Java: Reverse words in a given string

Last update on December 21 2024 09:43:47 (UTC/GMT +8 hours)

Write a Java program to reverse words in a given string.

Visual Presentation:

Java String Exercises: Reverse words in a given string

Sample Solution-1:

Java Code:

// Importing necessary Java utilities.
import java.util.*;

// Define a class named Main.
public class Main {
    
    // Method to reverse words in a given string.
    public static String WordsInReverse(String str1) {
        // Create a StringBuilder object and reverse the entire string.
        StringBuilder sb = new StringBuilder(str1);
        String StrRev = sb.reverse().toString();

        // Split the reversed string into words.
        String[] words = StrRev.split(" ");
        
        // Create a StringBuilder to store the reversed words.
        StringBuilder reverse = new StringBuilder();
        
        // Iterate through each word, reverse it, and append it to the result string.
        for (String tmp: words) {
            sb = new StringBuilder(tmp);
            reverse.append(sb.reverse() + " ");
        }
        
        // Remove the trailing space and return the reversed words string.
        reverse.deleteCharAt(reverse.length() - 1);
        return reverse.toString();
    }

    // Main method to execute the program.
    public static void main(String[] args) {
        String str1 = "Reverse words in a given string"; // Given input string.
        
        // Display the given string.
        System.out.println("The given string is: " + str1);
        
        // Display the string after reversing the words.
        System.out.println("The new string after reversing the words: " + WordsInReverse(str1));
    }
}

Sample Output:

The given string is: Reverse words in a given string
The new string after reversed the words: string given a in words Reverse

Flowchart:

Sample Solution-2:

Main.java Code:

//MIT License: https://bit.ly/35gZLa3
import java.util.concurrent.TimeUnit;

public class Main {

    private static final String TEXT = "My high school, the Illinois Mathematics and Science Academy, "
            + "showed me that anything is possible and that you're never too young to think big. "
            + "At 15, I worked as a computer programmer at the Fermi National Accelerator Laboratory, "
            + "or Fermilab. After graduating, I attended Stanford for a degree in economics and "
            + "computer science.";

    public static void main(String[] args) {

        System.out.println("Input text: \n" + TEXT + "\n");
        
        System.out.println("Reverse words in String via StringBuilder:");
        long startTimeV1 = System.nanoTime();
        
        String reversedV1 = Strings.reverseWordsV1(TEXT);
        
        displayExecutionTime(System.nanoTime() - startTimeV1);
        System.out.println("Reversed: \n" + reversedV1);

        System.out.println();        
        System.out.println("Reverse words in String using Java 8 functional-style:");
        long startTimeV2 = System.nanoTime();
        
        String reversedV2 = Strings.reverseWordsV2(TEXT);
        
        displayExecutionTime(System.nanoTime() - startTimeV2);
        System.out.println("Reversed: \n" + reversedV2);
        
        System.out.println();
        System.out.println("Reverse String via StringBuilder:");
        long startTimeV3 = System.nanoTime();
        
        String reversedV3 = Strings.reverse(TEXT);
        
        displayExecutionTime(System.nanoTime() - startTimeV3);
        System.out.println("Reversed: \n" + reversedV3);       
    }

    private static void displayExecutionTime(long time) {
        System.out.println("Execution time: " + time + " ns" + " ("
                + TimeUnit.MILLISECONDS.convert(time, TimeUnit.NANOSECONDS) + " ms)");
    }

}

Strings.java Code:

//MIT License: https://bit.ly/35gZLa3
import java.util.regex.Pattern;
import java.util.stream.Collectors;

public final class Strings {

    private static final Pattern PATTERN = Pattern.compile(" +");
    private static final String WHITESPACE = " ";

    private Strings() {
        throw new AssertionError("Cannot be instantiated");
    }

    public static String reverseWordsV1(String str) {

        if (str == null || str.isEmpty()) {
            // or throw IllegalArgumentException
            return "";
        }

        String[] words = str.split(WHITESPACE);
        StringBuilder reversedString = new StringBuilder();

        for (String word : words) {

            StringBuilder reverseWord = new StringBuilder();

            for (int i = word.length() - 1; i >= 0; i--) {
                reverseWord.append(word.charAt(i));
            }

            reversedString.append(reverseWord).append(WHITESPACE);
        }

        return reversedString.toString();
    }

    public static String reverseWordsV2(String str) {

        // or throw IllegalArgumentException
        if (str == null || str.isEmpty()) {
            return "";
        }

        return PATTERN.splitAsStream(str)
                .map(w -> new StringBuilder(w).reverse())
                .collect(Collectors.joining(WHITESPACE));
    }

    public static String reverse(String str) {

        // or throw IllegalArgumentException
        if (str == null || str.isEmpty()) {
            return "";
        }

        return new StringBuilder(str).reverse().toString();
    }

}

Sample Output:

Input text: 
My high school, the Illinois Mathematics and Science Academy, showed me that anything is possible and that you're never too young to think big. At 15, I worked as a computer programmer at the Fermi National Accelerator Laboratory, or Fermilab. After graduating, I attended Stanford for a degree in economics and computer science.

Reverse words in String via StringBuilder:
Execution time: 2670238 ns (2 ms)
Reversed: 
yM hgih ,loohcs eht sionillI scitamehtaM dna ecneicS ,ymedacA dewohs em taht gnihtyna si elbissop dna taht er'uoy reven oot gnuoy ot kniht .gib tA ,51 I dekrow sa a retupmoc remmargorp ta eht imreF lanoitaN rotareleccA ,yrotarobaL ro .balimreF retfA ,gnitaudarg I dednetta drofnatS rof a eerged ni scimonoce dna retupmoc .ecneics 

Reverse words in String using Java 8 functional-style:
Execution time: 143280080 ns (143 ms)
Reversed: 
yM hgih ,loohcs eht sionillI scitamehtaM dna ecneicS ,ymedacA dewohs em taht gnihtyna si elbissop dna taht er'uoy reven oot gnuoy ot kniht .gib tA ,51 I dekrow sa a retupmoc remmargorp ta eht imreF lanoitaN rotareleccA ,yrotarobaL ro .balimreF retfA ,gnitaudarg I dednetta drofnatS rof a eerged ni scimonoce dna retupmoc .ecneics

Reverse String via StringBuilder:
Execution time: 91373 ns (0 ms)
Reversed: 
.ecneics retupmoc dna scimonoce ni eerged a rof drofnatS dednetta I ,gnitaudarg retfA .balimreF ro ,yrotarobaL rotareleccA lanoitaN imreF eht ta remmargorp retupmoc a sa dekrow I ,51 tA .gib kniht ot gnuoy oot reven er'uoy taht dna elbissop si gnihtyna taht em dewohs ,ymedacA ecneicS dna scitamehtaM sionillI eht ,loohcs hgih yM

Flowchart:

Java Code Editor:

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