使用IndexOf获取带有空格和引号的字符串的子字符串

Question

I have the following String and would like to extract the contents of rule ie My Rule Description Looks Like This:

rule "My Rule Description Looks Like This"      
        followed by some white space other characters such as quotes".

When i use the following I get a java.lang.StringIndexOutOfBoundsException: String index out of range: -2:

String ruleName = rule.substring(rule.indexOf("rule \"" + 7, rule.indexOf("\""));

and when i use lastIndexOf:

String ruleName = rule.substring(rule.indexOf("rule \"" + 7, rule.lastIndexOf("\""));

the code executes OK but the output looks like:

My Rule Description Looks Like This"        
        followed by some white space other characters and quotes

Any ideas why the first option throws an exception using indexOf?

Answer 1

For any sort of complex text extraction, you might want to consider using regular expressions. Here is a short script which can extract the rule, and it avoids nasty string manipulations, which, as you have seen, can be prone to error.

String line = "rule \"My Rule Description Looks Like This\"\n";
line += "followed by some white space other characters such as quotes\".";
String pattern = "rule\\s+\"(.*?)\".*";

Pattern r = Pattern.compile(pattern, Pattern.DOTALL);
Matcher m = r.matcher(line);
if (m.find()) {
   System.out.println("Found a rule: " + m.group(1) );
} else {
   System.out.println("Could not find a rule.");
}

Output:

My Rule Description Looks Like This

Demo here:

Rextester

Answer 2

From the documentation :

public String substring(int beginIndex, int endIndex)

if the beginIndex is negative, or endIndex is larger than the length of this String object, or beginIndex is larger than endIndex.

You are calling rule.substring(rule.indexOf("rule \\"" + 7, rule.indexOf("\\"")) . The first parameter gives you the index of the first rule + quote , let's say x , + 7. The second parameter gives you the index of the first quote, which is x + 6 ( x - the number of characters in rule . So you are calling substring (x + 7, x +6) , which falls in the exception case:

first parameter biger than the second.

In you rsecond case, using lastIndexOf , you are getting the second quote, so you don't have this problem.

Answer 3

indexOf returns the index of the first occurrence of the specified String.

So your first example would try to substring starting from index 7 (0 is the index of where your String is found, and then you add 7), and ending at index 5 (where the first " is found).

The substring(int beginIndex, int endIndex) method has some logic in it where if the begin index subtracted from the end index is < 0 it will throw a StringIndexOutOfBoundsException with the value:

int subLen = endIndex - beginIndex;
if (subLen < 0) {
    throw new StringIndexOutOfBoundsException(subLen);
}

Your second example doesn't throw an exception, but because you are using lastIndexOf() it will substring from 7 to the end of the String (where there is a ").

The best solution would be to use a regex pattern like shown in @Tim Biegeleisen's answer