Java-替换字符串：charAt VS子字符串

Question

I'm making a simple test, just removing a char from string. It goes like this:

String str = "kitten";
    int i = 2;
    //substring version - works good 
    System.out.println(str.replaceFirst(str.substring(i, i+1), ""));
    //charAt (tried as regex):
    System.out.println(str.replaceFirst("[str.charAt(i)]", ""));
    //charAt (tried as char):
    System.out.println(str.replaceFirst("str.charAt(i)", ""));

Substring version works good, charAt works good only if i=1. What is wrong here ?

Answer 1

In your second and third snippet, you're replacing not the result of charAt() call, but "charAt(i)" string. See, it is in quotes. Also, charAt() returns char so you have to convert it to String before using.

Try this:

System.out.println(str.replaceFirst("[" + String.valueOf(str.charAt(i)) + "]", ""));
System.out.println(str.replaceFirst(String.valueOf(str.charAt(i)), ""));

Answer 2

System.out.println(str.replaceFirst("str.charAt(i)", ""));

doesn't do what you think it does. It isn't looking for a character at i . It is looking for the first instance that matches the regex pattern "str.charAt(i)". Similar issues exist with your other "replaceFirst" implementation.

That means that "strAchar(i)" matches "str.charAt(i)" but when i happens to equal 2, "i" does not match "str.charAt(i)". The stuff between the double quotes is not interpreted as Java code.

Answer 3

System.out.println(str.replaceFirst("str.charAt(i)", ""));

This line will replace the string "str,charAt(i)" by "" (if it occurs) in the string str . You need to read more about replace() and charAt() here .

Answer 4

Using the first example, which you say "works good", I'd expect this output:

kiten

ktten

kitten

str.substring(i+1) returns 't' (i+1th or "3rd" character). You then pass this into str.replaceFirst which replaces the first occurrence of 't' with "", effectively erasing it.

What you are doing in the second one is weird: You are invoking replaceFirst with the regex "[str.charAt(i)]" which basically means "replace the first of any of the characters in the square brackets (except the round brackets I think" so you may as well be saying "match any of the characters a,A,c,h,i,r,s,t" (I alphabetised, removed duplicates and '(', ')' and '.'), and the first of these characters that matches "kitten" just so happens to be 'i' so that charachter is removed.

The final example is looking for a complete match on the string "str.charAt(i)" which is of course nowhere to be found in "kitten" you may as well be searching for "dog".

The following code is equivalent to what you have just done:

String str = "kitten";
int i = 2;
// Eliminated redundant regex replacement:
System.out.println(new StringBuffer(str.substring(0, i)).append(str.substring(i+1)));
// Search for any of the characters in "str.substring(i)"
System.out.println(str.replaceFirst("[aAchirst]", ""));
// Search for non-matching string
System.out.println(str.replaceFirst("dog", ""));