invoking method via reference variable

Question

I'm trying to understand how System.out can invoke println(). All the searches say System.out is an instance of PrintStream class therefore it can invoke a method in that class.

BUT...

When I try to duplicate this with the following a NullPointerException results.

public class Main {
    public static void main(String[] args){
        Tasker.calc.add(1, 2);
    }
}

class Tasker{
    static Calc calc;
}

class Calc {
    public void add(int x, int y){
        System.out.println(x+y);
    }
}

Isn't Tasker.calc a similar instance of Calc? How is System.out not a NullPointerException?

To be clear, this is not a question about NullPointerException... this is a question about why System.out doesn't require initialization.

Thanks a bunch!

You never intialized `Tasker.calc`, hence you are getting this exception. You might want to read a good Java tutorial before you get started, Oracle's trails are a good start. — Tim Biegeleisen, Apr 19 '17 at 05:08
You have not initialized the `calc` variable. Therefore it has the value `null`. (By contrast, the Java runtime automatically initializes `System.out` to a non-null value.) — Stephen C, Apr 19 '17 at 05:08
I am thinking what to dup-close this too... What is Nullpointerexception? Thoughts anybody? — GhostCat, Apr 19 '17 at 05:10
Because the system takes care of that. It is special. (And besides, there is no way that you *could* correctly initialize it. That variable is set to point to the JVM's standard output stream, and there is no way to find out what it should be ...) Hint: *read the javadocs!* — Stephen C, Apr 19 '17 at 05:12
[What does your step debugger tell you?](http://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) — , Apr 19 '17 at 05:13
@JarrodRoberson - this is nothing to do with static methods. It is about the (default) initialization of fields ... and the unusual behavior of `System.xxx`. — Stephen C, Apr 19 '17 at 05:15
@TimBiegeleisen et al. This is not really about NPEs either. — Stephen C, Apr 19 '17 at 05:16
@StephenC Agree with your Javadoc comment, disagree about closing as a dupe, but since you outrank me I won't touch this question again. — Tim Biegeleisen, Apr 19 '17 at 05:19
`System.out` is a `static reference to an outputstream` so it has everything to do with `static` variables. — , Apr 19 '17 at 13:57

score 2 · Accepted Answer · answered Apr 19 '17 at 05:21

Why doesn't System.out require initialisation? Well ...

Deep inside the System class, there's a method called initializeSystemClass which the JVM calls. It sets System.out to the right value, so you don't have to.

Note that System.out is NOT an instance of PrintStream - it's a REFERENCE to an instance of PrintStream.

score 1 · Answer 2 · answered Apr 19 '17 at 05:08

1

You haven't _initialized calc. You could have written

static Calc calc = new Calc();

and it would have worked (depending on the definition of Calc).

answered Apr 19 '17 at 05:08

Louis Wasserman

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Why wouldn't I answer this or any question? – Louis Wasserman Apr 19 '17 at 05:09

Stephen C · Answer 3 · 2017-04-19T05:32:26.027

As @LouisWasserman points out, the reason that your code gives an NPE is that you haven't initialized the calc variable. As a result, it gets default initialized to null. The solution: initialize it!

But the deeper question is why System.out doesn't need to be initialized. That is because the Java runtime initializes it behind the scenes to point to a PrintStream that refers to your application's (external) standard output.

You don't need to initialize System.out because the system does it. The javadocs say so.
You couldn't initialize it in the nromal way because System.out is declared as final. (But there is System.setOut(...))
Even with System.setOut(...) there is no way for an application to find out what System.out should be set to. (Consider this java Test | wc )

invoking method via reference variable

3 Answers3