Merge branch 'eugenp:master' into master

algorithms-modules/algorithms-miscellaneous-7/pom.xml

@@ -13,4 +13,22 @@
         <version>1.0.0-SNAPSHOT</version>
     </parent>
 
+    <properties>
+        <jmh.version>1.35</jmh.version>
+    </properties>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.openjdk.jmh</groupId>
+            <artifactId>jmh-core</artifactId>
+            <version>${jmh.version}</version>
+        </dependency>
+
+        <dependency>
+            <groupId>org.openjdk.jmh</groupId>
+            <artifactId>jmh-generator-annprocess</artifactId>
+            <version>${jmh.version}</version>
+        </dependency>
+    </dependencies>
+
 </project>

algorithms-modules/algorithms-miscellaneous-7/src/main/java/com/baeldung/algorithms/perfectnumber/PerfectNumber.java

@@ -0,0 +1,50 @@
+package com.baeldung.algorithms.perfectnumber;
+
+import java.util.stream.IntStream;
+
+class PerfectNumber {
+
+    public static boolean isPerfectBruteForce(int number) {
+        int sum = 0;
+        for (int i = 1; i <= number / 2; i++) {
+            if (number % i == 0) {
+                sum += i;
+            }
+        }
+        return sum == number;
+    }
+
+    public static boolean isPerfectStream(int number) {
+        int sum = IntStream.rangeClosed(2, (int) Math.sqrt(number))
+                .filter(test -> number % test == 0)
+                .reduce(1, (s, test) -> s + test + (number / test));
+        return sum == number;
+    }
+
+    public static boolean isPerfectEuclidEuler(int number) {
+        int p = 2;
+        int perfectNumber = (int) (Math.pow(2, p - 1) * (Math.pow(2, p) - 1));
+        while (perfectNumber <= number) {
+            if (perfectNumber == number) {
+                return true;
+            }
+            p++;
+            perfectNumber = (int) (Math.pow(2, p - 1) * (Math.pow(2, p) - 1));
+        }
+        return false;
+    }
+
+    public static boolean isPerfectEuclidEulerUsingShift(int number) {
+        int p = 2;
+        int perfectNumber = (2 << (p - 1)) * ((2 << p) - 1);
+        while (perfectNumber <= number) {
+            if (perfectNumber == number) {
+                return true;
+            }
+            p++;
+            perfectNumber = (2 << (p - 1)) * ((2 << p) - 1);
+        }
+        return false;
+    }
+
+}

algorithms-modules/algorithms-miscellaneous-7/src/main/java/com/baeldung/algorithms/perfectnumber/PerfectNumberBenchmark.java

@@ -0,0 +1,41 @@
+package com.baeldung.algorithms.perfectnumber;
+
+import org.openjdk.jmh.annotations.Benchmark;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.runner.Runner;
+import org.openjdk.jmh.runner.options.Options;
+import org.openjdk.jmh.runner.options.OptionsBuilder;
+
+@State(Scope.Benchmark)
+public class PerfectNumberBenchmark {
+
+    @Benchmark
+    public boolean bruteForceBenchmark() {
+        return PerfectNumber.isPerfectBruteForce(33550336);
+    }
+
+    @Benchmark
+    public boolean streamBenchmark() {
+        return PerfectNumber.isPerfectStream(33550336);
+    }
+
+    @Benchmark
+    public boolean euclidEulerBenchmark() {
+        return PerfectNumber.isPerfectEuclidEuler(33550336);
+    }
+
+    @Benchmark
+    public boolean euclidEulerUsingShiftBenchmark() {
+        return PerfectNumber.isPerfectEuclidEulerUsingShift(33550336);
+    }
+
+    public static void main(String[] args) throws Exception {
+        Options options = new OptionsBuilder()
+                .include(PerfectNumberBenchmark.class.getSimpleName())
+                .forks(1)
+                .build();
+
+        new Runner(options).run();
+    }
+}

algorithms-modules/algorithms-miscellaneous-7/src/test/java/com/baeldung/algorithms/perfectnumber/PerfectNumberUnitTest.java

@@ -0,0 +1,68 @@
+package com.baeldung.algorithms.perfectnumber;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class PerfectNumberUnitTest {
+    @Test
+    void givenPerfectNumber_whenCheckingIsPerfectBruteForce_thenReturnTrue() {
+        assertTrue(PerfectNumber.isPerfectBruteForce(6));
+    }
+
+    @Test
+    void givenNonPerfectNumber_whenCheckingIsPerfectBruteForce_thenReturnFalse() {
+        assertFalse(PerfectNumber.isPerfectBruteForce(10));
+    }
+
+    @Test
+    void givenNegativeNumber_whenCheckingIsPerfectBruteForce_thenReturnFalse() {
+        assertFalse(PerfectNumber.isPerfectBruteForce(-28));
+    }
+
+    @Test
+    void givenPerfectNumber_whenCheckingIsPerfectStream_thenReturnTrue() {
+        assertTrue(PerfectNumber.isPerfectStream(28));
+    }
+
+    @Test
+    void givenNonPerfectNumber_whenCheckingIsPerfectStream_thenReturnFalse() {
+        assertFalse(PerfectNumber.isPerfectStream(10));
+    }
+
+    @Test
+    void givenNegativeNumber_whenCheckingIsPerfectStream_thenReturnFalse() {
+        assertFalse(PerfectNumber.isPerfectStream(-6));
+    }
+
+    @Test
+    void givenPerfectNumber_whenCheckingIsPerfectEuclidEuler_thenReturnTrue() {
+        assertTrue(PerfectNumber.isPerfectEuclidEuler(28));
+    }
+
+    @Test
+    void givenNonPerfectNumber_whenCheckingIsPerfectEuclidEuler_thenReturnFalse() {
+        assertFalse(PerfectNumber.isPerfectEuclidEuler(10));
+    }
+
+    @Test
+    void givenNegativeNumber_whenCheckingIsPerfectEuclidEuler_thenReturnFalse() {
+        assertFalse(PerfectNumber.isPerfectEuclidEuler(-6));
+    }
+
+    @Test
+    void givenPerfectNumber_whenCheckingIsPerfectEuclidEulerUsingShift_thenReturnTrue() {
+        assertTrue(PerfectNumber.isPerfectEuclidEulerUsingShift(28));
+    }
+
+    @Test
+    void givenNonPerfectNumber_whenCheckingIsPerfectEuclidEulerUsingShift_thenReturnFalse() {
+        assertFalse(PerfectNumber.isPerfectEuclidEulerUsingShift(10));
+    }
+
+    @Test
+    void givenNegativeNumber_whenCheckingIsPerfectEuclidEulerUsingShift_thenReturnFalse() {
+        assertFalse(PerfectNumber.isPerfectEuclidEulerUsingShift(-6));
+    }
+}

core-java-modules/core-java-collections-5/README.md

@@ -9,7 +9,6 @@
 - [Skipping the First Iteration in Java](https://www.baeldung.com/java-skip-first-iteration)
 - [Remove Elements From a Queue Using Loop](https://www.baeldung.com/java-remove-elements-queue)
 - [Intro to Vector Class in Java](https://www.baeldung.com/java-vector-guide)
-- [HashSet toArray() Method in Java](https://www.baeldung.com/java-hashset-toarray)
 - [Time Complexity of Java Collections Sort in Java](https://www.baeldung.com/java-time-complexity-collections-sort)
 - [Check if List Contains at Least One Enum](https://www.baeldung.com/java-list-check-enum-presence)
 - [Comparison of for Loops and Iterators](https://www.baeldung.com/java-for-loops-vs-iterators)

core-java-modules/core-java-collections-5/pom.xml

@@ -55,8 +55,8 @@
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>
                 <configuration>
-                    <source>9</source>
-                    <target>9</target>
+                    <source>16</source>
+                    <target>16</target>
                 </configuration>
             </plugin>
         </plugins>

core-java-modules/core-java-collections-6/README.md

@@ -0,0 +1,5 @@
+=========
+
+## Core Java Collections Cookbooks and Examples
+
+### Relevant Articles:

core-java-modules/core-java-collections-6/pom.xml

@@ -0,0 +1,71 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns="http://maven.apache.org/POM/4.0.0"
+    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>
+    <artifactId>core-java-collections-6</artifactId>
+    <packaging>jar</packaging>
+    <name>core-java-collections-6</name>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.platform</groupId>
+            <artifactId>junit-platform-runner</artifactId>
+            <version>${junit-platform.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>${junit.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.vintage</groupId>
+            <artifactId>junit-vintage-engine</artifactId>
+            <version>${junit.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.roaringbitmap</groupId>
+            <artifactId>RoaringBitmap</artifactId>
+            <version>${roaringbitmap.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.openjdk.jmh</groupId>
+            <artifactId>jmh-generator-annprocess</artifactId>
+            <version>${jmh.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.openjdk.jmh</groupId>
+            <artifactId>jmh-core</artifactId>
+            <version>${jmh.version}</version>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <source>9</source>
+                    <target>9</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <properties>
+        <junit.version>5.9.2</junit.version>
+        <roaringbitmap.version>0.9.38</roaringbitmap.version>
+        <jmh.version>1.36</jmh.version>
+    </properties>
+
+</project>

core-java-modules/core-java-collections-6/src/test/java/com/baeldung/listiteration/ListIterationUnitTest.java

@@ -0,0 +1,71 @@
+package com.baeldung.listiteration;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.*;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.assertIterableEquals;
+
+public class ListIterationUnitTest {
+
+    List<String> programmingLanguages = new ArrayList<>(List.of("Java", "Python", "C++"));
+    List<Integer> numbers = new ArrayList<>(List.of(1, 2, 3));
+
+   @Test
+    public void givenStringList_whenAddElementWithListIterator_thenModifiedList() {
+        ListIterator<String> listIterator = programmingLanguages.listIterator();
+        while (listIterator.hasNext()) {
+            String language = listIterator.next();
+            if (language.equals("Python")) {
+                listIterator.add("JavaScript");
+            }
+        }
+
+        assertIterableEquals(Arrays.asList("Java", "Python", "JavaScript", "C++"), programmingLanguages);
+    }
+
+    @Test
+    public void givenNumericalList_whenMultiplyElementWithListIterator_thenModifiedList() {
+        ListIterator<Integer> listIterator = numbers.listIterator();
+        while (listIterator.hasNext()) {
+            int num = listIterator.next();
+            if (num == 2) {
+                listIterator.add(num * 10);
+            }
+        }
+        assertIterableEquals(Arrays.asList(1, 2, 20, 3), numbers);
+    }
+
+    @Test
+    public void givenStringList_whenAddElementWithEnhancedForLoopAndCopy_thenModifiedList() {
+        List<String> copyOfWords = new ArrayList<>(programmingLanguages);
+        for (String word : copyOfWords) {
+            programmingLanguages.add(word.toUpperCase()); // Modified: Convert to uppercase
+        }
+        assertIterableEquals(Arrays.asList("Java", "Python", "C++", "JAVA", "PYTHON", "C++"), programmingLanguages);
+    }
+
+    @Test
+    public void givenNumericalList_whenMultiplyElementWithEnhancedForLoopAndCopy_thenModifiedList() {
+        List<Integer> copyOfNumbers = new ArrayList<>(numbers);
+        for (int num : copyOfNumbers) {
+            numbers.add(num * 2);
+        }
+        assertIterableEquals(Arrays.asList(1, 2, 3, 2, 4, 6), numbers);
+    }
+
+    @Test
+    public void givenStringList_whenConvertToUpperCaseWithJava8Stream_thenModifiedList() {
+        programmingLanguages = programmingLanguages.stream().map(String::toUpperCase).collect(Collectors.toList());
+        assertIterableEquals(Arrays.asList("JAVA", "PYTHON", "C++"), programmingLanguages);
+    }
+
+    @Test
+    public void givenNumericalList_whenMultiplyByThreeWithJava8Stream_thenModifiedList() {
+        numbers = numbers.stream().map(num -> num * 3).collect(Collectors.toList());
+        assertIterableEquals(Arrays.asList(3, 6, 9), numbers);
+    }
+
+}

core-java-modules/core-java-collections-conversions-3/README.md

@@ -5,3 +5,4 @@ This module contains articles about conversions among Collection types in Java.
 ### Relevant Articles:
 - [Converting HashMap Values to an ArrayList in Java](https://www.baeldung.com/java-hashmap-arraylist)
 - [Joining a List<String> in Java With Commas and “and”](https://www.baeldung.com/java-string-concatenation-natural-language)
+- [HashSet toArray() Method in Java](https://www.baeldung.com/java-hashset-toarray)

core-java-modules/core-java-collections-list-2/src/test/java/com/baeldung/java/listInitialization/ListInitializationUnitTest.java

@@ -1,59 +0,0 @@
-package com.baeldung.java.listInitialization;
-
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.List;
-import java.util.stream.Collectors;
-import java.util.stream.Stream;
-
-import lombok.extern.java.Log;
-
-import org.junit.Assert;
-import org.junit.Test;
-
-@Log
-public class ListInitializationUnitTest {
-
-    @Test
-    public void givenAnonymousInnerClass_thenInitialiseList() {
-        List<String> cities = new ArrayList() {
-            {
-                add("New York");
-                add("Rio");
-                add("Tokyo");
-            }
-        };
-
-        Assert.assertTrue(cities.contains("New York"));
-    }
-
-    @Test
-    public void givenArraysAsList_thenInitialiseList() {
-        List<String> list = Arrays.asList("foo", "bar");
-
-        Assert.assertTrue(list.contains("foo"));
-    }
-
-    @Test(expected = UnsupportedOperationException.class)
-    public void givenArraysAsList_whenAdd_thenUnsupportedException() {
-        List<String> list = Arrays.asList("foo", "bar");
-
-        list.add("baz");
-    }
-
-    @Test
-    public void givenArraysAsList_whenCreated_thenShareReference() {
-        String[] array = { "foo", "bar" };
-        List<String> list = Arrays.asList(array);
-        array[0] = "baz";
-        Assert.assertEquals("baz", list.get(0));
-    }
-
-    @Test
-    public void givenStream_thenInitializeList() {
-        List<String> list = Stream.of("foo", "bar")
-            .collect(Collectors.toList());
-
-        Assert.assertTrue(list.contains("foo"));
-    }
-}

core-java-modules/core-java-collections-list-2/src/test/java/com/baeldung/java/listinitialization/ListInitializationUnitTest.java

@@ -0,0 +1,91 @@
+package com.baeldung.java.listinitialization;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.junit.Test;
+
+import lombok.extern.java.Log;
+
+@Log
+public class ListInitializationUnitTest {
+
+    @Test
+    public void givenAnonymousInnerClass_thenInitialiseList() {
+        List<String> cities = new ArrayList() {
+            {
+                add("New York");
+                add("Rio");
+                add("Tokyo");
+            }
+        };
+
+        assertTrue(cities.contains("New York"));
+    }
+
+    @Test
+    public void givenArraysAsList_thenInitialiseList() {
+        List<String> list = Arrays.asList("foo", "bar");
+
+        assertTrue(list.contains("foo"));
+    }
+
+    @Test(expected = UnsupportedOperationException.class)
+    public void givenArraysAsList_whenAdd_thenUnsupportedException() {
+        List<String> list = Arrays.asList("foo", "bar");
+
+        list.add("baz");
+    }
+
+    @Test
+    public void givenArraysAsList_whenUsingArrayListConstructor_thenWeCanAddOrRemove() {
+        List<String> list = new ArrayList<>(Arrays.asList("foo", "bar"));
+
+        list.add("baz");
+        assertEquals(List.of("foo", "bar","baz"), list);
+        
+        list.remove("baz");
+        assertEquals(List.of("foo", "bar"), list);
+    }
+
+    @Test
+    public void givenArraysAsList_whenCreated_thenShareReference() {
+        String[] array = { "foo", "bar" };
+        List<String> list = Arrays.asList(array);
+        array[0] = "baz";
+        assertEquals("baz", list.get(0));
+    }
+
+    @Test
+    public void givenIntNumbers_whenRequiredLong_thenCastAutomatically() {
+        int intNum = 42;
+        long longNum = intNum;
+
+        assertEquals(42L, longNum);
+    }
+
+    @Test
+    public void givenArrayAsList_whenRequiredLongList_thenGetExpectedResult() {
+        List<Long> listOfLongFixedSize = Arrays.asList(1L, 2L, 3L);
+        List<Long> listOfLong = new ArrayList<>(Arrays.asList(1L, 2L, 3L));
+
+        List<Long> expected = List.of(1L, 2L, 3L);
+
+        assertEquals(expected, listOfLongFixedSize);
+        assertEquals(expected, listOfLong);
+    }
+
+    @Test
+    public void givenStream_thenInitializeList() {
+        List<String> list = Stream.of("foo", "bar")
+            .collect(Collectors.toList());
+
+        assertTrue(list.contains("foo"));
+    }
+}

core-java-modules/core-java-concurrency-advanced-3/pom.xml

@@ -86,7 +86,7 @@
         <maven.compiler.target>1.8</maven.compiler.target>
         <jcabi-aspects.version>0.22.6</jcabi-aspects.version>
         <aspectjrt.version>1.9.20.1</aspectjrt.version>
-        <cactoos.version>0.43</cactoos.version>
+        <cactoos.version>0.55.0</cactoos.version>
         <ea-async.version>1.2.3</ea-async.version>
         <jcabi-maven-plugin.version>0.14.1</jcabi-maven-plugin.version>
         <aspectjtools.version>1.9.20.1</aspectjtools.version>

core-java-modules/core-java-lang-math-3/README.md

@@ -7,15 +7,11 @@
 - [Evaluating a Math Expression in Java](https://www.baeldung.com/java-evaluate-math-expression-string)
 - [Swap Two Variables in Java](https://www.baeldung.com/java-swap-two-variables)
 - [Java Program to Find the Roots of a Quadratic Equation](https://www.baeldung.com/roots-quadratic-equation)
-- [Create a BMI Calculator in Java](https://www.baeldung.com/java-body-mass-index-calculator)
 - [Java Program to Calculate the Standard Deviation](https://www.baeldung.com/java-calculate-standard-deviation)
 - [Java Program to Print Pascal’s Triangle](https://www.baeldung.com/java-pascal-triangle)
 - [Java Money and the Currency API](http://www.baeldung.com/java-money-and-currency)
 - [Clamp Function in Java](https://www.baeldung.com/java-clamp-function)
 - [Creating a Magic Square in Java](https://www.baeldung.com/java-magic-square)
-- [Check if a Point Is Between Two Points Drawn on a Straight Line in Java](https://www.baeldung.com/java-check-point-straight-line)
 - [Validate if a String Is a Valid Geo Coordinate](https://www.baeldung.com/java-geo-coordinates-validation)
-- [Rotate a Vertex Around a Certain Point in Java](https://www.baeldung.com/java-rotate-vertex-around-point)
 - [Calculating the Power of Any Number in Java Without Using Math pow() Method](https://www.baeldung.com/java-calculating-the-power-without-math-pow)
-- [Solving Rod Cutting Problem in Java](https://www.baeldung.com/java-rod-cutting-problem)
 - More articles: [[<-- Prev]](/core-java-modules/core-java-lang-math-2)

core-java-modules/core-java-lang-math-4/README.md

@@ -2,3 +2,7 @@
 
 ### Relevant articles:
 - [Calculate Percentiles in Java](https://www.baeldung.com/java-compute-percentiles)
+- [Solving Rod Cutting Problem in Java](https://www.baeldung.com/java-rod-cutting-problem)
+- [Rotate a Vertex Around a Certain Point in Java](https://www.baeldung.com/java-rotate-vertex-around-point)
+- [Create a BMI Calculator in Java](https://www.baeldung.com/java-body-mass-index-calculator)
+- [Check if a Point Is Between Two Points Drawn on a Straight Line in Java](https://www.baeldung.com/java-check-point-straight-line)

core-java-modules/core-java-lang-math-4/pom.xml

@@ -12,4 +12,8 @@
         <version>0.0.1-SNAPSHOT</version>
     </parent>
 
+    <properties>
+        <java.version>17</java.version>
+    </properties>
+
 </project>

core-java-modules/core-java-lang-oop-others/src/main/java/com/baeldung/passclassasparameter/Example.java

@@ -0,0 +1,13 @@
+package com.baeldung.passclassasparameter;
+
+public class Example {
+    public static void processClass(Class<?> clazz) {
+        System.out.println("Processing class: " + clazz.getName());
+    }
+
+    public static void main(String[] args) {
+        processClass(String.class);
+        processClass(Integer.class);
+        processClass(Double.class);
+    }
+}

core-java-modules/core-java-lang-oop-others/src/main/java/com/baeldung/passclassasparameter/GenericExample.java

@@ -0,0 +1,22 @@
+package com.baeldung.passclassasparameter;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class GenericExample {
+    public static <T> void printListElements(Class<T> clazz, List<T> list) {
+        System.out.println("Elements of " + clazz.getSimpleName() + " list:");
+        for (T element : list) {
+            System.out.println(element);
+        }
+    }
+
+    public static void main(String[] args) {
+        List<String> stringList = new ArrayList<>();
+        stringList.add("Java");
+        stringList.add("is");
+        stringList.add("awesome");
+
+        printListElements(String.class, stringList);
+    }
+}

core-java-modules/core-java-lang-oop-others/src/main/java/com/baeldung/passclassasparameter/ReflectionExample.java

@@ -0,0 +1,21 @@
+package com.baeldung.passclassasparameter;
+
+import java.lang.reflect.Method;
+
+public class ReflectionExample {
+    public static void processClass(Class<?> clazz, String methodName) throws Exception {
+        Method method = clazz.getMethod(methodName);
+        Object instance = clazz.getDeclaredConstructor().newInstance();
+        method.invoke(instance);
+    }
+
+    public static void main(String[] args) throws Exception {
+        processClass(ReflectionTarget.class, "sayHello");
+    }
+}
+
+class ReflectionTarget {
+    public void sayHello() {
+        System.out.println("Hello, Reflection!");
+    }
+}

core-java-modules/core-java-numbers-7/src/main/java/com/baeldung/convertphonenumberinwordstonumber/UseHashMapToConvertPhoneNumberInWordsToNumber.java

@@ -0,0 +1,43 @@
+import java.util.Map;
+
+public class UseHashMapToConvertPhoneNumberInWordsToNumber {
+    private static Map<String, Integer> multipliers = Map.of("double",2,
+            "triple", 3,
+            "quadruple", 4);
+    private static Map<String, String> digits = Map.of("zero","1",
+            "one", "1",
+            "two", "2",
+            "three", "3",
+            "four", "4",
+            "five", "5",
+            "six", "6",
+            "seven", "7",
+            "eight", "8",
+            "nine", "9");
+
+
+    public static String convertPhoneNumberInWordsToNumber(String phoneNumberInWord) {
+
+        StringBuilder output = new StringBuilder();
+        Integer currentMultiplier = null;
+        String[] words = phoneNumberInWord.split(" ");
+
+        for (String word : words) {
+            Integer multiplier = multipliers.get(word);
+            if (multiplier != null) {
+                if (currentMultiplier != null) {
+                    throw new IllegalArgumentException("Cannot have consecutive multipliers, at: " + word);
+                }
+                currentMultiplier = multiplier;
+            } else {
+                String digit = digits.get(word);
+                if (digit == null) {
+                    throw new IllegalArgumentException("Invalid word: " + word);
+                }
+                output.append(digit.repeat(currentMultiplier != null ? currentMultiplier : 1));
+                currentMultiplier = null;
+            }
+        }
+        return output.toString();
+    }
+}

core-java-modules/core-java-numbers-7/src/main/java/com/baeldung/convertphonenumberinwordstonumber/UseSwitchToConvertPhoneNumberInWordsToNumber.java

@@ -0,0 +1,64 @@
+public class UseSwitchToConvertPhoneNumberInWordsToNumber {
+
+    public static String convertPhoneNumberInWordsToNumber(String phoneNumberInWord) {
+
+        StringBuilder output = new StringBuilder();
+        Integer currentMultiplier = null;
+        String[] words = phoneNumberInWord.split(" ");
+
+        for (String word : words) {
+            Integer multiplier = getWordAsMultiplier(word);
+            if (multiplier != null) {
+                if (currentMultiplier != null) {
+                    throw new IllegalArgumentException("Cannot have consecutive multipliers, at: " + word);
+                }
+                currentMultiplier = multiplier;
+            } else {
+                output.append(getWordAsDigit(word).repeat(currentMultiplier != null ? currentMultiplier : 1));
+                currentMultiplier = null;
+            }
+        }
+        return output.toString();
+    }
+
+    public static Integer getWordAsMultiplier(String word) {
+        switch (word) {
+            case "double":
+                return 2;
+            case "triple":
+                return 3;
+            case "quadruple":
+                return 4;
+            default:
+                return null;
+        }
+    }
+
+    public static String getWordAsDigit(String word) {
+        switch (word) {
+            case "zero":
+                return "0";
+            case "one":
+                return "1";
+            case "two":
+                return "2";
+            case "three":
+                return "3";
+            case "four":
+                return "4";
+            case "five":
+                return "5";
+            case "six":
+                return "6";
+            case "seven":
+                return "7";
+            case "eight":
+                return "8";
+            case "nine":
+                return "9";
+            default:
+                throw new IllegalArgumentException("Invalid word: " + word);
+        }
+    }
+
+}