Length Of The String Without Using Strlen () Function In Java Program

Strings are fundamental data structures in programming, and determining their length is a common operation. While Java provides a straightforward length() method for String objects, understanding alternative approaches can deepen your grasp of string manipulation and exception handling.

In this article, you will learn how to find the length of a string in Java without relying on the built-in length() method.

Problem Statement

The challenge is to programmatically determine the number of characters in a given string without invoking the String.length() method. This scenario often arises in technical interviews to assess a candidate's problem-solving skills, understanding of Java's String API beyond its most obvious methods, and ability to handle common exceptions. It pushes you to think about how string length might be determined at a lower level.

Example

Consider the string "Hello Java". If we were to manually count its characters, including spaces, we would find it has a length of 10. Our goal is to achieve this programmatically using methods other than String.length().

Desired output for "Hello Java":

String: "Hello Java"
Length: 10

Background & Knowledge Prerequisites

To understand the solutions presented in this article, you should have a basic understanding of:

• Java Fundamentals: Variables, data types, control flow (loops, if-else statements).
• String Basics: What a String is in Java, its immutability.
• Arrays: How to declare, initialize, and iterate over arrays.
• Exception Handling: The concepts of try-catch blocks for managing runtime errors.
• Java Stream API (Optional for some approaches): Basic understanding of streams and terminal operations.

Use Cases or Case Studies

While String.length() is almost always the preferred and most efficient method, exploring alternatives offers valuable insights:

• Interview Questions: A very common scenario to test fundamental understanding and problem-solving.
• Educational Purposes: Helps to understand the underlying mechanics of how string length might be computed internally.
• Custom String Implementations: If you were building your own MyString class, you wouldn't have a String.length() to rely on and would need to implement such logic.
• Understanding String Internals: Demonstrates how Java strings are sequences of characters and how iterating over them works.

Solution Approaches

Here, we will explore three distinct approaches to determine string length without using the length() method.

Approach 1: Iterating with charAt() and try-catch

This approach leverages Java's exception handling mechanism. We attempt to access characters at increasing indices using charAt(). When charAt() tries to access an index beyond the string's bounds, it throws a StringIndexOutOfBoundsException, which we can catch to determine the string's length.

• Summary: Continuously access characters using charAt() within a loop, incrementing a counter until a StringIndexOutOfBoundsException is caught. The counter's value at that point is the string's length.

• Code Example:

// Get String Length using charAt() and try-catch
    import java.util.Scanner;

    // Main class containing the entry point of the program
    public class Main {
        public static void main(String[] args) {
            // Step 1: Initialize a string and a counter
            String str = "Hello Java";
            int lengthCounter = 0;

            // Step 2: Use a try-catch block to iterate and detect end
            try {
                while (true) {
                    str.charAt(lengthCounter); // Attempt to access character
                    lengthCounter++; // Increment counter if successful
                }
            } catch (StringIndexOutOfBoundsException e) {
                // Step 3: The exception is caught when lengthCounter exceeds string bounds
                // The value of lengthCounter at this point is the string's length
                System.out.println("String: \\"" + str + "\\"");
                System.out.println("Length (using charAt and try-catch): " + lengthCounter);
            }
        }
    }

Run

• Sample Output:

String: "Hello Java"
    Length (using charAt and try-catch): 10

• Stepwise Explanation:

1. A String variable str is initialized, along with an integer lengthCounter set to 0.
2. A while(true) loop attempts to run indefinitely within a try block.
3. Inside the loop, str.charAt(lengthCounter) attempts to retrieve the character at the current index.
4. If successful, lengthCounter is incremented.
5. This continues until lengthCounter becomes equal to the actual string length, causing str.charAt(lengthCounter) to throw a StringIndexOutOfBoundsException.
6. The catch block intercepts this exception. At this precise moment, lengthCounter holds the value of the string's length (since it was incremented *after* the last valid character access).

Approach 2: Converting to a Character Array and Using its length Property

This method leverages the fact that a String can be converted into a char array, and arrays in Java have a public length field (not a method) that represents their size. This fulfills the requirement of not using the String.length() *method*.

• Summary: Convert the string into a character array using toCharArray(), then access the length property of the resulting array.

• Code Example:

// Get String Length by Converting to Character Array
    import java.util.Scanner;

    // Main class containing the entry point of the program
    public class Main {
        public static void main(String[] args) {
            // Step 1: Initialize a string
            String str = "Programming";

            // Step 2: Convert the string to a character array
            char[] charArray = str.toCharArray();

            // Step 3: Access the 'length' property of the character array
            int length = charArray.length;

            System.out.println("String: \\"" + str + "\\"");
            System.out.println("Length (using toCharArray().length): " + length);
        }
    }

Run

• Sample Output:

String: "Programming"
    Length (using toCharArray().length): 11

• Stepwise Explanation:

1. A String variable str is initialized.
2. The str.toCharArray() method is called to convert the string into an array of characters.
3. The length field of the charArray (e.g., charArray.length) is directly accessed to get the number of elements in the array, which corresponds to the string's length.

Approach 3: Using Java Stream API with codePoints() and count()

For modern Java development, the Stream API offers a functional way to process sequences. While count() is a method, str.codePoints().count() operates on the stream of character code points rather than directly on the String object's length property, making it a distinct approach.

• Summary: Obtain an IntStream of code points from the string using codePoints() and then use the count() terminal operation to get the total number of code points.

• Code Example:

// Get String Length using Stream API (codePoints)
    import java.util.Scanner; // Included for consistent structure, though not strictly used here

    // Main class containing the entry point of the program
    public class Main {
        public static void main(String[] args) {
            // Step 1: Initialize a string
            String str = "Hello World";

            // Step 2: Get an IntStream of code points from the string and count them
            long length = str.codePoints().count(); // count() returns a long

            System.out.println("String: \\"" + str + "\\"");
            System.out.println("Length (using Stream API codePoints().count()): " + length);
        }
    }

Run

• Sample Output:

String: "Hello World"
    Length (using Stream API codePoints().count()): 11

• Stepwise Explanation:

1. A String variable str is initialized.
2. str.codePoints() returns an IntStream representing the code points of the characters in the string. For most common characters, a code point corresponds to a single char.
3. The count() terminal operation on the IntStream returns the number of elements (code points) in the stream as a long. This value is the string's length.

Conclusion

Determining the length of a string without using the String.length() method in Java reveals different facets of the language, from exception handling to array manipulation and the modern Stream API. While these methods are generally less efficient or more verbose than the direct length() method, they are valuable for understanding fundamental programming concepts and for specific constrained scenarios like technical interviews. Each approach demonstrates a unique way to iterate over or infer the size of a string's underlying character sequence.

Summary

• The String.length() method is the standard and most efficient way to get a string's length.
• Approach 1 (charAt() with try-catch): Manually iterates, using StringIndexOutOfBoundsException to detect the end of the string.
• Approach 2 (toCharArray().length): Converts the String to a char array and uses the array's length field.
• Approach 3 (codePoints().count()): Utilizes the Java Stream API to count the number of code points in the string.
• These alternative methods are primarily useful for educational purposes, interview preparation, or specific scenarios where direct String.length() usage is restricted.