Question

When we want to print line in console we write System.out.println(data), what will be difference if we use PrintStream object out and use out.println(data).

Thanks.

Answer 1

Why do we need System class to call out.println() method?

We don’t. It is just a convenient way to get hold of the PrintStream object for writing to standard output.

… what will be difference if we use PrintStream object out and use out.println(data).

It depends on how you obtained the PrintStream object.

• If you originally got the PrintStream object from System.out or System.err, then there is no difference; e.g. the following have the same effect:

  public void sayHello(PrintStream ps) {
      ps.println("hello");
  }
  sayHello(System.out);
  System.out.println("hello");
  

• If you created the PrintStream on a stream that opened yourself, the behaviour will be analogous, but the output will (most likely) go to a different place; e.g.

  sayHello(new PrintStream(new FileOutputStream("foo.txt"));
  

  will write “hello” to the named file.

There is nothing special about System.in/out/err. They are just public static final variables that are initialized to specific stream objects when the JVM starts. (OK, there is something slightly unusual about them … but it doesn’t manifest here.)