Reverse a string without using string function in java

1. How to reverse a string in java without using reverse function
2. Reverse a string in java
3. Write a program to reverse string without using any function java

• First of all "there is no direct built in function or method in string class to reverse a string" but still it is frequently asked question in java interviews.
• It's not possible to reverse a string without using string methods. (length(),chgarAt())

public static String reverseString(String s) {

    char[] chars = s.toCharArray();

    int left = 0;

    int right = chars.length - 1;

    while (left < right) {

        char temp = chars[left];

        chars[left] = chars[right];

        chars[right] = temp;

        left++;

        right--;

    }

    return new String(chars);

}

This method converts the input string to a character array using the toCharArray() method. Then, it initializes two pointers, left and right, to the first and last positions in the array, respectively.

Then, it enters a while loop that continues as long as the left pointer is less than the right pointer. In each iteration of the loop, the characters at the positions indicated by the left and right pointers are swapped using a temporary variable. Then, the left and right pointers are incremented and decremented, respectively.

Finally, the reversed character array is converted back to a string and returned.

This method is more efficient than using string functions because strings are immutable in Java and any operation that modifies the string creates a new object, whereas a character array can be modified in place.

• Lets discuss on how to reverse a string in java with using any built in string functions or methods.

1. Using charAt() function/method

1:Using charAt():

• Take a for loop and iterate string by checking length of the string from n to 1.
• Use charAt() method of string and take each  character and append to another string.
• By the end of the loop you will get all characters in reverse order.
• Like this we can reverse a string in java with using string method.

#1: Reverse a string in java with using inbuilt function

1. package com.instaceofjava;
2.  
3. public class ReverseString {
4.  
5. public static void main(String[] args) {
6.  
7. String str="Hello world";
8. String revstring="";
9.  
10. for(int i=str.length()-1;i>=0;--i){
11. revstring +=str.charAt(i);
12. }
13.  
14. System.out.println(revstring);
15. }
16. }

OutPut:

1. dlrow olleH

• This program reads a string from the user, and then uses a loop to iterate over the characters in the string in reverse order. It appends each character to a new string called reversed, and then prints out the reversed string after the loop has finished.
• Note that this program does not modify the original string, but creates a new reversed string instead. If you want to reverse the original string in place, you can use an array of characters instead of a string, and swap the elements at opposite ends of the array until you reach the middle.

Here is a simple Java program that reverses a string without using any string functions:

• This program works by first converting the input string to a character array using the toCharArray method. 
• It then uses two pointers, i and j, to traverse the array from opposite ends. On each iteration, it swaps the characters at indices i and j, and increments i and decrements j until they meet in the middle. 
• Finally, it converts the reversed character array back to a string using the String constructor.

2: Using StringBuffer Object:

Another way to reverse a string in Java without using any built-in string functions is to use a StringBuilder or StringBuffer object.

Here is an example using a StringBuilder:

public static String reverseString(String s) {

    StringBuilder sb = new StringBuilder(s);

    return sb.reverse().toString();

}

Here, the input string is passed to the constructor of the StringBuilder object. Then, the reverse() method is called on the object, which reverses the order of the characters in the string. Finally, the toString() method is called on the StringBuilder object to convert it back to a string and return it.

StringBuffer is similar to StringBuilder, it is thread safe, whereas StringBuilder is not.

public static String reverseString(String s) {

    StringBuffer sb = new StringBuffer(s);

    return sb.reverse().toString();

}

The above approach is more efficient than the previous one that used a character array because it avoids the need to create a temporary array and copy the characters to it.

In both cases, the method takes a single string parameter and returns the reversed version of it.

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