Java - String.lastIndexOf(str) logic I cannot understand

Question

I used two different string to test the last index of "\t", but they both return 4. I thought it should be 5 and 4. I checked the oracle docs and I could not understand why. Could someone please tell me why? Thank you!

System.out.println("abc\t\tsubdir".lastIndexOf("\t"));
System.out.println("abct\tsubdir".lastIndexOf("\t"));

Are you confused by the length of `\t`? That's a String literal, its length at runtime will be one, a character representing a tab. See https://stackoverflow.com/questions/34393312/get-actual-length-of-the-defined-string-in-java — Sotirios Delimanolis, Aug 23 '18 at 19:58

score 10 · Answer 1 · answered Aug 23 '18 at 20:04

Lets make the number of indexes to understand it better :

String 1

a b c \t \t s u b d i r
0 1 2  3  4 5 6 7 8 9 10
          ^-----------------------------------last index of \t (for that you get 4)

String 2

a b c t \t s u b d i r
0 1 2 3  4 5 6 7 8 9 10
         ^-----------------------------------last index of \t (for that you get 4)

There are some special characters that should be escaped by \ (tab \t, breadline \n, quote \" ...) In Java, so it count as one character and not 2

Steven Spungin · Answer 2 · 2018-08-23T20:35:58.450

4

In the first line, the last tab is at 4 a b c <tab> <tab>

In the second line, the last tab is at 4 also a b c t <tab>

\t counts as 1 character

edited Aug 23 '18 at 20:35

answered Aug 23 '18 at 20:00

Steven Spungin

27,002
5
88
78

Emmanuel H · Answer 3 · 2018-08-23T20:05:53.220

3

This is because the \t does not count as two characters, it is an escape sequence and counts for only one character.

You can find the full list of escape sequences here : https://docs.oracle.com/javase/tutorial/java/data/characters.html

edited Aug 23 '18 at 20:05

answered Aug 23 '18 at 20:03

Emmanuel H

82
8

score 2 · Answer 4 · answered Aug 23 '18 at 20:08

2

It's important to note that the count starts from zero and '\t' only counts as one character. This can be confusing at times especially if you forget to start form zero.

0|1|2| 3| 4
a|b|c|\t|\t
a|b|c| t|\t

answered Aug 23 '18 at 20:08

John Ciociola

97
7

This answer did not exist when I started typing my answer and I have edited mine to reflect it. Thanks! – John Ciociola Aug 23 '18 at 20:11

score 0 · Answer 5 · answered Aug 23 '18 at 21:15

The count start with zero in java, that the reason why first and sysout returns 4. for your better understanding i added 3rd sysout where you can find the last index of \t will returns zero.

/**
 * @author itsection
 */
public class LastIndexOf {
    public static void main(String[] args) {
        System.out.println("abc\t\tsubdir".lastIndexOf("\t"));
        System.out.println("abct\tsubdir".lastIndexOf("\t"));
        System.out.println("\tabctsubdir".lastIndexOf("\t"));
    }
}// the output will be 4,4,0

Java - String.lastIndexOf(str) logic I cannot understand

5 Answers5