Java Program to find longest substring without repeating characters

In this tutorial, you will learn how to write a java program to find longest substring without repeating characters. For example, if a string is “TEXT”, the longest substring in “TEXT” without repeating character is: “TEX”. Similarly longest substring with non-repeating characters in a string “AAAAA” is “A”. Here we will write a java program that finds the longest substring with non-repeating character and display the length of this substring.

Program to find longest substring without repeating characters

In this program, we have created a user defined method longestSubstrWithoutRepeat(). This method takes a string as an input and returns the length of the longest substring with no repeating characters.

public class JavaExample {
  public static int longestSubstrWithoutRepeat(String str)
  {
    String longestSubstr = "";

    // Result
    int maxLength = -1;

    // Return 0 if the input string is empty
    if (str.isEmpty()) {
      return 0;
    }
    // Return 1 is if the input string contains only one char
    else if (str.length() == 1) {
      return 1;
    }
    for (char c : str.toCharArray()) {
      String current = String.valueOf(c);
      if (longestSubstr.contains(current)) {
        longestSubstr =
                longestSubstr.substring(
                        longestSubstr.indexOf(current) + 1);
      }
      longestSubstr = longestSubstr + String.valueOf(c);
      maxLength = Math.max(longestSubstr.length(), maxLength);
    }

    return maxLength;
  }

  public static void main(String[] args)
  {
    String str = "beginnersbook";
    System.out.println("The given string is " + str);

    int len = longestSubstrWithoutRepeat(str);
    System.out.println("The length of the longest " +
            "substring without repeating " +
            "characters is: "+ len);
  }
}

Output: Here, the longest substring with no reoccurring character would be: “nersbo” (length: 6, as number of characters in this string is 6).

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