Java String's split method ignores empty substrings

Question

It occured to me today the behavior of java String.split() is very strange.

Actually I want to split a string \"aa,bb,cc,dd,,,ee\" to array

Answer 1

Use String.split(String regex, int limit) with negative limit (e.g. -1).

"aa,bb,cc,dd,,,,".split(",", -1)

When String.split(String regex) is called, it is called with limit = 0, which will remove all trailing empty strings in the array (in most cases, see below).

The actual behavior of String.split(String regex) is quite confusing:

• Splitting an empty string will result in an array of length 1. Empty string split will always result in length 1 array containing the empty string.
• Splitting ";" or ";;;" with regex being ";" will result in an empty array. Non-empty string split will result in all trailing empty strings in the array removed.

The behavior above can be observed from at least Java 5 to Java 8.

There was an attempt to change the behavior to return an empty array when splitting an empty string in JDK-6559590. However, it was soon reverted in JDK-8028321 when it causes regression in various places. The change never makes it into the initial Java 8 release.

Answer 2

You can use public String[] split(String regex, int limit):

The limit parameter controls the number of times the pattern is applied and therefore affects the length of the resulting array. If the limit n is greater than zero then the pattern will be applied at most n - 1 times, the array's length will be no greater than n, and the array's last entry will contain all input beyond the last matched delimiter. If n is non-positive then the pattern will be applied as many times as possible and the array can have any length. If n is zero then the pattern will be applied as many times as possible, the array can have any length, and trailing empty strings will be discarded.

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String st = "aa,bb,cc,dd,,,,";
System.out.println(Arrays.deepToString(st.split(",",-1)));
                                                    ↑

Prints:

[aa, bb, cc, dd, , , , ]