为什么contains（）方法在Java中的非空字符串中找到空字符串

Question

This is a weird behavior I found with Java's String.indexOf() and String.contains() methods. If I have a non-empty string says blablabla and I try to look for an empty string inside it, it always returns true whereas I would expect it to return false .

So basically, why does the code below return true and 0 ?

String testThis = "";
String fileName = "blablablabla";
System.out.println(fileName.contains(testThis));
System.out.println(fileName.indexOf(testThis));

Logically (at least to me) "" does not occur in blablablabla but indexOf("") says it does, why?

Answer 1

An empty string occurs in every string. Specifically, a contiguous subset of the string must match the empty string. Any empty subset is contiguous and any string has an empty string as such a subset.

Returns true if and only if this string contains the specified sequence of char values.

An empty set of char values exists in any string, at the beginning, end, and between characters. Out of anything, you can extract nothing. From a physical piece of string/yarn I can say that a zero-length portion exists within it.

If contains returns true there is a possible substring( invocation to get the string to find. "aaa".substring(1,1) should return "" , but don't quote me on that as I don't have an IDE at the moment.

Answer 2

""出现在每个角色之间以及开始和结束之间。