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Rewriting the String in Reverse [duplicate]

I have "Hello World" kept in a String variable named hi.
I need to print it, but reversed.
How can I do this? I understand there is some kind of a function already built-in into Java that does that.
Related: Reverse each individual word of “Hello World” string with Java

You can use this:
new StringBuilder(hi).reverse().toString()
StringBuilder was added in Java 5. For versions prior to Java 5, the StringBuffer class can be used instead — it has the same API.

For Online Judges problems that does not allow StringBuilder or StringBuffer, you can do it in place using char[] as following:
public static String reverse(String input){
char[] in = input.toCharArray();
int begin=0;
int end=in.length-1;
char temp;
while(end>begin){
temp = in[begin];
in[begin]=in[end];
in[end] = temp;
end--;
begin++;
}
return new String(in);
}

public static String reverseIt(String source) {
int i, len = source.length();
StringBuilder dest = new StringBuilder(len);
for (i = (len - 1); i >= 0; i--){
dest.append(source.charAt(i));
}
return dest.toString();
}
http://www.java2s.com/Code/Java/Language-Basics/ReverseStringTest.htm

String string="whatever";
String reverse = new StringBuffer(string).reverse().toString();
System.out.println(reverse);

I am doing this by using the following two ways:
Reverse string by CHARACTERS:
public static void main(String[] args) {
// Using traditional approach
String result="";
for(int i=string.length()-1; i>=0; i--) {
result = result + string.charAt(i);
}
System.out.println(result);
// Using StringBuffer class
StringBuffer buffer = new StringBuffer(string);
System.out.println(buffer.reverse());
}
Reverse string by WORDS:
public static void reverseStringByWords(String string) {
StringBuilder stringBuilder = new StringBuilder();
String[] words = string.split(" ");
for (int j = words.length-1; j >= 0; j--) {
stringBuilder.append(words[j]).append(' ');
}
System.out.println("Reverse words: " + stringBuilder);
}

Take a look at the Java 6 API under StringBuffer
String s = "sample";
String result = new StringBuffer(s).reverse().toString();

Here is an example using recursion:
public void reverseString() {
String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
String reverseAlphabet = reverse(alphabet, alphabet.length()-1);
}
String reverse(String stringToReverse, int index){
if(index == 0){
return stringToReverse.charAt(0) + "";
}
char letter = stringToReverse.charAt(index);
return letter + reverse(stringToReverse, index-1);
}

Here is a low level solution:
import java.util.Scanner;
public class class1 {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
String inpStr = in.nextLine();
System.out.println("Original String :" + inpStr);
char temp;
char[] arr = inpStr.toCharArray();
int len = arr.length;
for(int i=0; i<(inpStr.length())/2; i++,len--){
temp = arr[i];
arr[i] = arr[len-1];
arr[len-1] = temp;
}
System.out.println("Reverse String :" + String.valueOf(arr));
}
}

I tried, just for fun, by using a Stack. Here my code:
public String reverseString(String s) {
Stack<Character> stack = new Stack<>();
StringBuilder sb = new StringBuilder();
for (int i = 0; i < s.length(); i++) {
stack.push(s.charAt(i));
}
while (!stack.empty()) {
sb.append(stack.pop());
}
return sb.toString();
}

Since the below method (using XOR) to reverse a string is not listed, I am attaching this method to reverse a string.
The Algorithm is based on :
1.(A XOR B) XOR B = A
2.(A XOR B) XOR A = B
Code snippet:
public class ReverseUsingXOR {
public static void main(String[] args) {
String str = "prateek";
reverseUsingXOR(str.toCharArray());
}
/*Example:
* str= prateek;
* str[low]=p;
* str[high]=k;
* str[low]=p^k;
* str[high]=(p^k)^k =p;
* str[low]=(p^k)^p=k;
*
* */
public static void reverseUsingXOR(char[] str) {
int low = 0;
int high = str.length - 1;
while (low < high) {
str[low] = (char) (str[low] ^ str[high]);
str[high] = (char) (str[low] ^ str[high]);
str[low] = (char) (str[low] ^ str[high]);
low++;
high--;
}
//display reversed string
for (int i = 0; i < str.length; i++) {
System.out.print(str[i]);
}
}
}
Output:
keetarp

As others have pointed out the preferred way is to use:
new StringBuilder(hi).reverse().toString()
But if you want to implement this by yourself, I'm afraid that the rest of responses have flaws.
The reason is that String represents a list of Unicode points, encoded in a char[] array according to the variable-length encoding: UTF-16.
This means some code points use a single element of the array (one code unit) but others use two of them, so there might be pairs of characters that must be treated as a single unit (consecutive "high" and "low" surrogates).
public static String reverseString(String s) {
char[] chars = new char[s.length()];
boolean twoCharCodepoint = false;
for (int i = 0; i < s.length(); i++) {
chars[s.length() - 1 - i] = s.charAt(i);
if (twoCharCodepoint) {
swap(chars, s.length() - 1 - i, s.length() - i);
}
twoCharCodepoint = !Character.isBmpCodePoint(s.codePointAt(i));
}
return new String(chars);
}
private static void swap(char[] array, int i, int j) {
char temp = array[i];
array[i] = array[j];
array[j] = temp;
}
public static void main(String[] args) throws Exception {
FileOutputStream fos = new FileOutputStream("C:/temp/reverse-string.txt");
StringBuilder sb = new StringBuilder("Linear B Syllable B008 A: ");
sb.appendCodePoint(65536); //http://unicode-table.com/es/#10000
sb.append(".");
fos.write(sb.toString().getBytes("UTF-16"));
fos.write("\n".getBytes("UTF-16"));
fos.write(reverseString(sb.toString()).getBytes("UTF-16"));
}

Using charAt() method
String name = "gaurav";
String reversedString = "";
for(int i = name.length()-1; i>=0; i--){
reversedString = reversedString + name.charAt(i);
}
System.out.println(reversedString);
Using toCharArray() method
String name = "gaurav";
char [] stringCharArray = name.toCharArray();
String reversedString = "";
for(int i = stringCharArray.length-1; i>=0; i--) {
reversedString = reversedString + stringCharArray[i];
}
System.out.println(reversedString);
Using reverse() method of the Stringbuilder
String name = "gaurav";
String reversedString = new StringBuilder(name).reverse().toString();
System.out.println(reversedString);
Check https://coderolls.com/reverse-a-string-in-java/

It is very simple in minimum code of lines
public class ReverseString {
public static void main(String[] args) {
String s1 = "neelendra";
for(int i=s1.length()-1;i>=0;i--)
{
System.out.print(s1.charAt(i));
}
}
}

This did the trick for me
public static void main(String[] args) {
String text = "abcdefghijklmnopqrstuvwxyz";
for (int i = (text.length() - 1); i >= 0; i--) {
System.out.print(text.charAt(i));
}
}

1. Using Character Array:
public String reverseString(String inputString) {
char[] inputStringArray = inputString.toCharArray();
String reverseString = "";
for (int i = inputStringArray.length - 1; i >= 0; i--) {
reverseString += inputStringArray[i];
}
return reverseString;
}
2. Using StringBuilder:
public String reverseString(String inputString) {
StringBuilder stringBuilder = new StringBuilder(inputString);
stringBuilder = stringBuilder.reverse();
return stringBuilder.toString();
}
OR
return new StringBuilder(inputString).reverse().toString();

System.out.print("Please enter your name: ");
String name = keyboard.nextLine();
String reverse = new StringBuffer(name).reverse().toString();
String rev = reverse.toLowerCase();
System.out.println(rev);
I used this method to turn names backwards and into lower case.

One natural way to reverse a String is to use a StringTokenizer and a stack. Stack is a class that implements an easy-to-use last-in, first-out (LIFO) stack of objects.
String s = "Hello My name is Sufiyan";
Put it in the stack frontwards
Stack<String> myStack = new Stack<>();
StringTokenizer st = new StringTokenizer(s);
while (st.hasMoreTokens()) {
myStack.push(st.nextToken());
}
Print the stack backwards
System.out.print('"' + s + '"' + " backwards by word is:\n\t\"");
while (!myStack.empty()) {
System.out.print(myStack.pop());
System.out.print(' ');
}
System.out.println('"');

public String reverse(String s) {
String reversedString = "";
for(int i=s.length(); i>0; i--) {
reversedString += s.charAt(i-1);
}
return reversedString;
}

public class Test {
public static void main(String args[]) {
StringBuffer buffer = new StringBuffer("Game Plan");
buffer.reverse();
System.out.println(buffer);
}
}

All above solution is too good but here I am making reverse string using recursive programming.
This is helpful for who is looking recursive way of doing reverse string.
public class ReversString {
public static void main(String args[]) {
char s[] = "Dhiral Pandya".toCharArray();
String r = new String(reverse(0, s));
System.out.println(r);
}
public static char[] reverse(int i, char source[]) {
if (source.length / 2 == i) {
return source;
}
char t = source[i];
source[i] = source[source.length - 1 - i];
source[source.length - 1 - i] = t;
i++;
return reverse(i, source);
}
}

You can also try this:
public class StringReverse {
public static void main(String[] args) {
String str = "Dogs hates cats";
StringBuffer sb = new StringBuffer(str);
System.out.println(sb.reverse());
}
}

Procedure :
We can use split() to split the string .Then use reverse loop and add the characters.
Code snippet:
class test
{
public static void main(String args[])
{
String str = "world";
String[] split= str.split("");
String revers = "";
for (int i = split.length-1; i>=0; i--)
{
revers += split[i];
}
System.out.printf("%s", revers);
}
}
//output : dlrow

It gets the value you typed and returns it reversed ;)
public static String reverse (String a){
char[] rarray = a.toCharArray();
String finalvalue = "";
for (int i = 0; i < rarray.length; i++)
{
finalvalue += rarray[rarray.length - 1 - i];
}
return finalvalue;
}

public String reverseWords(String s) {
String reversedWords = "";
if(s.length()<=0) {
return reversedWords;
}else if(s.length() == 1){
if(s == " "){
return "";
}
return s;
}
char arr[] = s.toCharArray();
int j = arr.length-1;
while(j >= 0 ){
if( arr[j] == ' '){
reversedWords+=arr[j];
}else{
String temp="";
while(j>=0 && arr[j] != ' '){
temp+=arr[j];
j--;
}
j++;
temp = reverseWord(temp);
reversedWords+=temp;
}
j--;
}
String[] chk = reversedWords.split(" ");
if(chk == null || chk.length == 0){
return "";
}
return reversedWords;
}
public String reverseWord(String s){
char[] arr = s.toCharArray();
for(int i=0,j=arr.length-1;i<=j;i++,j--){
char tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
}
return String.valueOf(arr);
}

public static void main(String[] args) {
String str = "Prashant";
int len = str.length();
char[] c = new char[len];
for (int j = len - 1, i = 0; j >= 0; j--, i++) {
c[i] = str.charAt(j);
}
str = String.copyValueOf(c);
System.out.println(str);
}

public void reverString(){
System.out.println("Enter value");
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
try{
String str=br.readLine();
char[] charArray=str.toCharArray();
for(int i=charArray.length-1; i>=0; i--){
System.out.println(charArray[i]);
}
}
catch(IOException ex){
}

recursion:
public String stringReverse(String string) {
if (string == null || string.length() == 0) {
return string;
}
return stringReverse(string.substring(1)) + string.charAt(0);
}

Sequence of characters (or) StringString's Family:
String testString = "Yashwanth#777"; // ~1 1⁄4→D800₁₆«2²⁰
Using Java 8 Stream API
First we convert String into stream by using method CharSequence.chars(), then we use the method IntStream.range to generate a sequential stream of numbers. Then we map this sequence of stream into String.
public static String reverseString_Stream(String str) {
IntStream cahrStream = str.chars();
final int[] array = cahrStream.map( x -> x ).toArray();
int from = 0, upTo = array.length;
IntFunction<String> reverseMapper = (i) -> ( Character.toString((char) array[ (upTo - i) + (from - 1) ]) );
String reverseString = IntStream.range(from, upTo) // for (int i = from; i < upTo ; i++) { ... }
.mapToObj( reverseMapper ) // array[ lastElement ]
.collect(Collectors.joining()) // Joining stream of elements together into a String.
.toString(); // This object (which is already a string!) is itself returned.
System.out.println("Reverse Stream as String : "+ reverseString);
return reverseString;
}
Using a Traditional for Loop
If you want to reverse the string then we need to follow these steps.
Convert String into an Array of Characters.
Iterate over an array in reverse order, append each Character to temporary string variable until the last character.
public static String reverseString( String reverse ) {
if( reverse != null && reverse != "" && reverse.length() > 0 ) {
char[] arr = reverse.toCharArray();
String temp = "";
for( int i = arr.length-1; i >= 0; i-- ) {
temp += arr[i];
}
System.out.println("Reverse String : "+ temp);
}
return null;
}
Easy way to Use reverse method provided form StringBuffer or StringBuilder Classes
StringBuilder and StringBuffer are mutable sequence of characters. That means one can change the value of these object's.
StringBuffer buffer = new StringBuffer(str);
System.out.println("StringBuffer - reverse : "+ buffer.reverse() );
String builderString = (new StringBuilder(str)).reverse().toString;
System.out.println("StringBuilder generated reverse String : "+ builderString );
StringBuffer has the same methods as the StringBuilder, but each method in StringBuffer is synchronized so it is thread safe.

public static String revString(String str){
char[] revCharArr = str.toCharArray();
for (int i=0; i< str.length()/2; i++){
char f = revCharArr[i];
char l = revCharArr[str.length()-i-1];
revCharArr[i] = l;
revCharArr[str.length()-i-1] = f;
}
String revStr = new String(revCharArr);
return revStr;
}

Simple For loop in java
public void reverseString(char[] s) {
int length = s.length;
for (int i = 0; i < s.length / 2; i++) {
// swaping character
char temp = s[length - i - 1];
s[length - i - 1] = s[i];
s[i] = temp;
}
}

Reverse String characters in java

Am trying to reverse a string using a method in java, I can fetch all the elements of the string and print them out in order via a loop, my problem is reversing the string such that the first comes last and the last comes first, I tried to find a reverse function to no avail... Here is what I have so far...
private static void palindrome() {
char[] name = new char[]{};
String name1;
System.out.println("Enter your name");
Scanner tim = new Scanner(System.in);
name1 = tim.next();
int len = name1.length();
for (int i = 0; i <= len; ++i) {
char b = name1.charAt(i);
System.out.println(b + " ");
}
}
That loop succeeds in printing out the single characters from the string.

You can use StringBuilder like this:
import java.lang.*;
import java.io.*;
import java.util.*;
class 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 = input1.reverse();
// print reversed String
System.out.println(input1);
}
}
You can also modify your code to do this:
1 -
for (int i = 0; i <= len; ++i) {
char b = name1[len - i];
System.out.println(b + " ");
}
2 -
for (int i = len; i >= 0; --i) {
char b = name1.charAt(i);
System.out.println(b + " ");
}

Using Java 9 codePoints stream you can reverse a string as follows. This example shows the reversal of a string containing surrogate pairs. It works with regular characters as well.
String str = "𝕙𝕖𝕝𝕝𝕠 𝕨𝕠𝕣𝕝𝕕";
String reversed = str.codePoints()
// Stream<String>
.mapToObj(Character::toString)
// concatenate in reverse order
.reduce((a, b) -> b + a)
.get();
System.out.println(reversed); // 𝕕𝕝𝕣𝕠𝕨 𝕠𝕝𝕝𝕖𝕙
See also: Reverse string printing method

You simply need to loop through the array backwards:
for (int i = len - 1; i >= 0; i--) {
char b = name1.charAt(i);
System.out.println(b + " ");
}
You start at the last element which has its index at the position length - 1 and iterate down to the first element (with index zero).
This concept is not specific to Java and also applies to other data structures that provide index based access (such as lists).

Use the built-in reverse() method of the StringBuilder class.
private static void palindrome() {
String name1;
StringBuilder input = new StringBuilder();
System.out.println("Enter your name");
Scanner tim = new Scanner(System.in);
name1 = tim.next();
input.append(name1);
input.reverse();
System.out.println(input);
}

Added reverse() function for your understanding
import java.util.Scanner;
public class P3 {
public static void main(String[] args) {
palindrome();
}
private static void palindrome() {
char[] name = new char[]{};
String name1;
System.out.println("Enter your name");
Scanner tim = new Scanner(System.in);
name1 = tim.next();
String nameReversed = reverse(name1);
int len = name1.length();
for (int i = 0; i < len; ++i) {
char b = name1.charAt(i);
System.out.println(b + " ");
}
}
private static String reverse(String name1) {
char[] arr = name1.toCharArray();
int left = 0, right = arr.length - 1;
while (left < right) {
//swap characters first and last positions
char temp = arr[left];
arr[left++] = arr[right];
arr[right--] = temp;
}
return new String(arr);
}
}

you can try the build-in function charAt()
private String reverseString2(String str) {
if (str == null) {
return null;
}
String result = "";
for (int i = str.length() - 1; i >= 0; i--) {
result = result + str.charAt(i);
}
return result;
}
public void test(){
System.out.println(reverseString2("abcd"));
}
see also rever a string in java

String reversed = new StringBuilder(originalString).reverse().toString();

How to reverse String in place in Java

How to reverse String in place in Java
input String : 1234
Output Should : 4321
what i have tried.
public static void main(String args[])
{
String number = "1234";
System.out.println("original String: " + number); String reversed = inPlaceReverse(number);
System.out.println("reversed String: " + reversed);
}
public static String inPlaceReverse(final String input)
{
final StringBuilder builder = new StringBuilder(input);
int length = builder.length();
for (int i = 0; i < length / 2; i++)
{
final char current = builder.charAt(i);
final int otherEnd = length - i - 1;
builder.setCharAt(i, builder.charAt(otherEnd)); // swap
builder.setCharAt(otherEnd, current);
}
return builder.toString();
}
i am getting answer like:
reversed String: 4231 as i expected 4321.

If your teacher wants to see your work then you should manipulate the chars directly. Something like the following should be enough to let you spot the mistake:
public static String reverse(String orig)
{
char[] s = orig.toCharArray();
final int n = s.length;
final int halfLength = n / 2;
for (int i=0; i<halfLength; i++)
{
char temp = s[i];
s[i] = s[n-1-i];
s[n-1-i] = temp;
}
return new String(s);
}

It can be even simpler using StringBuilder's reverse() function:
public static String inPlaceReverse(String input) {
StringBuilder builder = new StringBuilder(input);
return builder.reverse().toString();
}

public static String inPlaceReverse(String number) {
char[] ch = number.toCharArray();
int i = 0;
int j = number.length()-1;
while (i < j) {
char temp = ch[i];
ch[i] = ch[j];
ch[j] = temp;
i++;
j--;
}
return String.valueOf(ch);
}

1. Using Character Array:
public String reverseSting(String inputString) {
char[] inputStringArray = inputString.toCharArray();
String reverseString = "";
for (int i = inputStringArray.length - 1; i >= 0; i--) {
reverseString += inputStringArray[i];
}
return reverseString;
}
2. Using StringBuilder:
public String reverseSting(String inputString) {
StringBuilder stringBuilder = new StringBuilder(inputString);
stringBuilder = stringBuilder.reverse();
return stringBuilder.toString();
}
OR
return new StringBuilder(inputString).reverse().toString();

This is an interview question.
Reverse a String in place :
public class Solution4 {
public static void main(String[] args) {
String a = "Protijayi";
System.out.println(reverse(a)); //iyajitorP
}
private static String reverse(String a) {
char[] ca = a.toCharArray();
int start = 0 ; int end = a.length()-1;
while(end > start) {
swap(ca,start,end);
start++;
end--;
}//while
return new String(ca);
}
private static void swap(char[] ca, int start, int end) {
char t = ca[start];
ca[start] = ca[end];
ca[end] = t ;
}
}

Mind also, that you can avoid using additional memory during the swap, though having some extra computation.
public class StringReverser {
public static String reverseStringInPlace(String toReverse) {
char[] chars = toReverse.toCharArray();
int inputStringLength = toReverse.length();
for (int i = 0; i < inputStringLength / 2; i++) {
int toMoveBack = toReverse.charAt(i);
int toMoveForward = toReverse.charAt(inputStringLength - i - 1);
//swap
toMoveForward = toMoveBack - toMoveForward;
toMoveBack -= toMoveForward;
toMoveForward += toMoveBack;
chars[i] = (char) toMoveBack;
chars[inputStringLength - i - 1] = (char) toMoveForward;
}
return String.valueOf(chars);
}
public static void main(String[] args) {
System.out.println(reverseStringInPlace("asd0")); // output: 0dsa
System.out.println(reverseStringInPlace("sd0")); // output: 0ds
System.out.println(reverseStringInPlace("")); // output: empty
System.out.println(reverseStringInPlace("-")); // output: -
System.out.println(reverseStringInPlace("ABD+C")); // output: C+DBA
System.out.println(reverseStringInPlace("勒")); // output: 勒
System.out.println(reverseStringInPlace("分歧。")); // output: 。歧分
System.out.println(reverseStringInPlace("Marítimo")); // output: omitíraM
}
}
Relevant to swap discussion can be found here:
How to swap two numbers without using temp variables or arithmetic operations?

Convert the string to a character array first and then use recursion.
public void reverseString(char[] s) {
helper(0, s.length - 1, s);
}
private void helper(int left, int right, char[] s){
if(left >= right) {
return;
}
char temp = s[left];
s[left++] = s[right];
s[right--] = temp;
helper(left, right, s);
}
So with the input [1,2,3,4], the helper function will be called as follows :
1. helper(0, 3, [1,2,3,4]), Swap 1 and 4
2. helper(1, 2, [1,2,3,4]), Swap 2 and 3
3. helper(2, 1, [1,2,3,4]) Terminates, left is now greater than right

After converting into char array. Just swap the both ends ( first index, last index) and move towards each other(first index to last index and from last index to first) until the crossing.
public void reverseString(char[] s) {
int start = 0;
int end = s.length-1;
char temp = ' ';
while((start)<(end)){
temp = s[start];
s[start] = s[end];
s[end] = temp;
start++;
end--;
}
System.out.println(s);
}

Reverseing a string in java [duplicate]

I have "Hello World" kept in a String variable named hi.
I need to print it, but reversed.
How can I do this? I understand there is some kind of a function already built-in into Java that does that.
Related: Reverse each individual word of “Hello World” string with Java

You can use this:
new StringBuilder(hi).reverse().toString()
StringBuilder was added in Java 5. For versions prior to Java 5, the StringBuffer class can be used instead — it has the same API.

For Online Judges problems that does not allow StringBuilder or StringBuffer, you can do it in place using char[] as following:
public static String reverse(String input){
char[] in = input.toCharArray();
int begin=0;
int end=in.length-1;
char temp;
while(end>begin){
temp = in[begin];
in[begin]=in[end];
in[end] = temp;
end--;
begin++;
}
return new String(in);
}

public static String reverseIt(String source) {
int i, len = source.length();
StringBuilder dest = new StringBuilder(len);
for (i = (len - 1); i >= 0; i--){
dest.append(source.charAt(i));
}
return dest.toString();
}
http://www.java2s.com/Code/Java/Language-Basics/ReverseStringTest.htm

String string="whatever";
String reverse = new StringBuffer(string).reverse().toString();
System.out.println(reverse);

I am doing this by using the following two ways:
Reverse string by CHARACTERS:
public static void main(String[] args) {
// Using traditional approach
String result="";
for(int i=string.length()-1; i>=0; i--) {
result = result + string.charAt(i);
}
System.out.println(result);
// Using StringBuffer class
StringBuffer buffer = new StringBuffer(string);
System.out.println(buffer.reverse());
}
Reverse string by WORDS:
public static void reverseStringByWords(String string) {
StringBuilder stringBuilder = new StringBuilder();
String[] words = string.split(" ");
for (int j = words.length-1; j >= 0; j--) {
stringBuilder.append(words[j]).append(' ');
}
System.out.println("Reverse words: " + stringBuilder);
}

Take a look at the Java 6 API under StringBuffer
String s = "sample";
String result = new StringBuffer(s).reverse().toString();

Here is an example using recursion:
public void reverseString() {
String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
String reverseAlphabet = reverse(alphabet, alphabet.length()-1);
}
String reverse(String stringToReverse, int index){
if(index == 0){
return stringToReverse.charAt(0) + "";
}
char letter = stringToReverse.charAt(index);
return letter + reverse(stringToReverse, index-1);
}

Here is a low level solution:
import java.util.Scanner;
public class class1 {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
String inpStr = in.nextLine();
System.out.println("Original String :" + inpStr);
char temp;
char[] arr = inpStr.toCharArray();
int len = arr.length;
for(int i=0; i<(inpStr.length())/2; i++,len--){
temp = arr[i];
arr[i] = arr[len-1];
arr[len-1] = temp;
}
System.out.println("Reverse String :" + String.valueOf(arr));
}
}

I tried, just for fun, by using a Stack. Here my code:
public String reverseString(String s) {
Stack<Character> stack = new Stack<>();
StringBuilder sb = new StringBuilder();
for (int i = 0; i < s.length(); i++) {
stack.push(s.charAt(i));
}
while (!stack.empty()) {
sb.append(stack.pop());
}
return sb.toString();
}

Since the below method (using XOR) to reverse a string is not listed, I am attaching this method to reverse a string.
The Algorithm is based on :
1.(A XOR B) XOR B = A
2.(A XOR B) XOR A = B
Code snippet:
public class ReverseUsingXOR {
public static void main(String[] args) {
String str = "prateek";
reverseUsingXOR(str.toCharArray());
}
/*Example:
* str= prateek;
* str[low]=p;
* str[high]=k;
* str[low]=p^k;
* str[high]=(p^k)^k =p;
* str[low]=(p^k)^p=k;
*
* */
public static void reverseUsingXOR(char[] str) {
int low = 0;
int high = str.length - 1;
while (low < high) {
str[low] = (char) (str[low] ^ str[high]);
str[high] = (char) (str[low] ^ str[high]);
str[low] = (char) (str[low] ^ str[high]);
low++;
high--;
}
//display reversed string
for (int i = 0; i < str.length; i++) {
System.out.print(str[i]);
}
}
}
Output:
keetarp

As others have pointed out the preferred way is to use:
new StringBuilder(hi).reverse().toString()
But if you want to implement this by yourself, I'm afraid that the rest of responses have flaws.
The reason is that String represents a list of Unicode points, encoded in a char[] array according to the variable-length encoding: UTF-16.
This means some code points use a single element of the array (one code unit) but others use two of them, so there might be pairs of characters that must be treated as a single unit (consecutive "high" and "low" surrogates).
public static String reverseString(String s) {
char[] chars = new char[s.length()];
boolean twoCharCodepoint = false;
for (int i = 0; i < s.length(); i++) {
chars[s.length() - 1 - i] = s.charAt(i);
if (twoCharCodepoint) {
swap(chars, s.length() - 1 - i, s.length() - i);
}
twoCharCodepoint = !Character.isBmpCodePoint(s.codePointAt(i));
}
return new String(chars);
}
private static void swap(char[] array, int i, int j) {
char temp = array[i];
array[i] = array[j];
array[j] = temp;
}
public static void main(String[] args) throws Exception {
FileOutputStream fos = new FileOutputStream("C:/temp/reverse-string.txt");
StringBuilder sb = new StringBuilder("Linear B Syllable B008 A: ");
sb.appendCodePoint(65536); //http://unicode-table.com/es/#10000
sb.append(".");
fos.write(sb.toString().getBytes("UTF-16"));
fos.write("\n".getBytes("UTF-16"));
fos.write(reverseString(sb.toString()).getBytes("UTF-16"));
}

Using charAt() method
String name = "gaurav";
String reversedString = "";
for(int i = name.length()-1; i>=0; i--){
reversedString = reversedString + name.charAt(i);
}
System.out.println(reversedString);
Using toCharArray() method
String name = "gaurav";
char [] stringCharArray = name.toCharArray();
String reversedString = "";
for(int i = stringCharArray.length-1; i>=0; i--) {
reversedString = reversedString + stringCharArray[i];
}
System.out.println(reversedString);
Using reverse() method of the Stringbuilder
String name = "gaurav";
String reversedString = new StringBuilder(name).reverse().toString();
System.out.println(reversedString);
Check https://coderolls.com/reverse-a-string-in-java/

It is very simple in minimum code of lines
public class ReverseString {
public static void main(String[] args) {
String s1 = "neelendra";
for(int i=s1.length()-1;i>=0;i--)
{
System.out.print(s1.charAt(i));
}
}
}

This did the trick for me
public static void main(String[] args) {
String text = "abcdefghijklmnopqrstuvwxyz";
for (int i = (text.length() - 1); i >= 0; i--) {
System.out.print(text.charAt(i));
}
}

1. Using Character Array:
public String reverseString(String inputString) {
char[] inputStringArray = inputString.toCharArray();
String reverseString = "";
for (int i = inputStringArray.length - 1; i >= 0; i--) {
reverseString += inputStringArray[i];
}
return reverseString;
}
2. Using StringBuilder:
public String reverseString(String inputString) {
StringBuilder stringBuilder = new StringBuilder(inputString);
stringBuilder = stringBuilder.reverse();
return stringBuilder.toString();
}
OR
return new StringBuilder(inputString).reverse().toString();

System.out.print("Please enter your name: ");
String name = keyboard.nextLine();
String reverse = new StringBuffer(name).reverse().toString();
String rev = reverse.toLowerCase();
System.out.println(rev);
I used this method to turn names backwards and into lower case.

One natural way to reverse a String is to use a StringTokenizer and a stack. Stack is a class that implements an easy-to-use last-in, first-out (LIFO) stack of objects.
String s = "Hello My name is Sufiyan";
Put it in the stack frontwards
Stack<String> myStack = new Stack<>();
StringTokenizer st = new StringTokenizer(s);
while (st.hasMoreTokens()) {
myStack.push(st.nextToken());
}
Print the stack backwards
System.out.print('"' + s + '"' + " backwards by word is:\n\t\"");
while (!myStack.empty()) {
System.out.print(myStack.pop());
System.out.print(' ');
}
System.out.println('"');

public String reverse(String s) {
String reversedString = "";
for(int i=s.length(); i>0; i--) {
reversedString += s.charAt(i-1);
}
return reversedString;
}

public class Test {
public static void main(String args[]) {
StringBuffer buffer = new StringBuffer("Game Plan");
buffer.reverse();
System.out.println(buffer);
}
}

All above solution is too good but here I am making reverse string using recursive programming.
This is helpful for who is looking recursive way of doing reverse string.
public class ReversString {
public static void main(String args[]) {
char s[] = "Dhiral Pandya".toCharArray();
String r = new String(reverse(0, s));
System.out.println(r);
}
public static char[] reverse(int i, char source[]) {
if (source.length / 2 == i) {
return source;
}
char t = source[i];
source[i] = source[source.length - 1 - i];
source[source.length - 1 - i] = t;
i++;
return reverse(i, source);
}
}

You can also try this:
public class StringReverse {
public static void main(String[] args) {
String str = "Dogs hates cats";
StringBuffer sb = new StringBuffer(str);
System.out.println(sb.reverse());
}
}

Procedure :
We can use split() to split the string .Then use reverse loop and add the characters.
Code snippet:
class test
{
public static void main(String args[])
{
String str = "world";
String[] split= str.split("");
String revers = "";
for (int i = split.length-1; i>=0; i--)
{
revers += split[i];
}
System.out.printf("%s", revers);
}
}
//output : dlrow

It gets the value you typed and returns it reversed ;)
public static String reverse (String a){
char[] rarray = a.toCharArray();
String finalvalue = "";
for (int i = 0; i < rarray.length; i++)
{
finalvalue += rarray[rarray.length - 1 - i];
}
return finalvalue;
}

public String reverseWords(String s) {
String reversedWords = "";
if(s.length()<=0) {
return reversedWords;
}else if(s.length() == 1){
if(s == " "){
return "";
}
return s;
}
char arr[] = s.toCharArray();
int j = arr.length-1;
while(j >= 0 ){
if( arr[j] == ' '){
reversedWords+=arr[j];
}else{
String temp="";
while(j>=0 && arr[j] != ' '){
temp+=arr[j];
j--;
}
j++;
temp = reverseWord(temp);
reversedWords+=temp;
}
j--;
}
String[] chk = reversedWords.split(" ");
if(chk == null || chk.length == 0){
return "";
}
return reversedWords;
}
public String reverseWord(String s){
char[] arr = s.toCharArray();
for(int i=0,j=arr.length-1;i<=j;i++,j--){
char tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
}
return String.valueOf(arr);
}

public static void main(String[] args) {
String str = "Prashant";
int len = str.length();
char[] c = new char[len];
for (int j = len - 1, i = 0; j >= 0; j--, i++) {
c[i] = str.charAt(j);
}
str = String.copyValueOf(c);
System.out.println(str);
}

public void reverString(){
System.out.println("Enter value");
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
try{
String str=br.readLine();
char[] charArray=str.toCharArray();
for(int i=charArray.length-1; i>=0; i--){
System.out.println(charArray[i]);
}
}
catch(IOException ex){
}

recursion:
public String stringReverse(String string) {
if (string == null || string.length() == 0) {
return string;
}
return stringReverse(string.substring(1)) + string.charAt(0);
}

Sequence of characters (or) StringString's Family:
String testString = "Yashwanth#777"; // ~1 1⁄4→D800₁₆«2²⁰
Using Java 8 Stream API
First we convert String into stream by using method CharSequence.chars(), then we use the method IntStream.range to generate a sequential stream of numbers. Then we map this sequence of stream into String.
public static String reverseString_Stream(String str) {
IntStream cahrStream = str.chars();
final int[] array = cahrStream.map( x -> x ).toArray();
int from = 0, upTo = array.length;
IntFunction<String> reverseMapper = (i) -> ( Character.toString((char) array[ (upTo - i) + (from - 1) ]) );
String reverseString = IntStream.range(from, upTo) // for (int i = from; i < upTo ; i++) { ... }
.mapToObj( reverseMapper ) // array[ lastElement ]
.collect(Collectors.joining()) // Joining stream of elements together into a String.
.toString(); // This object (which is already a string!) is itself returned.
System.out.println("Reverse Stream as String : "+ reverseString);
return reverseString;
}
Using a Traditional for Loop
If you want to reverse the string then we need to follow these steps.
Convert String into an Array of Characters.
Iterate over an array in reverse order, append each Character to temporary string variable until the last character.
public static String reverseString( String reverse ) {
if( reverse != null && reverse != "" && reverse.length() > 0 ) {
char[] arr = reverse.toCharArray();
String temp = "";
for( int i = arr.length-1; i >= 0; i-- ) {
temp += arr[i];
}
System.out.println("Reverse String : "+ temp);
}
return null;
}
Easy way to Use reverse method provided form StringBuffer or StringBuilder Classes
StringBuilder and StringBuffer are mutable sequence of characters. That means one can change the value of these object's.
StringBuffer buffer = new StringBuffer(str);
System.out.println("StringBuffer - reverse : "+ buffer.reverse() );
String builderString = (new StringBuilder(str)).reverse().toString;
System.out.println("StringBuilder generated reverse String : "+ builderString );
StringBuffer has the same methods as the StringBuilder, but each method in StringBuffer is synchronized so it is thread safe.

public static String revString(String str){
char[] revCharArr = str.toCharArray();
for (int i=0; i< str.length()/2; i++){
char f = revCharArr[i];
char l = revCharArr[str.length()-i-1];
revCharArr[i] = l;
revCharArr[str.length()-i-1] = f;
}
String revStr = new String(revCharArr);
return revStr;
}

Simple For loop in java
public void reverseString(char[] s) {
int length = s.length;
for (int i = 0; i < s.length / 2; i++) {
// swaping character
char temp = s[length - i - 1];
s[length - i - 1] = s[i];
s[i] = temp;
}
}

Remove repeated characters in a string

I need to write a static method that takes a String as a parameter and returns a new String obtained by replacing every instance of repeated adjacent letters with a single instance of that letter without using regular expressions. For example if I enter "maaaakkee" as a String, it returns "make".
I already tried the following code, but it doesn't seem to display the last character.
Here's my code:
import java.util.Scanner;
public class undouble {
public static void main(String [] args){
Scanner console = new Scanner(System.in);
System.out.println("enter String: ");
String str = console.nextLine();
System.out.println(removeSpaces(str));
}
public static String removeSpaces(String str){
String ourString="";
int j = 0;
for (int i=0; i<str.length()-1 ; i++){
j = i+1;
if(str.charAt(i)!=str.charAt(j)){
ourString+=str.charAt(i);
}
}
return ourString;
}
}

You could use regular expressions for that.
For instance:
String input = "ddooooonnneeeeee";
System.out.println(input.replaceAll("(.)\\1{1,}", "$1"));
Output:
done
Pattern explanation:
"(.)\\1{1,}" means any character (added to group 1) followed by itself at least once
"$1" references contents of group 1

maybe:
for (int i=1; i<str.length() ; i++){
j = i+1;
if(str.charAt(i)!=str.charAt(j)){
ourString+=str.charAt(i);
}
}

The problem is with your condition. You say compare i and i+1 in each iteration and in last iteration you have both i and j pointing to same location so it will never print the last character. Try this unleass you want to use regex to achive this:
EDIT:
public void removeSpaces(String str){
String ourString="";
for (int i=0; i<str.length()-1 ; i++){
if(i==0){
ourString = ""+str.charAt(i);
}else{
if(str.charAt(i-1) != str.charAt(i)){
ourString = ourString +str.charAt(i);
}
}
}
System.out.println(ourString);
}

if you cannot use replace or replaceAll, here is an alternative. O(2n), O(N) for stockage and O(N) for creating the string. It removes all repeated chars in the string put them in a stringbuilder.
input : abcdef , output : abcdef
input : aabbcdeef, output : cdf
private static String remove_repeated_char(String str)
{
StringBuilder result = new StringBuilder();
HashMap<Character, Integer> items = new HashMap<>();
for (int i = 0; i < str.length(); i++)
{
Character current = str.charAt(i);
Integer ocurrence = items.get(current);
if (ocurrence == null)
items.put(current, 1);
else
items.put(current, ocurrence + 1);
}
for (int i = 0; i < str.length(); i++)
{
Character current = str.charAt(i);
Integer ocurrence = items.get(current);
if (ocurrence == 1)
result.append(current);
}
return result.toString();
}

import java.util.*;
public class string2 {
public static void main(String[] args) {
//removes repeat character from array
Scanner sc=new Scanner(System.in);
StringBuffer sf=new StringBuffer();
System.out.println("enter a string");
sf.append(sc.nextLine());
System.out.println("string="+sf);
int i=0;
while( i<sf.length())
{
int j=1+i;
while(j<sf.length())
{
if(sf.charAt(i)==sf.charAt(j))
{
sf.deleteCharAt(j);
}
else
{
j=j+1;
}
}
i=i+1;
}
System.out.println("string="+sf);
}
}

Input AABBBccDDD, Output BD
Input ABBCDDA, Outout C
private String reducedString(String s){
char[] arr = s.toCharArray();
String newString = "";
Map<Character,Integer> map = new HashMap<Character,Integer>();
map.put(arr[0],1);
for(int index=1;index<s.length();index++)
{
Character key = arr[index];
int value;
if(map.get(key) ==null)
{
value =0;
}
else
{
value = map.get(key);
}
value = value+1;
map.put(key,value);
}
Set<Character> keyset = map.keySet();
for(Character c: keyset)
{
int value = map.get(c);
if(value%2 !=0)
{
newString+=c;
}
}
newString = newString.equals("")?"Empty String":newString;
return newString;
}

public class RemoveDuplicateCharecterInString {
static String input = new String("abbbbbbbbbbbbbbbbccccd");
static String output = "";
public static void main(String[] args)
{
// TODO Auto-generated method stub
for (int i = 0; i < input.length(); i++) {
char temp = input.charAt(i);
boolean check = false;
for (int j = 0; j < output.length(); j++) {
if (output.charAt(j) == input.charAt(i)) {
check = true;
}
}
if (!check) {
output = output + input.charAt(i);
}
}
System.out.println(" " + output);
}
}
Answer : abcd

public class RepeatedChar {
public static void main(String[] args) {
String rS = "maaaakkee";
String outCome= rS.charAt(0)+"";
int count =0;
char [] cA =rS.toCharArray();
for(int i =0; i+1<cA.length; ++i) {
if(rS.charAt(i) != rS.charAt(i+1)) {
outCome += rS.charAt(i+1);
}
}
System.out.println(outCome);
}
}

TO WRITE JAVA PROGRAM TO REMOVE REPEATED CHARACTERS:
package replace;
public class removingrepeatedcharacters
{
public static void main(String...args){
int i,j=0,count=0;
String str="noordeen";
String str2="noordeen";
char[] ch=str.toCharArray();
for(i=0;i<=5;i++)
{
count=0;
for(j=0;j<str2.length();j++)
{
if(ch[i]==str2.charAt(j))
{
count++;
System.out.println("at the index "+j +"position "+ch[i]+ "+ count is"+count);
if(count>=2){
str=str2;
str2=str.replaceFirst(Character.toString(ch[j]),Character.toString(' '));
}
System.out.println("after replacing " +str2);
}
}
}
}
}

String outstr = "";
String outstring = "";
for(int i = 0; i < str.length() - 1; i++) {
if(str.charAt(i) != str.charAt(i + 1)) {
outstr = outstr + str.charAt(i);
}
outstring = outstr + str.charAt(i);
}
System.out.println(outstring);

public static void remove_duplicates(String str){
String outstr="";
String outstring="";
for(int i=0;i<str.length()-1;i++) {
if(str.charAt(i)!=str.charAt(i+1)) {
outstr=outstr+str.charAt(i);
}
outstring=outstr+str.charAt(i);
}
System.out.println(outstring);
}

More fun with java 7:
System.out.println("11223344445555".replaceAll("(?<nums>.+)\\k<nums>+","${nums}"));
No more cryptic numbers in regexes.

public static String removeDuplicates(String str) {
String str2 = "" + str.charAt(0);
for (int i = 1; i < str.length(); i++) {
if (str.charAt(i - 1) == str.charAt(i) && i != 0) {
continue;
}
str2 = str2 + str.charAt(i);
}
return str2;
}