为什么contains（）方法在Java中的非空字符串中找到空字符串 [英] Why does contains() method find empty string in non-empty string in Java

问题描述

这是我在Java的 String.indexOf（）和 String.contains（）方法中发现的奇怪行为。如果我有非空字符串说 blablabla 并且我尝试在其中查找空字符串，它总是返回 true 而我希望它返回 false 。

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.

所以基本上，为什么下面的代码是否返回true和0？

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));

逻辑上（至少对我来说）不会出现在 blablablabla 但是 indexOf（）这样说，为什么？

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

推荐答案

每个字符串都会出现一个空字符串。具体而言，字符串的连续子集必须与空字符串匹配。任何空子集都是连续的，任何字符串都有一个空字符串作为这样的子集。

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.

当且仅当此字符串包含指定的字符串时返回true char值的序列。

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

任何字符串，开头，结尾和字符之间都存在一组空的char值。出于任何事情，你什么都不提取。从一个物理的字符串/纱线我可以说它内部存在零长度部分。

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.

如果包含返回true，则可能存在 substring（调用以获取要查找的字符串。aaa.substring（1,1）应返回 ，但请不要引用我，因为我目前还没有IDE。

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.

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