Java, splitting string into array-CodePudding

I am trying to split a string into string array. And I have stumbled to something strange to me. I don't understand why it works like this.

   String one, two;
    
    one = "";
    two = ":";
    
    
    String[] devided1 = one.trim().split(":");
    String[] devided2 = two.trim().split(":");
    
    System.out.println("size: "  devided1.length);
    System.out.println("size: "  devided2.length);

I get output:

   size: 1
   size: 0

Why is empty string giving me size of one, but string that only has the delimiter gives my array size of 0? I saw more confusing things like: that size of "::" is 0, but size of ": :" is 2, not 3. Can someone please explain it to me?

CodePudding user response：

See the doc comment in source code or documentation for public String[] split(String regex, int limit) method.

Case 1:

String one = "";
String[] devided1 = one.trim().split(":");

The resulting array will have 1 element = original string String[1] [""], because expresion ":" was not match any part of the input string.

According to documentation:

If the * expression does not match any part of the input then the resulting array * has just one element, namely this string.

Case 2:

String two = ":";
String[] devided2 = two.trim().split(":");

The split(":") has default limit = 0. It means that from the resulting array trailing empty strings will be removed. So method splits ":" string to array with two empty strings and then remove them and as result we get empty array.

According to documentation:

If limit 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.

Case 3:

String two = ":";
String[] devided2 = two.trim().split(":", -1);

We will get an array with two empty strings.

According to documentation:

If limit is non-positive then the pattern will be applied as many times as possible and the array can have any length

Case 4:

String two = "::";
String[] devided2 = two.trim().split(":");

We will get empty array. It is the same like Case 2.

Case 5:

String one = ": :";
String[] devided1 = one.trim().split(":");

The method will split string to three array elements ["", " ", ""] and then remove empty strings from the end of array, because limit = 0. We will get String[2] ["", " "].

According to documentation:

If limit 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.

CodePudding user response：

This link is helpful:

https://konigsberg.blogspot.com/2009/11/final-thoughts-java-puzzler-splitting.html

Basically, it is for perl compatibility.

You can use split(":", -1) here if you don't want that behavior.

Otherwise, split(":") defaults to split(":", 0), and the difference is:

https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#split(java.lang.String,int)

If the limit 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.
If the limit is negative then the pattern will be applied as many times as possible and the array can have any length.

In case of ":" being splitted, it would result in {"" , ""}, but empty traling spaces will be discarded, so it will return an empty array.