From 63953b74f3cef3f6989bffa257ec32e0578c64fc Mon Sep 17 00:00:00 2001
From: Kai Yuan <kent.yuan@Gmail.com>
Date: Mon, 6 Nov 2023 02:26:25 +0100
Subject: [PATCH] Nth substring (#15120)

* [nth-substring] nth substring in a string

* [nth-substring] find n-th substring
---
 .../FindNthSubstringIndexUnitTest.java        | 106 ++++++++++++++++++
 1 file changed, 106 insertions(+)
 create mode 100644 core-java-modules/core-java-string-apis-2/src/test/java/com/baeldung/nthsubstring/FindNthSubstringIndexUnitTest.java

diff --git a/core-java-modules/core-java-string-apis-2/src/test/java/com/baeldung/nthsubstring/FindNthSubstringIndexUnitTest.java b/core-java-modules/core-java-string-apis-2/src/test/java/com/baeldung/nthsubstring/FindNthSubstringIndexUnitTest.java
new file mode 100644
index 0000000000..779585ffb0
--- /dev/null
+++ b/core-java-modules/core-java-string-apis-2/src/test/java/com/baeldung/nthsubstring/FindNthSubstringIndexUnitTest.java
@@ -0,0 +1,106 @@
+package com.baeldung.nthsubstring;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class FindNthSubstringIndexUnitTest {
+    //                                  "0       8       16      24    "
+    private final static String INPUT = "a word, a word, a word, a word";
+
+    @Test
+    void whenCallingIndexOfTwice_thenGetTheSecondSubstringIndex() {
+        int firstIdx = INPUT.indexOf("a");
+        int result = INPUT.indexOf("a", firstIdx + "a".length());
+        assertEquals(8, result);
+    }
+
+    // the recursive approach
+    static int nthIndexOf(String input, String substring, int nth) {
+        if (nth == 1) {
+            return input.indexOf(substring);
+        } else {
+            return input.indexOf(substring, nthIndexOf(input, substring, nth - 1) + substring.length());
+        }
+    }
+
+    @Test
+    void whenCallingRecursiveMethod_thenGetTheExpectedResult() {
+        int result1 = nthIndexOf(INPUT, "a", 1);
+        assertEquals(0, result1);
+
+        int result2 = nthIndexOf(INPUT, "a", 2);
+        assertEquals(8, result2);
+
+        int result3 = nthIndexOf(INPUT, "a", 3);
+        assertEquals(16, result3);
+
+        int result4 = nthIndexOf(INPUT, "a", 4);
+        assertEquals(24, result4);
+
+        int result5 = nthIndexOf(INPUT, "a", 5);
+        assertEquals(-1, result5);
+    }
+
+    // loop-based approach
+    static int nthIndexOf2(String input, String substring, int nth) {
+        int index = -1;
+        while (nth > 0) {
+            index = input.indexOf(substring, index + substring.length());
+            if (index == -1) {
+                return -1;
+            }
+            nth--;
+        }
+        return index;
+    }
+
+    @Test
+    void whenCallingLoopBasedMethod_thenGetTheExpectedResult() {
+        int result1 = nthIndexOf2(INPUT, "a", 1);
+        assertEquals(0, result1);
+
+        int result2 = nthIndexOf2(INPUT, "a", 2);
+        assertEquals(8, result2);
+
+        int result3 = nthIndexOf2(INPUT, "a", 3);
+        assertEquals(16, result3);
+
+        int result4 = nthIndexOf2(INPUT, "a", 4);
+        assertEquals(24, result4);
+
+        int result5 = nthIndexOf2(INPUT, "a", 5);
+        assertEquals(-1, result5);
+    }
+
+    static int nthOccurrenceIndex(String input, String regexPattern, int nth) {
+        Matcher matcher = Pattern.compile(regexPattern).matcher(INPUT);
+        for (int i = 0; i < nth; i++) {
+            if (!matcher.find()) {
+                return -1;
+            }
+        }
+        return matcher.start();
+    }
+
+    @Test
+    void whenCallingRegexBasedMethod_thenGetTheExpectedResult() {
+        int result1 = nthOccurrenceIndex(INPUT, "a", 1);
+        assertEquals(0, result1);
+
+        int result2 = nthOccurrenceIndex(INPUT, "a", 2);
+        assertEquals(8, result2);
+
+        int result3 = nthOccurrenceIndex(INPUT, "a", 3);
+        assertEquals(16, result3);
+
+        int result4 = nthOccurrenceIndex(INPUT, "a", 4);
+        assertEquals(24, result4);
+
+        int result5 = nthOccurrenceIndex(INPUT, "a", 5);
+        assertEquals(-1, result5);
+    }
+}
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