This article discusses different ways to reverse a string in Java with examples.
Examples:
Prerequisite: String vs StringBuilder vs StringBuffer in Java
Following are some interesting facts about String and StringBuilder classes :
- Objects of String are immutable.
- String class in Java does not have reverse() method, however, the StringBuilder class has built-in reverse() method.
- StringBuilder class do not have toCharArray() method, while String class does have toCharArray() method.
1. The idea is to traverse the length of the string
2. Extract each character while traversing
3. Add each character in front of the existing string
Implementation:
Java
// java program to reverse a wordimport java.io.*;import java.util.Scanner;class GFG { public static void main (String[] args) { String str= "Geeks", nstr=""; char ch; System.out.print("Original word: "); System.out.println("Geeks"); //Example word for (int i=0; i<str.length(); i++) { ch= str.charAt(i); //extracts each character nstr= ch+nstr; //adds each character in front of the existing string } System.out.println("Reversed word: "+ nstr); }}//Contributed by Tiyasa |
Output
Original word: Geeks Reversed word: skeeG
Converting String into Bytes: getBytes() method is used to convert the input string into bytes[].
Method:
1. Create a temporary byte[] of length equal
to the length of the input string.
2. Store the bytes (which we get by using
getBytes() method) in reverse order into
the temporary byte[] .
3. Create a new String abject using byte[] to
store result.
Implementation:
Java
// Java program to ReverseString using ByteArray.import java.lang.*;import java.io.*;import java.util.*;// Class of ReverseStringclass ReverseString { public static void main(String[] args) { String input = "Lazyroar"; // getBytes() method to convert string // into bytes[]. byte[] strAsByteArray = input.getBytes(); byte[] result = new byte[strAsByteArray.length]; // Store result in reverse order into the // result byte[] for (int i = 0; i < strAsByteArray.length; i++) result[i] = strAsByteArray[strAsByteArray.length - i - 1]; System.out.println(new String(result)); }} |
Output
skeeGrofskeeG
Using built in reverse() method of the StringBuilder class:
String class does not have reverse() method, we need to convert the input string to StringBuilder, which is achieved by using the append method of StringBuilder. After that, print out the characters of the reversed string by scanning from the first till the last index.
Implementation:
Java
// Java program to ReverseString using StringBuilderimport java.lang.*;import java.io.*;import java.util.*;// Class of ReverseStringclass ReverseString { public static void main(String[] args) { String input = "Geeks for Geeks"; StringBuilder input1 = new StringBuilder(); // append a string into StringBuilder input1 input1.append(input); // reverse StringBuilder input1 input1.reverse(); // print reversed String System.out.println(input1); }} |
Output
skeeG rof skeeG
Converting String to character array: The user input the string to be reversed.
Method:
1. First, convert String to character array
by using the built in Java String class
method toCharArray().
2. Then, scan the string from end to start,
and print the character one by one.
Implementation:
Java
// Java program to Reverse a String by// converting string to characters one// by oneimport java.lang.*;import java.io.*;import java.util.*;// Class of ReverseStringclass ReverseString { public static void main(String[] args) { String input = "GeeksForGeeks"; // convert String to character array // by using toCharArray char[] try1 = input.toCharArray(); for (int i = try1.length - 1; i >= 0; i--) System.out.print(try1[i]); }} |
Output
skeeGroFskeeG
- Convert the input string into character array by using the toCharArray(): Convert the input string into character array by using the toCharArray() – built in method of the String Class. Then, scan the character array from both sides i.e from the start index (left) as well as from last index(right) simultaneously.
1. Set the left index equal to 0 and right
index equal to the length of the string -1.
2. Swap the characters of the start index
scanning with the last index scanning
one by one. After that, increase the left
index by 1 (left++) and decrease the right
by 1 i.e., (right--) to move on to the next
characters in the character array .
3. Continue till left is less than or equal to
the right.
Implementation:
Java
// Java program to Reverse a String using swapping// of variablesimport java.lang.*;import java.io.*;import java.util.*;// Class of ReverseStringclass ReverseString { public static void main(String[] args) { String input = "Geeks For Geeks"; char[] temparray = input.toCharArray(); int left, right = 0; right = temparray.length - 1; for (left = 0; left < right; left++, right--) { // Swap values of left and right char temp = temparray[left]; temparray[left] = temparray[right]; temparray[right] = temp; } for (char c : temparray) System.out.print(c); System.out.println(); }} |
Output
skeeG roF skeeG
- Using ArrayList object: Convert the input string into the character array by using toCharArray() built in method. Then, add the characters of the array into the ArrayList object. Java also has built in reverse() method for the Collections class. Since Collections class reverse() method takes a list object, to reverse the list, we will pass the ArrayList object which is a type of list of characters.
1. We copy String contents to an object
of ArrayList.
1. We create a ListIterator object by using
the listIterator() method on the ArrayList
object.
2. ListIterator object is used to iterate over
the list.
3. ListIterator object helps us to iterate
over the reversed list and print it one
by one to the output screen.
Implementation:
Java
// Java program to Reverse a String using ListIteratorimport java.lang.*;import java.io.*;import java.util.*;// Class of ReverseStringclass ReverseString { public static void main(String[] args) { String input = "Geeks For Geeks"; char[] hello = input.toCharArray(); List<Character> trial1 = new ArrayList<>(); for (char c : hello) trial1.add(c); Collections.reverse(trial1); ListIterator li = trial1.listIterator(); while (li.hasNext()) System.out.print(li.next()); }} |
Output
skeeG roF skeeG
Using StringBuffer:
String class does not have reverse() method, we need to convert the input string to StringBuffer, which is achieved by using the reverse method of StringBuffer.
Implementation:
Java
// Java program to demonstrate conversion from// String to StringBuffer and reverse of stringimport java.lang.*;import java.io.*;import java.util.*;public class Test { public static void main(String[] args) { String str = "Geeks"; // conversion from String object to StringBuffer StringBuffer sbr = new StringBuffer(str); // To reverse the string sbr.reverse(); System.out.println(sbr); }} |
Output
skeeG
- Reversing String by taking input from user-
Java
/*package whatever //do not write package name here */import java.io.*;import java.util.Scanner;class GFG { public static void main (String[] args) { Scanner scanner = new Scanner(System.in); String Str = scanner.nextLine(); char[] arr = Str.toCharArray(); String rev = ""; for(int i = Str.length() - 1; i >= 0; i--) { rev = rev + Str.charAt(i); } System.out.println(rev); }} |
Output
0
In the above code, we are essentially reading a String from the user before starting an iteration loop to create a new, inverted String. The “charAt” function of the String class is used to retrieve each character of the original String individually from the end, and the “+” operator is used to concatenate them into a new String.
Using Stack:
The idea of stack is that we will be accessing only top element of a stack so it includes 2 steps as follows:
- push all the characters in stack.
- pop all the characters from stack and append to temp string.
Below is the implementation of the above approach.
Java
import java.util.*;class GFG { public static void main(String[] args) { String s="Geeks For Geeks"; //initializing a stack of type char Stack<Character> stack=new Stack<>(); for(char c:s.toCharArray()) { //pushing all the characters stack.push(c); } String temp=""; while(!stack.isEmpty()) { //popping all the chars and appending to temp temp+=stack.pop(); } System.out.println("Reversed string is : "+temp); }}//This code is contributed by aeroabrar_31 |
Output
Reversed string is : skeeG roF skeeG
Time Complexity : O(N) N is length of string
Auxiliary Space: O(N) for stack
Related Article: Different methods to reverse a string in C/C++
This article is contributed by Mr. Somesh Awasthi. If you like Lazyroar and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the Lazyroar main page and help other Geeks
