You can use :

str.replaceAll("\n+", "\n");

Here is an example :

String str = "Hello!\n"
        + "\n"
        + "\n"
        + "\n"
        + "\n"
        + "\n"
        + "\n"
        + "\n"
        + "Hello again!";
str = str.replaceAll("\n+", "\n");
System.out.println(str);

Output

Hello!
Hello again!

Try Bart Kiers's answer to remove empty lines within a string.

String removedEmptyLines = text.replaceAll("(?m)^[ \t]*\r?\n", "");

You can make a Queue . Read line by line If first character of line is not a new line character , include it in the queue. Otherwise if its a space just peek last element is queue, dont add it if the queue last element was also new line .

So when you print also , you have the liberty you want to print new line character or not .