Java using substring() to reverse String

Question

How is the String passed to this method getting reversed? If you pass "ABCD" the string become "DCBA", but I was thinking reverse(str.substring(1)) will call it self until it return "D" which will be concatenated with str.substring(0,1) which is D and the string returned to be "DD".JAVA programming Language.

private static String reverse(String str) {
    if (str.length() <= 1) {
        return str;
    }
    return reverse(str.substring(1)) + str.substring(0, 1);
}

Hi, welcome to SO. Do you know how recursion works? reverse(str.substring(1)) will call it self until it return "D" which will be concatinated with str.substring(0,1) wich is D and the string returned to be "DD" - No. Did you try debugging? — BackSlash
– BackSlash, Commented Sep 11, 2018 at 6:57

T.J. Crowder · Accepted Answer · 2018-09-11 07:00:20Z

7

Your best bet with things like this is to work it through on paper and/or step through it statement by statement in the debugger built into your IDE.

reverse("ABCD") returns reverse("BCD") + "A". This is recursion. The call hasn't returned yet, because it needs the result of reverse("BCD") before it can add "A" to it and return.
- reverse("BCD") returns reverse("CD") + "B", which waits for reverse("CD") to return
  - reverse("CD") returns reverse("D") + "C", which waits for reverse("D") to return
    - reverse("D") returns "D"
  - The result of reverse("D") + "C" is "DC", so now reverse("CD") can return "DC"
- The result of reverse("CD") + "B" is "DCB", so now reverse("BCD") can return "DCB"
The result of reverse("BCD") + "A" is "DCBA", so now reverse("ABCD") can return "DCBA"

answered Sep 11, 2018 at 7:00

T.J. Crowder

1.1m201 gold badges2k silver badges2k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Ndosho Mbwana Over a year ago

Thanks for the Quick response.

Danial Jalalnouri · Accepted Answer · 2018-09-11 16:07:09Z

0

You can also use the following procedure to solve your problem:

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

Also, according to the requirements about thread safety you can use StringBuffer or StringBuilder

answered Sep 11, 2018 at 16:07

Danial Jalalnouri

6401 gold badge8 silver badges19 bronze badges

Collectives™ on Stack Overflow

Java using substring() to reverse String

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related