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class Echo {
public static void main (String[] args) {
System.out.println(args[0]);
System.out.println(args[0]);
}
}
Line-by-line the code reads as follows:
class keywordEcho. It is good practice and often obligatory to let
the name of the
source file (Echo.java) correspond with the name of
the main class (Echo) defined. Also use separate files for separate
classes.
main methodmain.
It refers to the main execution loop and is invoked by the Java
interpreter at start-up.
public, static, and
voidmain method that looks like:
public static void main(String[] args) {
...
}
The keyword public indicates that the method can be
called by any object. The keyword static turns main
in what is called a class method, which does not need an object to
be invoked. void is the type of the value
returned by the method; in this case, no value is returned.
These keywords will be discussed later in more detail.
String[] argsmain method has one parameter:
an array of strings. This array will enable your program to operate
on actual arguments passed to application in the command-line.
System.out.printlnSystem.out.println prints out its argument, which in this
case is the first of the command-line arguments, on standard output
(mostly your terminal screen).
System.out.println as follows: along with the Java system
comes the class System that contains a variable out
of type PrintStream; its full name is
java.lang.System.out. This object or instance
implements the standard output stream. On this object we apply
the println belonging to the PrintStream class.
Soon you will see easier examples of the use of periods.
()[]{}