[count-substr] count seq in a string (#18814)

* [count-substr] count seq in a string

* [count-substr] move to -6 module

Author: sk1418

core-java-modules/core-java-string-algorithms-6/pom.xml

@@ -0,0 +1,27 @@
+<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>
+    <artifactId>core-java-string-algorithms-6</artifactId>
+    <packaging>jar</packaging>
+    <name>core-java-string-algorithms-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.apache.commons</groupId>
+            <artifactId>commons-lang3</artifactId>
+            <version>${common-lang3.version}</version>
+        </dependency>
+    </dependencies>
+
+    <properties>
+        <common-lang3.version>3.18.0</common-lang3.version>
+    </properties>
+
+</project>

core-java-modules/core-java-string-algorithms-6/src/test/java/com/baeldung/countseq/CountSequenceInStringUnitTest.java

@@ -0,0 +1,78 @@
+package com.baeldung.countseq;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.apache.commons.lang3.StringUtils;
+import org.junit.jupiter.api.Test;
+
+public class CountSequenceInStringUnitTest {
+
+    private final static String INPUT = "This is a test string. This test is for testing the count of a sequence in a string. This string has three sentences.";
+
+    int countSeqByIndexOf(String input, String seq) {
+        int count = 0;
+        int index = input.indexOf(seq);
+        while (index != -1) {
+            count++;
+            index = input.indexOf(seq, index + seq.length());
+        }
+        return count;
+    }
+
+    @Test
+    void whenUsingIndexOf_thenCorrect() {
+        assertEquals(3, countSeqByIndexOf(INPUT, "string"));
+        assertEquals(2, countSeqByIndexOf(INPUT, "string."));
+    }
+
+    int countSeqByRegexFind(String input, String seq) {
+        // Alternative: Pattern pattern = Pattern.compile(seq, Pattern.LITERAL);
+        Matcher matcher = Pattern.compile(Pattern.quote(seq))
+            .matcher(input);
+        int count = 0;
+        while (matcher.find()) {
+            count++;
+        }
+        return count;
+    }
+
+    @Test
+    void whenUsingRegexFind_thenCorrect() {
+        assertEquals(3, countSeqByRegexFind(INPUT, "string"));
+        assertEquals(2, countSeqByRegexFind(INPUT, "string."));
+    }
+
+    int countSeqByRegexSplit(String input, String seq) {
+        Pattern pattern = Pattern.compile(seq, Pattern.LITERAL);
+        return pattern.split(input, -1).length - 1;
+    }
+
+    @Test
+    void whenUsingRegexSplit_thenCorrect() {
+        assertEquals(3, countSeqByRegexSplit(INPUT, "string"));
+        assertEquals(2, countSeqByRegexSplit(INPUT, "string."));
+    }
+
+    int countSeqByStream(String input, String seq) {
+        long count = Pattern.compile(Pattern.quote(seq))
+            .matcher(input)
+            .results()
+            .count();
+        return Math.toIntExact(count);
+    }
+
+    @Test
+    void whenUsingStream_thenCorrect() {
+        assertEquals(3, countSeqByStream(INPUT, "string"));
+        assertEquals(2, countSeqByStream(INPUT, "string."));
+    }
+
+    @Test
+    void whenUsingApacheCommonsLangCountMatches_thenCorrect() {
+        assertEquals(3, StringUtils.countMatches(INPUT, "string"));
+        assertEquals(2, StringUtils.countMatches(INPUT, "string."));
+    }
+}

core-java-modules/pom.xml

@@ -243,6 +243,7 @@
         <module>core-java-string-algorithms-3</module>
         <module>core-java-string-algorithms-4</module>
         <module>core-java-string-algorithms-5</module>
+        <module>core-java-string-algorithms-6</module>
         <module>core-java-string-apis</module>
         <module>core-java-string-apis-2</module>
         <module>core-java-swing</module>