Printing reverse of any String without using any predefined function?

Question

How to print the reverse of the String java is object orientated language without using any predefined function like reverse()?

Answer 1

I had this a while back, and having answered with the obvious StringBuffer.reverse() answer, they then asked 'Can you reverse a char array without using those API methods and achieve the result without spooling into a new char array?'

At the time I recognised that I only needed to iterate over half the length of the char array, but made a bit of a hash of explaining the actual code that needed to go into it (it was a verbal question). Anyway, I tried it when I got home and came up with this:

public class StringReverse {

public static void main(String[] args){

    String a = "String";

    char[] aChar = a.toCharArray();

    for (int i = (aChar.length-1)/2 ; i >= 0 ; i--){
        int posA = i;
        int posB = (aChar.length-1-i);
        char tmpA = aChar[posA];
        char tmpB = aChar[posB];
        System.out.println("Setting " + posA + " to " + tmpB);
        System.out.println("Setting " + posB + " to " + tmpA);

        aChar[posA] = tmpB;
        aChar[posB] = tmpA;
    }

    System.out.println(aChar);
}

}

You can obviously achieve this with less code, but I think the temporary assignments in the method make it more clear what the code is doing.

Outputs something like:

Setting 2 to i
Setting 3 to r
Setting 1 to n
Setting 4 to t
Setting 0 to g
Setting 5 to S

gnirtS

More of an interview question than a homework question, I'd say.

Answer 2

public String reverse(String arg)
{
    String tmp = null;
    if (arg.length() == 1)
    {
        return arg;
    }

    else
    {

        String lastChar = arg.substring(arg.length()-1,arg.length());

        String remainingString = arg.substring(0, arg.length() -1);

        tmp = lastChar + reverse(remainingString);
        return tmp;


    }
}

Answer 3

public static void main(String[] args) {


    String str = "hello world here I am";

    StringTokenizer strToken = new StringTokenizer(str);
    int token = strToken.countTokens();
    String str1 [] = new String[token];

    char chr[] = new char[str.length()];
    int counter = 0;

    for(int j=0; j < str.length(); j++) {

        if(str.charAt(j) != ' ') {
            chr[j] = str.charAt(j);
        }else {
            str1[counter++] = new String(chr).trim();
            chr = new char[str.length()];
        }
    }
    str1[counter++] = new String(chr).trim();

    for(int i=str1.length-1; i >= 0 ; i--) {
        System.out.println(str1[i]);
    }
}

O/P is: am I here world hello

Answer 4

 public class ReverseWithoutStringAPI {

public static void main(String[] args) {

      String st="hello";

        StringBuffer b=new StringBuffer();

        for(int i=st.length()-1;i>=0;i--){

            b.append(st.charAt(i)); }

        System.out.println("reverse:::"+b);
}
}

Answer 5

package com.ofs;

public class ReverseWordsInString{
public static void main(String[] args) {

    String str = "welcome to the new world and how are you feeling ?";

    // Get the Java runtime
    Runtime runtime = Runtime.getRuntime();
    // Run the garbage collector
    runtime.gc();
    // Calculate the used memory
    long firstUsageMemory = runtime.totalMemory() - runtime.freeMemory();
    System.out.println("Used memory in bytes: " + firstUsageMemory);
    System.out.println(str);
    str = new StringBuffer(str).reverse().toString();
    int count = 0;
    int preValue = 0;
    int lastspaceIndexVal = str.lastIndexOf(" ");
    int strLen = str.length();
    for (int i = 0; i < strLen - 1; i++) {
        if (Character.isWhitespace(str.charAt(i))) {
            if (i - preValue == 1 && count == 0) {
                str = str.substring(0, preValue) + str.charAt(i - 1)
                        + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 2 && count == 0) {
                str = str.substring(0, preValue) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 3 && count == 0) {
                str = str.substring(0, preValue) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 4 && count == 0) {
                str = str.substring(0, preValue) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.charAt(i - 4) + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 5 && count == 0) {
                str = str.substring(0, preValue) + str.charAt(i - 1)
                        + str.substring(i - 2, i - 1) + str.charAt(i - 3)
                        + str.charAt(i - 3) + str.charAt(i - 5)
                        + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 6 && count == 0) {
                str = str.substring(0, preValue) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.charAt(i - 4) + str.charAt(i - 5)
                        + str.charAt(i - 6) + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 7 && count == 0) {
                str = str.substring(0, preValue) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.charAt(i - 4) + str.charAt(i - 5)
                        + str.charAt(i - 6) + str.charAt(i - 7)
                        + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 8 && count == 0) {
                str = str.substring(0, preValue) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.charAt(i - 4) + str.charAt(i - 5)
                        + str.charAt(i - 6) + str.charAt(i - 7)
                        + str.charAt(i - 8) + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 2 && count != 0) {
                str = str.substring(0, preValue) + str.charAt(i - 1)
                        + str.substring(i, strLen);
                preValue = i;
            } else if (i - preValue == 3 && count != 0) {
                str = str.substring(0, preValue + 1) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.substring(i, strLen);
                preValue = i;
            } else if (i - preValue == 4 && count != 0) {
                str = str.substring(0, preValue + 1) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.substring(i, strLen);
                preValue = i;
            } else if (i - preValue == 5 && count != 0) {
                str = str.substring(0, preValue + 1) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.charAt(i - 4) + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 6 && count != 0) {
                str = str.substring(0, preValue + 1) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.charAt(i - 4) + str.charAt(i - 5)
                        + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 7 && count != 0) {
                str = str.substring(0, preValue + 1) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.charAt(i - 4) + str.charAt(i - 5)
                        + str.charAt(i - 6) + str.substring(i, strLen);
                preValue = i;
                count++;
            } else if (i - preValue == 8 && count != 0) {
                str = str.substring(0, preValue + 1) + str.charAt(i - 1)
                        + str.charAt(i - 2) + str.charAt(i - 3)
                        + str.charAt(i - 4) + str.charAt(i - 5)
                        + str.charAt(i - 6) + str.charAt(i - 7)
                        + str.substring(i, strLen);
                preValue = i;
                count++;
            }
            if (lastspaceIndexVal == preValue) {
                if (strLen - lastspaceIndexVal == 2 && count != 0) {
                    str = str.substring(0, preValue + 1)
                            + str.charAt(strLen - 1);
                    preValue = i;
                } else if (strLen - lastspaceIndexVal == 3 && count != 0) {
                    str = str.substring(0, preValue + 1)
                            + str.charAt(strLen - 1)
                            + str.charAt(strLen - 2);
                    preValue = i;
                } else if (strLen - lastspaceIndexVal == 4 && count != 0) {
                    str = str.substring(0, preValue + 1)
                            + str.charAt(strLen - 1)
                            + str.charAt(strLen - 2)
                            + str.charAt(strLen - 3);
                    preValue = i;
                    count++;
                } else if (strLen - lastspaceIndexVal == 5 && count != 0) {
                    str = str.substring(0, preValue + 1)
                            + str.charAt(strLen - 1)
                            + str.charAt(strLen - 2)
                            + str.charAt(strLen - 3)
                            + str.charAt(strLen - 4);
                    preValue = i;
                } else if (strLen - lastspaceIndexVal == 6 && count != 0) {
                    str = str.substring(0, preValue + 1)
                            + str.charAt(strLen - 1)
                            + str.charAt(strLen - 2)
                            + str.charAt(strLen - 3)
                            + str.charAt(strLen - 4)
                            + str.charAt(strLen - 5);
                    preValue = i;
                    count++;
                } else if (strLen - lastspaceIndexVal == 7 && count != 0) {
                    str = str.substring(0, preValue + 1)
                            + str.charAt(strLen - 1)
                            + str.charAt(strLen - 2)
                            + str.charAt(strLen - 3)
                            + str.charAt(strLen - 4)
                            + str.charAt(strLen - 5)
                            + str.charAt(strLen - 6);
                    preValue = i;
                } else if (strLen - lastspaceIndexVal == 8 && count != 0) {
                    str = str.substring(0, preValue + 1)
                            + str.charAt(strLen - 1)
                            + str.charAt(strLen - 2)
                            + str.charAt(strLen - 3)
                            + str.charAt(strLen - 4)
                            + str.charAt(strLen - 5)
                            + str.charAt(strLen - 6)
                            + str.charAt(strLen - 7);
                    preValue = i;
                }
            }
        }
    }
    runtime.gc();
    // Calculate the used memory
    long SecondaryUsageMemory = runtime.totalMemory()
            - runtime.freeMemory();
    System.out.println("Used memory in bytes: " + SecondaryUsageMemory);
    System.out.println(str);
}
}

Answer 6

this is the best solution for this

public class String_rev {
public static void main(String[] args) {
    String str="Karan Rajput";
    int ln=str.length();
    for (int i = ln; i > 0; i--) {
        System.out.print(str.charAt(i-1));
    }
}

}