toCharacterList(value);
+        assertThat(expected).isEqualTo(actual);
+    }
+
+    @ParameterizedTest
+    @MethodSource("stringProvider")
+    void toStringTest(String expected, List<Character> value) {
+        final String actual = Helper.toString(value);
+        assertThat(expected).isEqualTo(actual);
+    }
+
+    static Stream<Arguments> stringProvider() {
+        return Stream.of(
+            Arguments.of("hello", Arrays.asList('h', 'e', 'l', 'l', 'o')),
+            Arguments.of("abc", Arrays.asList('a','b','c')),
+            Arguments.of("12345", Arrays.asList('1', '2', '3', '4', '5'))
+        );
+    }
+
+}

algorithms-modules/algorithms-miscellaneous-4/src/test/java/com/baeldung/algorithms/stringpermutation/StringPermutationsApacheUnitTest.java

@@ -2,22 +2,19 @@ package com.baeldung.algorithms.stringpermutation;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
-import java.util.Collection;
 import java.util.List;
-import org.apache.commons.collections4.iterators.PermutationIterator;
-import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.CsvSource;
 
 class StringPermutationsApacheUnitTest {
 
+    @ParameterizedTest
     @CsvSource({"abc, 6",
                 "hello, 120",
                 "aaaaaa, 720"})
-    @DisplayName("Apache permutation for ")
     void testPermutationsWithRepetitions(String string, int numberOfPermutations) {
         StringPermutationsApache permutationGenerator = new StringPermutationsApache();
-        final Collection<List<Character>> permutations = permutationGenerator.eagerPermutationWithRepetitions(string);
+        final List<String> permutations = permutationGenerator.eagerPermutationWithRepetitions(string);
         final int size = permutations.size();
         assertThat(permutations)
             .as("\"%s\" should have %d permutation, but had %d", string, numberOfPermutations, size)
@@ -30,12 +27,8 @@ class StringPermutationsApacheUnitTest {
         "aaaaaa, 720"})
     void testPermutationsWithoutRepetitions(String string, int numberOfPermutations) {
         StringPermutationsApache permutationGenerator = new StringPermutationsApache();
-        final PermutationIterator<Character> permutations = permutationGenerator.lazyPermutationWithoutRepetitions(string);
+        final List<String> permutations = permutationGenerator.lazyPermutationWithoutRepetitions(string);
-        int size = 0;
+        int size = permutations.size();
-        while (permutations.hasNext()) {
-            permutations.next();
-            ++size;
-        }
         assertThat(size)
             .as("\"%s\" should have %d permutation, but had %d", string, numberOfPermutations, size)
             .isEqualTo(numberOfPermutations);

algorithms-modules/algorithms-miscellaneous-4/src/test/java/com/baeldung/algorithms/stringpermutation/StringPermutationsCombinatoricsLibUnitTest.java

@@ -14,7 +14,7 @@ class StringPermutationsCombinatoricsLibUnitTest {
                 "aaaaaa, 720"})
     void testPermutationsWithRepetitions(String string, int numberOfPermutations) {
         StringPermutationsCombinatoricsLib permutationGenerator = new StringPermutationsCombinatoricsLib();
-        final List<List<Character>> permutations = permutationGenerator.permutationWithRepetitions(string);
+        final List<String> permutations = permutationGenerator.permutationWithRepetitions(string);
         final int size = permutations.size();
         assertThat(permutations)
             .as("\"%s\" should have %d permutation, but had %d", string, numberOfPermutations, size)
@@ -27,7 +27,7 @@ class StringPermutationsCombinatoricsLibUnitTest {
                 "aaaaaa, 1"})
     void testPermutationsWithoutRepetitions(String string, int numberOfPermutations) {
         StringPermutationsCombinatoricsLib permutationGenerator = new StringPermutationsCombinatoricsLib();
-        final List<List<Character>> permutations = permutationGenerator.permutationWithoutRepetitions(string);
+        final List<String> permutations = permutationGenerator.permutationWithoutRepetitions(string);
         final int size = permutations.size();
         assertThat(permutations)
             .as("\"%s\" should have %d permutation, but had %d", string, numberOfPermutations, size)

algorithms-modules/algorithms-miscellaneous-4/src/test/java/com/baeldung/algorithms/stringpermutation/StringPermutationsGuavaUnitTest.java

@@ -2,7 +2,6 @@ package com.baeldung.algorithms.stringpermutation;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
-import java.util.Collection;
 import java.util.List;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.CsvSource;
@@ -15,7 +14,7 @@ class StringPermutationsGuavaUnitTest {
         "aaaaaa, 720"})
     void testPermutationsWithRepetitions(String string, int numberOfPermutations) {
         StringPermutationsGuava permutationGenerator = new StringPermutationsGuava();
-        final Collection<List<Character>> permutations = permutationGenerator.permutationWithRepetitions(string);
+        final List<String> permutations = permutationGenerator.permutationWithRepetitions(string);
         final int size = permutations.size();
         assertThat(permutations)
             .as("\"%s\" should have %d permutation, but had %d", string, numberOfPermutations, size)
@@ -28,7 +27,7 @@ class StringPermutationsGuavaUnitTest {
         "aaaaaa, 1"})
     void testPermutationsWithoutRepetitions(String string, int numberOfPermutations) {
         StringPermutationsGuava permutationGenerator = new StringPermutationsGuava();
-        final Collection<List<Character>> permutations = permutationGenerator.permutationWithoutRepetitions(string);
+        final List<String> permutations = permutationGenerator.permutationWithoutRepetitions(string);
         final int size = permutations.size();
         assertThat(permutations)
             .as("\"%s\" should have %d permutation, but had %d", string, numberOfPermutations, size)