Reversing a string word by word in java

How to Reverse a String in Java is one of the popular interview questions, but the interviewer might add some twist to it by asking you to write the code without using the reverse() method, recursion, etc. In this article, we will learn the possible ways of reversing a string in Java. We will look into techniques of reversing a single word and group of words in a sentence [Word by Word]

Method 1: Using reverse() function — Using StringBuilder (or) StringBuffer

This is the easiest way of reversing a String, we can use reverse() method of either StringBuilder (or) StringBuffer.

Note: StringBuilder can give a better performance as it is not Synchronized. StringBuffer is Synchronized

import java.util.Scanner;

public class StringReverse
{
    public static void main(String[] args)
    {
        String reverseString = "";
        
        System.out.println("Enter string to reversed");
        
        //Read the input from the user
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        
        //Pass input to constructor of StringBuilder
        StringBuilder inputStr = new StringBuilder(input);
        
        //Use reverse() method to reverse the String
        reverseString = inputStr.reverse().toString();
        
        System.out.println("Original String : "+input);
        System.out.println("Reversed String : "+reverseString);
    }
}

• • Get the input string from the user and pass it to the constructor of the StringBuilder.
  • Use the reverse() method of the StringBuilder class to get the reversed String.

Output:

Method 2: Reverse String Using charAt()

import java.util.Scanner;

public class StringReverse
{
    public static void main(String[] args)
    {
        String reverseString = "";
        
        System.out.println("Enter string to reversed");
        
        //Read the input from the user
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        
        //Convert input to inputArray using toCharArray()
        char[] inputArray = input.toCharArray();
        
        for(int i=inputArray.length-1;i>=0;i--)
        {
            reverseString = reverseString+inputArray[i];
        }
        
        System.out.println("Original String : "+input);
        System.out.println("Reversed String : "+reverseString);
    }
}

• Get the input string from the user and convert the input string into a character array inputArray using toCharArray() method
• Iterate the inputArray from end to start, each time append it to the reverseString.

Output:

Enter string to reversed
JavaInterviewPoint
Original String : JavaInterviewPoint
Reversed String : tnioPweivretnIavaJ 

Method 3: using Recursion

package com.javainterviewpoint;

import java.util.Scanner;

public class StringReverse
{
    public static void main(String[] args)
    {
        String reversedString = "";
        
        System.out.println("Enter string to reversed");
        
        //Read the input from the user
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        
        reversedString = reverseString(input);
        
        System.out.println("Original String : "+input);
        System.out.println("Reversed String : "+reversedString);
    }
    public static String reverseString(String input)
    {
        if(input.isEmpty())
            return input;
        
        //Call reverseString() function recursively
        return reverseString(input.substring(1)) + input.charAt(0);
    }
}

• Get the input string from the user and pass the input string to the reverseString() method.
• In the reverseString() method, we will be recursively subString() the first character of the input String and append it to the end using charAt() method.

Method 4: Using Stack

import java.util.Scanner;
import java.util.Stack;

public class StringReverse
{
    public static void main(String[] args)
    {
        StringBuilder reverseString = new StringBuilder();

        System.out.println("Enter string to reversed");

        // Read the input from the user
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();

        // Create a stack of characters
        Stack<Character> stack = new Stack<Character>();

        // Push each character into the stack
        for (int i = 0; i < input.length(); i++)
        {
            stack.push(input.charAt(i));
        }

        // pop each characters from the stack until it is empty
        while (!stack.empty())
        {
            reverseString.append(stack.pop());
        }

        System.out.println("Original String : "+input);
        System.out.println("Reversed String : "+reverseString);
    }
}

• Get the input string from the user.
• Create a Stack of Character and push each character of the input string into the stack.
• Pop each character from the stack until it is empty and append it to the reverseString.

Method 5: Using Collections reverse() method

• Get the input string from the user
• Create a List of Character (characterList) and push each character of the input string into the characterList.
• Use Collections.reverse() method to reverse the elements of the characterList
• Iterate the characterList from end to start, each time append it to the reverseString

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class StringReverse
{
    public static void main(String[] args)
    {
        StringBuilder reverseString = new StringBuilder();

        System.out.println("Enter string to reversed");

        // Read the input from the user
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();

        //Create a list of characters
        List<Character> characterList =  new ArrayList<Character>();
        
        //Push each characters into the characterList
        for(Character c : input.toCharArray())
        {
           characterList.add(c);
        }
        
        //Reverse the List using Collections.reverse()
        Collections.reverse(characterList);
        
        //Convert ArrayList to String
        for(Character c : characterList)
        {
            reverseString.append(c);
        }

        System.out.println("Original String : "+input);
        System.out.println("Reversed String : "+reverseString);
    }
}

Java Reverse String Word by Word

All the above methods will work good for a single word, lets now learn how to reverse a string in Java word by word,
We will reverse each word in a sentence.

Method 1: Using StringBuffer

• Get the input string from the user
• Using split() method split the sentence into words and save them to a String array (words)
• Iterate through the String array and use reverse() method of the StringBuffer class to reverse the String and keep appending the reversedString.

package com.javainterviewpoint;

import java.util.Scanner;

public class StringReverse
{
	public static void main(String[] args)
    {
        String reverseString = "";
        
        System.out.println("Enter string to reversed");
        
        //Read the input from the user
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        
        // Split sentence into seperate words
        String[] words = input.split(" "); 
        
        // Iterate the String array
        for(String word : words)
        {
        	// Append each reversed Words
        	reverseString = reverseString + new StringBuilder(word).reverse().toString()+" ";
        }
        
        System.out.println("Original String : "+input);
        System.out.println("Reversed String : "+reverseString);
    }
}

Output:

Enter string to reversed
Hello World
Original String : Hello World
Reversed String : olleH dlroW

Method 2: Using charAt()

• Get the input string from the user
• Using split() method split the sentence into words and save them to a String array (words)
• Iterate through the String array and convert each word into a character array using toCharArray() method
• Iterate through the character array in the decreasing order and each time append the character to the reverseString

package com.javainterviewpoint;

import java.util.Scanner;

public class StringReverse
{
	public static void main(String[] args)
    {
        String reverseString = "";
        
        System.out.println("Enter string to reversed");
        
        //Read the input from the user
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        
        // Split sentence into seperate words
        String[] words = input.split(" "); 
        
        // Iterate the String array
        for(String word : words)
        {
            //Convert input to inputArray using toCharArray()
            char[] inputArray = word.toCharArray();
            
            for(int i=inputArray.length-1; i>=0; i--)
            {
                reverseString = reverseString+inputArray[i];
            }
            reverseString = reverseString + " ";
            
        }
        
        System.out.println("Original String : "+input);
        System.out.println("Reversed String : "+reverseString);
    }
}