Java out.println() how is this possible?

Question

I've seen some code such as:

out.println("print something");

I tried import java.lang.System;

but it's not working. How do you use out.println() ?

score 103 · Accepted Answer · edited Sep 26 '12 at 07:43

103

static imports do the trick:

import static java.lang.System.out;

or alternatively import every static method and field using

import static java.lang.System.*;

Addendum by @Steve C: note that @sfussenegger said this in a comment on my Answer.

"Using such a static import of System.out isn't suited for more than simple run-once code."

So please don't imagine that he (or I) think that this solution is Good Practice.

edited Sep 26 '12 at 07:43

Stephen C

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answered Mar 23 '10 at 22:37

sfussenegger

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with most ide's these days it will fix it up for you anyway, in eclipse do an organise imports and it will change your java.lang.System.*; to java.lang.System.out; for you (assuming you are only using out) – hhafez Mar 23 '10 at 22:58

tangens · Answer 2 · 2010-03-24T05:57:24.167

31

PrintStream out = System.out;
out.println( "hello" );

edited Mar 24 '10 at 05:57

answered Mar 23 '10 at 22:34

tangens

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Stephen C · Answer 3 · 2021-03-31T07:25:41.390

17

@sfussenegger's answer explains how to make this work. But I'd say don't do it!

Experienced Java programmers use, and expect to see

        System.out.println(...);

and not

        out.println(...);

A static import of System.out or System.err is (IMO) bad style because:

it breaks the accepted idiom, and
it makes it harder to track down unwanted trace prints that were added during testing and not removed.

If you find yourself doing lots of output to System.out or System.err, I think it is a better to abstract the streams into attributes, local variables or methods. This will make your application or library more maintainable and more reusable.

(Obviously, if your Java program is a once-off thing that you intend to throw away when you have completed the current task, then maintainability is not a concern. But the flip side is that "throw away" code is often NOT thrown away.)

edited Mar 31 '21 at 07:25

answered Mar 23 '10 at 23:26

Stephen C

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I agree a missing "System." might be confusing at first look. Using local variables or attributes doesn't really change anything though, does it? Using a local `protected void println(Object o) { System.out.println(o);}` might be a good idea though as the output destination could easily be changed, say to `log.info(o)` for instance. – sfussenegger Mar 23 '10 at 23:35
1

@sfussenegger - use of a local variable or an attribute *does* make it easier to change the destination than using System.out all over the place ... whether System.out is imported or not. – Stephen C Mar 24 '10 at 06:15
1

okay, that's true. But it's also possible to replace a static import of System.out by a field called out - no need to assign System.out to out. But generally, I agree with you. Using such a static import of System.out isn't suited for more than simple run-once code. – sfussenegger Mar 24 '10 at 07:57

Casey · Answer 4 · 2010-03-23T23:09:00.043

7

Well, you would typically use

System.out.println("print something");

which doesn't require any imports. However, since out is a static field inside of System, you could write use a static import like this:

import static java.lang.System.*;

class Test {
    public static void main(String[] args) {
        out.println("print something");
    }
}

Take a look at this link. Typically you would only do this if you are using a lot of static methods from a particular class, like I use it all the time for junit asserts, and easymock.

edited Mar 23 '10 at 23:09

answered Mar 23 '10 at 22:38

Casey

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`out` isn't a method, it's a field. – sfussenegger Mar 23 '10 at 22:41
doh! Thanks for catching that. I meant to say field. – Casey Mar 23 '10 at 23:09

score 3 · Answer 5 · edited Jan 30 '13 at 17:46

out is a PrintStream type of static variable(object) of System class and println() is function of the PrintStream class.

class PrintStream
{
    public void println(){}    //member function
    ...
}

class System
{
    public static final PrintStream out;   //data member
    ...
}

That is why the static variable(object) out is accessed with the class name System which further invokes the method println() of it's type PrintStream (which is a class).

Oskar Kjellin · Answer 6 · 2010-03-23T23:12:16.363

3

You will have to create an object out first. More about this here:

    // write to stdout
    out = System.out;
    out.println("Test 1");
    out.close();

edited Mar 23 '10 at 23:12

answered Mar 23 '10 at 22:35

Oskar Kjellin

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and now you don't "create" an object :) – sfussenegger Mar 23 '10 at 22:48
The official java documentation uses the term "Create object" aswell as the one i guess you are referring to "instantiate" http://java.sun.com/docs/books/tutorial/java/javaOO/objectcreation.html – Oskar Kjellin Mar 23 '10 at 22:51
You don't create or instantiate an object at all. You simply copy the reference. Hence `System.out` and `out` will reference the same object, i.e. `System.out == out` will be `true` – sfussenegger Mar 23 '10 at 22:57
oh, missed your "now" part :P By the way I haven't programmed java that much as you might notice – Oskar Kjellin Mar 23 '10 at 23:10

score 1 · Answer 7 · answered Aug 27 '12 at 13:58

1

you can see this also in sockets ...

PrintWriter out = new PrintWriter(socket.getOutputStream());

out.println("hello");

answered Aug 27 '12 at 13:58

Exorcismus

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score 1 · Answer 8 · edited Jan 26 '17 at 07:58

1

simply import :

import static java.lang.System.*;

edited Jan 26 '17 at 07:58

Tharif

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answered Oct 13 '13 at 23:35

score 0 · Answer 9 · answered Mar 23 '10 at 22:38

0

Or simply:

System.out.println("Some text");

answered Mar 23 '10 at 22:38

Chris Knight

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Java out.println() how is this possible?

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