Question about String.replace()

[Davie Lin]

Hello everyone,

Thanks for reading my question. According to String.java in 5, the implementation for String.replace(CharSequence, CharSequence) is

public String replace(CharSequence target, CharSequence replacement) {
        return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
            this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
    }

while the implementation for String.replace( char, char ) is

public String replace(char oldChar, char newChar) {
	if (oldChar != newChar) {
	    int len = count;
	    int i = -1;
	    char[] val = value; /* avoid getfield opcode */
	    int off = offset;   /* avoid getfield opcode */

	    while (++i < len) {
		if (val[off + i] == oldChar) {
		    break;
		}
	    }
	    if (i < len) {
		char buf[] = new char[len];
		for (int j = 0 ; j < i ; j++) {
		    buf[j] = val[off+j];
		}
		while (i < len) {
		    char c = val[off + i];
		    buf[i] = (c == oldChar) ? newChar : c;
		    i++;
		}
		return new String(0, len, buf);
	    }
	}
	return this;
    }

I don't quite understand how Pattern and Matcher object work, my question is does the

String.replace(CharSequence, CharSequence)

iterate the entire string and replace the char just like

String.replace(char, char)

Thanks
Davie

[fred rosenberger]

According to the API:

Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence. The replacement proceeds from the beginning of the string to the end, for example, replacing "aa" with "b" in the string "aaa" will result in "ba" rather than "ab".

Of course, you could always write a quick program to test various cases and see for yourself what it does.

[Henry Wong]

Davie Lin wrote:
Thanks for reading my question. According to String.java in 5, the implementation for String.replace(CharSequence, CharSequence) is

public String replace(CharSequence target, CharSequence replacement) {
        return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
            this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
    }

I don't quite understand how Pattern and Matcher object work, my question is does the

String.replace(CharSequence, CharSequence)

iterate the entire string and replace the char just like

String.replace(char, char)

Well, we are not going to explain regular expressions here .... there are whole books dedicated to the subject. It is arguably a language itself.

Basically, Java has a regular expression engine as part of the core libraries -- and the Pattern and Matcher classes is how you use it. As for the LITERAL and quoteReplacement() stuff, that is used to disable practically all the special features of the regular expression engine.... basically, it is using the regular expression engine to do simple string replacement.

Henry

[Davie Lin]

fred rosenberger wrote:According to the API:

Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence. The replacement proceeds from the beginning of the string to the end, for example, replacing "aa" with "b" in the string "aaa" will result in "ba" rather than "ab".

Of course, you could always write a quick program to test various cases and see for yourself what it does.

That sounds to me like the API is saying it check the literal target one at a time, thus iterate the entire string if the literal target is at the end of the String.
Does that sound right?

Thanks
Davie

[Winston Gutkowski]

Davie Lin wrote:That sounds to me like the API is saying it check the literal target one at a time, thus iterate the entire string if the literal target is at the end of the String. Does that sound right?

Sounds about right; and if you want to replace every occurrence of a substring, I can't imagine how it could do otherwise.

String does have a method called lastIndexOf() if you're only interested in the last match, and I suspect it works from the right-hand end. It doesn't uses regexes though.

Winston