27 January 2020

Easiest Way to Reverse a String in Java

development

Reversing a string is one of the most frequently asked questions in a Java technical interview. The Interviewers may ask you to write different ways to reverse a string, or they may ask you to reverse a string without using in-built methods, or they may even ask you to reverse a string using recursion.

Below are various methods you can use to reverse a string in Java.

Reverse String in Java, Easiest Way

The easiest way to reverse a string in Java is to use the built-in reverse() function of the StringBuilder class.

Example:

package io.devqa.tutorials;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

class ReverseString {

    String reverse(String inputString) {
        return new StringBuilder(inputString).reverse().toString();
    }

    @Test
    public void testAWord() {
        assertEquals("tobor", new ReverseString().reverse("robot"));
    }
}

Reverse String using Recursion

Another way to reverse a string in java is to use recursion and utilizing the charAt() method of the String class

Example:

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

class ReverseString {

    String reverse(String inputString) {

        StringBuilder reverseStringBuilder = new StringBuilder();

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

        return reverseStringBuilder.toString();
    }

    @Test
    public void testAWord() {
        assertEquals("tobor", new ReverseString().reverse("robot"));
    }
}

A variation of the above is to use the toCharArray() and loop over the characters, for example:

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

class ReverseString {

    String reverse(String inputString) {
        String outString = "";
        for(char c : inputString.toCharArray()) {
            outString = c + outString;
        }
        return outString;
    }

    @Test
    public void testAWord() {
        assertEquals("tobor", new ReverseString().reverse("robot"));
    }
}

Reverse String in Java 8

import org.junit.jupiter.api.Test;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import static org.junit.jupiter.api.Assertions.assertEquals;

class ReverseString {

    String reverse(String inputString) {
        return IntStream.range(0, inputString.length())
            .mapToObj(x-> inputString.charAt((inputString.length()-1) - x))
            .map(character -> String.valueOf(character))
            .collect(Collectors.joining(""));
    }

    @Test
    public void testAWord() {
        assertEquals("tobor", new ReverseString().reverse("robot"));
    }
}

27 January 2020