Chapter 22 Exercise 4, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

22.4 (Pattern matching) Write a program that prompts the user to enter two strings and tests whether the second string is a substring of the first string. (Don’t use the indexOf method in the String class.) Analyze the time complexity of your algorithm.

import java.util.Scanner;

public class Exercise_22_04 {
 public static void main(String[] args) {
  Scanner input = new Scanner(System.in);

  // Prompt the user to enter two strings
  System.out.print("Enter a string s1: ");
  String s1 = input.nextLine();
  System.out.print("Enter a string s2: ");
  String s2 = input.nextLine();

  int index = -1; // Index of sub string
  int count = 0; // Count matching characters
  boolean matched = false; 

  // tests whether the second string 
  // is a substring of the first string
  for (int i = 0; i < s1.length(); i++) {
   if (s1.charAt(i) != s2.charAt(count)){
    count = 0;
   }

   if (s1.charAt(i) == s2.charAt(count)) {
    if (count == 0) 
     index = i;  
    count++;
   }

   if (count == s2.length()) {
    matched = true;
    break;
   }
  }

  // Display result
  if (matched)
   System.out.println("matched at index " + index);
  else
   System.out.println("\"" + s2 + 
    "\" is not a substring of \"" + s1 + "\""+ ".");
 }
}