Java out.println() how is this possible?

问题

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() ?

回答1:

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.

回答2:

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

回答3:

@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 more reusable.

回答4:

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.

回答5:

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).

回答6:

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();

回答7:

you can see this also in sockets ...

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

out.println("hello");

回答8:

simply import :

import static java.lang.System.*;

回答9:

Or simply:

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

来源：https://stackoverflow.com/questions/2504078/java-out-println-how-is-this-possible

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