Develop Reference

Is there a way of alter first iteration of for loop for adding strings?

I currently have a for loop, and I am looking for a quick and elegant method to remove the plus sign + from the first "adding" of the loop, so the results is not +example+input+string but rather example+input+string preferably without the need of rewriting the loop much.
String inpputString = "example input string";
String outputString= "";
String[] stringPieces= mainString.split(" ");
for (String strTemp : stringPieces){
outputString = outputString + "+" + strTemp;
}

Based on the observation that you don't want a separator the first time - use an empty separator the first time!
String sep = "";
for (String strTemp : stringPieces) {
outputString += sep + strTemp;
sep = "+";
}

Personally, I would proffer the solution using String.join() proposed by #shmosel in the comment.
Here is another way of achieving this with Stream API:
String outputString =
Arrays.stream(stringPieces).collect(Collectors.joining("+"));

Java String TRIM function not working

I am receiving a string from server trailing one or two lines of spaces like below given string.
String str = "abc*******
********";
Consider * as spaces after my string
i have tried a few methods like
str = str.trim();
str = str.replace(String.valueOf((char) 160), " ").trim();
str = str.replaceAll("\u00A0", "");
but none is working.
Why i am not able to remove the space?

You should try like this:
str = str.replaceAll("\n", "").trim();
You can observe there is a new line in that string . first replace new line "\n" with space("") and than trim

You should do:
str = str.replaceAll("\n", "");
In my case use to work the function trim()

Try this:
str = str.replaceAll("[.]*[\\s\t]+$", "");

I have tried your 3 methods, and them all work. I think your question describing not correctly or complete, in fact, a String in java would not like
String str = "abc*******
********";
They must like
String str = "abc*******"
+ "********";
So I think you should describe your question better to get help.

Java RegEx replace all characters in string except for a word

I am using the code in Java:
String word = "hithere";
String str = "123hithere12345hi";
output(str.replaceAll("(?!"+word+")", "x"));
However, rather than outputting: xxxhitherexxxxxxx like I want it to, it outputs: x1x2x3hxixtxhxexrxex1x2x3x4x5xhxix x, I've tried a load of different regex patterns to try to do this, but I can't seem to figure out how to do this :(
Any help would be much appreciated.

Well this technically works. Using only replace all and only one line, and it's assuming you string does not contain a deprecated ASCII character (BEL)
String string = "hithere";
String string2 = "asdfasdfasdfasdfhithereasasdf";
System.out.println(string2.replaceAll(string,"" + (char)string.length()).replaceAll("[^" + (char)string.length() + "]", "x").replaceAll("" + (char)string.length(), string));

I think this is what you're looking for, if I'm not mistaken:
String pattern = "(\\d)|(hi$)";
System.out.println("123hithere12345hi".replaceAll(pattern, "X"));
The pattern replaces any numeric digits and the word "hi".

This lookaround based code will work for you:
String word = "hithere";
String string = "123hithere12345hi";
System.out.println(string.replaceAll(
".(?=.*?\\Q" + word + "\\E)|(?<=\\Q" + word + "\\E(.){0,99}).", "x"));
//=> xxxhitherexxxxxxx

how to check if a space is followed by a certain character?

I am really confused on this regex things. I have tried to understand it, went no where.
Basically, i am trying to replace all spaces followed by every character but a space to be replaced with "PM".
" sd"
" sd"
however
" sd"
" sd"

This will replace the space and the following character with "PM":
String s = "123 axy cq23 dasd"; //your string
String newString = s.replaceAll(" [^ ]","PM");
Since I'm not sure if you want to replace only the space or the space and the following character, too, here is a slightly modified version that replaces only the space:
String s = "123 axy cq23 dasd"; //your string
String newString = s.replaceAll(" ([^ ])", "PM$1")

You need to use non-capturing pattern:
String res = oldString.replaceAll(" (?:[^ ])", "PM");

Java - New Line Character Issue

I have a code like,
String str = " " ;
while( cond ) {
str = str + "\n" ;
}
Now, I don't know why at the time of printing, the output string is not printing the newline character. However, when I add any other character like ( str = str + "c"), it is printing properly. Can anybody help me, how to solve this problem and why this happening ?

The newline character is considered a control character, which doesn't print a special character to the screen by default.
As an example, try this:
String str = "Hi";
while (cond) {
str += "\n"; // Syntactically equivalent to your code
}
str += "Bye";
System.out.println(str);

Looks like you are trying to run the above code on Windows. Well the line separator or new line is different on Windows ( '\r\n' ) and Unix flavors ('\n').
So, instead of hard coding and using '\n' as new line. Try getting new line from the system like:
String newLine = System.getProperty("line.separator");
String str = " " ;
while( cond ) {
str = str + newLine ;
}

If you really want \n, to get printed, do it like this.
String first = "C:/Mine/Java" + "\\n";
System.out.println(first);
OUTPUT is as follows :
For a good reference as to why is this happening, visit JAVA Tutorials
As referred in that TUTORIAL : A character preceded by a backslash is an escape sequence, and has a special meaning to the compiler. When an escape sequence is encountered in a print statement, the compiler interprets it accordingly
Hope this might help.
Regards

Based on your sample, the only reason it would not show a new line character is that cond is never true and thus the while loop never runs...