Java 空字符串的 String IndexOf 行为

Question

对于字符串来说，String str = "abc" str.indexOf("a") 和 str.indexOf("") return 0。这种行为有效吗？

Answer 1

String.indexof("")的return值为0，也就是起始索引，当你传入一个空字符串时，因为空字符串"" 确实位于那里。 将 "abc" 视为 "" + "abc".

否则，请参考此文档：

http://docs.oracle.com/javase/7/docs/api/java/lang/String.html#indexOf(int)

indexOf "returns:the index of the first occurrence of the character in the character sequence represented by this object, or -1 if the character does not occur."

因此，str.indexOf("a") returns 0.

Answer 2

是的。概念上的原因与数学中加 0 非常相似。所以""+"a"+"bc" = "abc" = ""+"a"+"b"+""+"c".

Answer 3

要是有某个地方可以记录方法的行为就好了。

indexOf(string)

Returns the index within this string of the first occurrence of the specified substring. The returned index is the smallest value k for which:

this.startsWith(str, k)

startsWith(string)

true if the character sequence represented by the argument is a prefix of the character sequence represented by this string; false otherwise. Note also that true will be returned if the argument is an empty string or is equal to this String object as determined by the equals(Object) method.

Answer 4

把它想象成

"" +"abc"="abc"

基本上

"abc" is ""+"abc"

Answer 5

空字符串实际上存在于任何非空的字符串中。