add unit-test class for the issue 5155 (#11217)

core-java-modules/core-java-string-operations-3/pom.xml

@@ -16,6 +16,16 @@
     </parent>
 
     <dependencies>
+        <dependency>
+            <groupId>org.springframework</groupId>
+            <artifactId>spring-core</artifactId>
+            <version>${spring-core.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>${apache-commons-lang3.version}</version>
+        </dependency>
         <dependency>
             <groupId>org.assertj</groupId>
             <artifactId>assertj-core</artifactId>
@@ -68,10 +78,12 @@
         <maven.compiler.source>11</maven.compiler.source>
         <maven.compiler.target>11</maven.compiler.target>
         <assertj.version>3.6.1</assertj.version>
+        <spring-core.version>5.3.9</spring-core.version>
+        <apache-commons-lang3.version>3.12.0</apache-commons-lang3.version>
         <maven-artifact.version>3.6.3</maven-artifact.version>
         <gradle-core.version>6.1.1</gradle-core.version>
         <jackson-core.version>2.11.1</jackson-core.version>
         <semver4j.version>3.1.0</semver4j.version>
     </properties>
 
-</project>
+</project>

core-java-modules/core-java-string-operations-3/src/test/java/com/baeldung/countspaces/CountSpacesInStringUnitTest.java

@@ -0,0 +1,65 @@
+package com.baeldung.countspaces;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import org.apache.commons.lang3.StringUtils;
+import org.junit.jupiter.api.Test;
+
+class CountSpacesInStringUnitTest {
+    private static final String INPUT_STRING = "  This string has nine spaces and a Tab:'	'";
+    private static final int EXPECTED_COUNT = 9;
+
+    @Test
+    void givenString_whenCountSpaceByLooping_thenReturnsExpectedCount() {
+        int spaceCount = 0;
+        for (char c : INPUT_STRING.toCharArray()) {
+            if (c == ' ') {
+                spaceCount++;
+            }
+        }
+        assertThat(spaceCount).isEqualTo(EXPECTED_COUNT);
+    }
+
+    @Test
+    void givenString_whenCountSpaceByJava8StreamFilter_thenReturnsExpectedCount() {
+        long spaceCount = INPUT_STRING.chars().filter(c -> c == (int) ' ').count();
+        assertThat(spaceCount).isEqualTo(EXPECTED_COUNT);
+    }
+
+    @Test
+    void givenString_whenCountSpaceByRegexMatcher_thenReturnsExpectedCount() {
+        Pattern pattern = Pattern.compile(" ");
+        Matcher matcher = pattern.matcher(INPUT_STRING);
+        int spaceCount = 0;
+        while (matcher.find()) {
+            spaceCount++;
+        }
+        assertThat(spaceCount).isEqualTo(EXPECTED_COUNT);
+    }
+
+    @Test
+    void givenString_whenCountSpaceByReplaceAll_thenReturnsExpectedCount() {
+        int spaceCount = INPUT_STRING.replaceAll("[^ ]", "").length();
+        assertThat(spaceCount).isEqualTo(EXPECTED_COUNT);
+    }
+
+    @Test
+    void givenString_whenCountSpaceBySplit_thenReturnsExpectedCount() {
+        int spaceCount = INPUT_STRING.split(" ").length - 1;
+        assertThat(spaceCount).isEqualTo(EXPECTED_COUNT);
+    }
+
+    @Test
+    void givenString_whenCountSpaceUsingApacheCommons_thenReturnsExpectedCount() {
+        int spaceCount = StringUtils.countMatches(INPUT_STRING, " ");
+        assertThat(spaceCount).isEqualTo(EXPECTED_COUNT);
+    }
+
+    @Test
+    void givenString_whenCountSpaceUsingSpring_thenReturnsExpectedCount() {
+        int spaceCount = org.springframework.util.StringUtils.countOccurrencesOf(INPUT_STRING, " ");
+        assertThat(spaceCount).isEqualTo(EXPECTED_COUNT);
+    }
+}