Reverse a Sentence in Java [2 Methods]

Summary: In this programming example, we will learn different ways to reverse a given sentence word by word in Java.

Example:

Input:  I am a programmer
Output: programmer a am I

The problem is to reverse a sentence without reversing its individual words.

We can solve this by extracting every word from the sentence and concatenate them in reverse order.

Here are two such ways to do so in Java:

Method 1: Using Loop

With the help of indexOf() and substring() methods, we extract the words from the input sentence and concatenate them to the reverse string in reverse order.

import java.util.Scanner;
 
public class Main {
  public static void main(String args[]){
    Scanner in =new Scanner(System.in);
    
    //Input sentence
    System.out.print("Enter a sentence: ");
    String str = in.nextLine();

    //Append whitespace for the extraction of last word
    str += " ";

    String reversed = "";

    //repeat until no word is left
    while(str.indexOf(" ") != -1){
      //Extract word
      int idx = str.indexOf(" ");
      String word = str.substring(0, idx);

      //Concatenate in reverse order
      reversed = word + " "+ reversed;

      //Remove word from the sentence
      str = str.substring(idx+1);
    }
   
    //Output the reverse
    System.out.print("Reverse: ");
    System.out.println(reversed);
  }
}

Output:

Enter a sentence: how are you
Reverse: you are how

Every time we append a word to the reverse string, we remove it from the input sentence.

We use the ‘while’ loop to repeat the process until no word in the input sentence is left to process.

Method 2: Using Recursion

What we did in the previous method can also be implemented using the recursion technique.

import java.util.Scanner;
 
public class Main {
  public static void main(String args[]){
    Scanner in =new Scanner(System.in);

    //Input sentence
    System.out.print("Enter a Sentence: ");
    String sentence = in.nextLine();

    //Reverse by calling the recursive method
    String reversed = reverse(sentence);

    //output the reversed word
    System.out.print("Reverse: ");
    System.out.println(reversed);
  }

  public static String reverse(String str){
    int idx = str.indexOf(" ");
    //Base condition - when str has only one word
    if(idx == -1)
      return str;
    
    //return after concatenating in reverse order
    return reverse(str.substring(idx+1)) +" "+ str.substring(0, idx);
  }
}

Output:

Enter a Sentence: Hello World!
Reverse: World! Hello

On each recursive call, we extract a word from the front of the sentence using str.substring(0, idx) and concatenate it to the reversed result (i.e., reverse(str.substring(idx+1))) in the reverse order.

The recursive call reverse(str.substring(idx+1)) passes the remaining sentence as an argument that in turn extracts the word from the front and concatenates it in reversed order.

These are the two methods using which we can reverse a sentence without reversing its individual words in Java programming language.