How to Find the Longest Common Prefix of Two Strings in Java?

In this article, we will find the longest common prefix of two Strings in Java.

Examples:

Input: String 1= geeksforgeeks, String 2 = geezer
Output: “gee”

Input: String 1= flower, String 2 = flight
Output: “fl”

Methods to Find the Longest common Prefix of two Strings in Java

Below are the methods by which we can find the longest common prefix of two Strings.

• Using a loop to compare characters
• Using the startsWith method
• Using a recursive approach

1. Loop to Compare Characters

Here we use a loop to perform this comparison and construct the longest common prefix by appending matching characters to a StringBuilder. Once a mismatch is encountered, the loop breaks and the method returns the constructed prefix.

Below is the program to Find the Longest common Prefix of two Strings:

Java

public class LongestCommonPrefix {
    public static String longestCommonPrefix(String str1, String str2) {
        // Find the smaller length between the
          // two strings to ensure comparison within bounds
        int minLength = Math.min(str1.length(), str2.length());
        
          // Use StringBuilder for efficient string manipulation
        StringBuilder prefix = new StringBuilder();
        
        // Loop through both strings until the minimum length
        for (int i = 0; i < minLength; i++) {
            
              // Break the loop if characters do not match
            if (str1.charAt(i) != str2.charAt(i)) {
                break;
            }
            
              // Append the matching character to the prefix
            prefix.append(str1.charAt(i));
        }
        
        // Convert StringBuilder to String and return
        return prefix.toString();
    }
    
    public static void main(String[] args) {
        // Prepare two strings to find their
          // longest common prefix
        String str1 = "flower";
        String str2 = "flight";
        
        // Determine the longest common
          // prefix and display it
        String longestPrefix = longestCommonPrefix(str1, str2);
        System.out.println("Longest Common Prefix: " + longestPrefix);
    }
}

Output

Longest Common Prefix: fl

2. startsWith Method

This method checks if one string starts with another string. It iterates through the characters until a mismatch is found, and then returns the prefix formed by the characters that matched until that index.

Below is the program to Find the Longest common Prefix of two Strings:

Java

public class LongestCommonPrefix {
    public static String longestCommonPrefix(String str1, String str2) {
        // Determine the minimum length
          // between the two strings to limit the comparison
        int minLength = Math.min(str1.length(), str2.length());
        
        // Iterate through the strings up to the minimum length
        for (int i = 0; i < minLength; i++) {
            // If characters at the current index are not
              // the same, return the substring up to that point
            if (str1.charAt(i) != str2.charAt(i)) {
                  // Return prefix until mismatch
                return str1.substring(0, i); 
            }
        }
        
        // If all characters match up to the
          // minimum length, return the entire substring
        return str1.substring(0, minLength); 
          // Return prefix if no mismatch
    }
    
    public static void main(String[] args) {
        // Initialize two strings for comparison
        String str1 = "flower";
        String str2 = "flight";
        
        // Call the method to find and
          // print the longest common prefix
        String longestPrefix = longestCommonPrefix(str1, str2);
        System.out.println("Longest Common Prefix: " + longestPrefix);
    }
}

Output

Longest Common Prefix: fl

3. Recursive approach

If the substrings at a certain length are equal, it returns that substring as the common prefix. Otherwise, it recursively calls itself with a smaller length until it finds the longest common prefix.

Below is the program to Find the Longest common Prefix of two Strings:

Java

public class LongestCommonPrefix {
    public static String longestCommonPrefix(String str1, String str2) {
        // Initialize the minimum length variable
          // with the minimum of the two string lengths
        int minLength = Math.min(str1.length(), str2.length());
        
          // Call the recursive method to find the longest common prefix
        return longestCommonPrefixRecursive(str1, str2, minLength);
    }
  
    private static String longestCommonPrefixRecursive(String str1, String str2, int length) {
        // if length is 0 
          // return an empty string
        if (length == 0) {
            return "";
        }
      
        // If the substrings of the current
          // length are equal, return the substring
        else if (str1.substring(0, length).equals(str2.substring(0, length))) {
            return str1.substring(0, length); 
        } else {
            // Otherwise, decrease the length
              // by 1 and call the method recursively
            return longestCommonPrefixRecursive(str1, str2, length - 1);
        }
    }
    
    public static void main(String[] args) {
        // Define two strings to find their longest common prefix
        String str1 = "flower";
        String str2 = "flight";
        
        // Call the method to find the longest common prefix and print it
        String longestPrefix = longestCommonPrefix(str1, str2);
        System.out.println("Longest Common Prefix: " + longestPrefix);
    }
}

Output

Longest Common Prefix: fl