CODEWITHSUNDEEP
java
alain.janinm
alain.janinm

Reputation: 20065

Explanation of replace method in Java

I often use the method replace(CharSequence,CharSequence) in Java, but today I encountered something I don't really understand.

When I do :

"|||".replace("||","| |");

Why it results in : | || and not | | |? Why the second | can't be include in two patterns?

To resolve my problem I have to write "|||".replace("||","| |").replace("||","| |"); which looks a bit redundant.

Any one have an explanation to help me understand better the mechanism behind?

EDIT

So it has been said that it's the particular case specify in the javadoc, but it's wrong the example is the following :

 * The replacement proceeds from the beginning of the string to the end, for
 * example, replacing "aa" with "b" in the string "aaa" will result in
 * "ba" rather than "ab".

So it's different from my case because the result "ba" is not a match for "aa" anymore. Whereas in my case the result "| ||" still contains the pattern.

Upvotes: 2

Views: 206

Answers (5)

Robin
Robin

Reputation: 36611

Not really an answer on the why, but it is a good thing that 

"|||".replace("||","| |");

does not result in "| | |". If that would be the result, it means the replacement is also performed on a part of the String where replacements already took place. That would make it very easy to write an infinite loop, e.g. "|".replace("|", "||" ); would result in "||||||...." until your String becomes so big you run out-of-memory

Upvotes: 1

Zecas
Zecas

Reputation: 647

What you are looking for is 

"|||".replace("|","| ");

or

"|||".replace("|","| ").trim();

if you don't want the last space.

The why i let for you to self study. Tip: search for regular expression replacement.

Upvotes: 0

NPE
NPE

Reputation: 500407

There are two possibilities for why replace() might behave the way you expect it to:

  1. It could consider overlapping matches (in your example: match characters 1 & 2, and then 2 & 3). It does not do that.

  2. It could substitute recursively, meaning that stuff that got added/changed as a result of a substitution would be eligible for further matches. It does not do this either.

Upvotes: 1

Guillaume Polet
Guillaume Polet

Reputation: 47608

The replace starts by looking at the charsequence to find in your original string and once found, it says, ok the match starts in start and ends in end, let's replace that part with the replacement and then keep on looking for the next match from the end + 1 position

Upvotes: 1

James Montagne
James Montagne

Reputation: 78650

There are two things going on here that could cause the confusion. First, replace will not process the same character twice, so the middle | can only apply to a single replace. Second, replace does not process the replacement characters, so after the first replace you will have | ||, but the 2nd | is part of the first replacement.

So basically what you get is after one replace, you have the first || is replaced and the remainder is | which does not match so the replace is complete.

Upvotes: 3

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