In system.out.println,what meant ln..?
System: It is a class made available by Java to let you manipulate various operating system related objects. It's part of java.lang namespace.
out: out being one of those objects, is a static publicly available object inside that class. The object itself, representing standard output, is an instance of java.io.PrintStream class. Standard output is, on most operating systems, console output.
println: It is a method of java.io.PrintStream class that lets you output some text into the stream and, as opposed to print() method, gets you to the new line after the text.