Question

I tried this example just interchanging two lines it gives different outputs why

String inputString = "[e-mail]"; 
String pattern="([a-z]+@)([a-z]+)(\\.[a-z]+)";
Pattern p = Pattern.compile(pattern); 
Matcher m = p.matcher(inputString); 

///changes happens here

if(m.find()) 
{
   String resultString = m.replaceAll("$1xxxx$3"); 
   System.out.println(resultString); 
}

System.out.println(m.matches());//line to be changed

output :

[e-mail]

true

System.out.println(m.matches());//line changed     
if(m.find()) 
{
   String resultString = m.replaceAll("$1xxxx$3"); 
   System.out.println(resultString); 
}

output :
true

Answer 1

Extracted from Matcher.find documentation

find

public boolean find()

Attempts to find the next subsequence of the
input sequence that matches the pattern. This method starts at the
beginning of this matcher’s region, or, if a previous invocation of
the method was successful and the matcher has not since been reset, at
the first character not matched by the previous match.

If the match succeeds then more information can be obtained via the
start, end, and group methods.

Returns: true if, and only if, a subsequence of the input sequence
matches this matcher’s pattern

So, since you called Matcher.matches which attempts to match the whole String, and you did not reset the matcher, it tried to find starting after the first match. As there is only one match, it does not find anything.