how to make multiple line string to single line string?

No regular expressions and operating system independent:

str1.replaceAll(System.lineSeparator(), " ");

Windows uses \r\n as a line breaker, while *nix systems use only \n.

Use String.replaceAll instead.

str1=str.replaceAll("[\r\n]+", " ");

If you want to use regex, you should use the String.replaceAll() method.