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How does it know?

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  How does it know? Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2014-09-12 08:11 -0700
    Re: How does it know? Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2014-09-12 08:36 -0700
      Re: How does it know? Steven Simpson <ss@domain.invalid> - 2014-09-12 17:36 +0100
        Re: How does it know? Donkey Hottie <donkey@fredriksson.dy.fi> - 2014-09-12 21:40 +0300
          Re: How does it know? Joerg Meier <joergmmeier@arcor.de> - 2014-09-13 11:57 +0200
    Re: How does it know? Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2014-09-12 16:18 -0700
      Re: How does it know? Mike Amling <mamling@chaff.us> - 2014-09-14 08:52 -0500
        Re: How does it know? Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2014-09-14 07:48 -0700
    Re: How does it know? Roedy Green <see_website@mindprod.com.invalid> - 2014-09-12 18:11 -0700
      Re: How does it know? Joerg Meier <joergmmeier@arcor.de> - 2014-09-13 11:59 +0200
        Re: How does it know? Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2014-09-14 07:51 -0700
          Re: How does it know? Arne Vajhøj <arne@vajhoej.dk> - 2014-09-14 22:24 -0400
      Re: How does it know? Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2014-09-14 07:50 -0700
        Re: How does it know? Arne Vajhøj <arne@vajhoej.dk> - 2014-09-14 22:26 -0400
          Re: How does it know? Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2014-09-17 14:52 -0700
    Re: How does it know? Roedy Green <see_website@mindprod.com.invalid> - 2014-09-12 18:13 -0700
    Re: How does it know? Mike Amling <mamling@chaff.us> - 2014-09-14 08:29 -0500
      Re: How does it know? Knute Johnson <nospam@rabbitbrush.frazmtn.com> - 2014-09-14 07:53 -0700
        Re: How does it know? Roedy Green <see_website@mindprod.com.invalid> - 2014-09-17 00:40 -0700
          Re: How does it know? Joerg Meier <joergmmeier@arcor.de> - 2014-09-18 00:10 +0200

#29950 — How does it know?

Give two classes:

public class test {
     public static void main(String... args) {
         System.out.println("calling test1.main()");
         test1.main();
     }
}

public class test1 {
     public static void main(String... args) {
         System.out.println("test1");
     }
}

Compiling test.jar and running test produces the following output:
C:\Users\Knute Johnson>java test
calling test1.main()
test1

Adding the .class files to a .jar and running it produces the same:

C:\Users\Knute Johnson>java -jar test.jar
calling test1.main()
test1

I have two questions, the first is how does the test1 class get loaded 
when test is run from the .jar?  When all the .class files are in the 
working directory they are in the CLASSPATH.  Are all of the classes in 
a .jar in the CLASSPATH too?

The second question is, what type is test.main()?  I want to be able to 
put it into a Map.

Thanks,

knute...

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#29951

On 9/12/2014 08:11, Knute Johnson wrote:
> The second question is, what type is test.main()?  I want to be able to
> put it into a Map.

Now that I think about it, I don't think that is possible.

k...

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#29953

On 12/09/14 16:36, Knute Johnson wrote:
> On 9/12/2014 08:11, Knute Johnson wrote:
>> The second question is, what type is test.main()?  I want to be able to
>> put it into a Map.
>
> Now that I think about it, I don't think that is possible.

import java.util.*;

public class TestApplication {
     public interface Application {
         void main(String... args) throws Exception;
     }

     public static void main(String[] args) {
         Map<String, Application> map = new HashMap<>();
         map.put("self", TestApplication::main);
     }
}



-- 
ss at comp dot lancs dot ac dot uk

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#29954

12.09.2014, 19:36, Steven Simpson kirjoitti:
> On 12/09/14 16:36, Knute Johnson wrote:
>> On 9/12/2014 08:11, Knute Johnson wrote:
>>> The second question is, what type is test.main()?  I want to be able to
>>> put it into a Map.
>>
>> Now that I think about it, I don't think that is possible.
> 
> import java.util.*;
> 
> public class TestApplication {
>     public interface Application {
>         void main(String... args) throws Exception;
>     }
> 
>     public static void main(String[] args) {
>         Map<String, Application> map = new HashMap<>();
>         map.put("self", TestApplication::main);
>     }
> }
> 
> 
> 

That does not compute. Or is this some Java 8 new feature? I thought
methods are not "first class citizens" in Java...

I would say

map.put("something", new Application {
   void main(String... args) throws Exception
   {
      throw new Exception("WTF?");
   }
});


-- 

Avert misunderstanding by calm, poise, and balance.

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#29966

On Fri, 12 Sep 2014 21:40:38 +0300, Donkey Hottie wrote:

> 12.09.2014, 19:36, Steven Simpson kirjoitti:
>> On 12/09/14 16:36, Knute Johnson wrote:
>>> On 9/12/2014 08:11, Knute Johnson wrote:
>>>> The second question is, what type is test.main()?  I want to be able to
>>>> put it into a Map.
>>> Now that I think about it, I don't think that is possible.
>> import java.util.*;

>> public class TestApplication {
>>     public interface Application {
>>         void main(String... args) throws Exception;
>>     }

>>     public static void main(String[] args) {
>>         Map<String, Application> map = new HashMap<>();
>>         map.put("self", TestApplication::main);
>>     }
>> }
> That does not compute. Or is this some Java 8 new feature? I thought
> methods are not "first class citizens" in Java...

Yes, that is part of the new Java 8 changes surrounding the Lambdas.

Liebe Gruesse,
		Joerg

-- 
Ich lese meine Emails nicht, replies to Email bleiben also leider
ungelesen.

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#29959

On 9/12/2014 09:18, Stefan Ram wrote:
> Knute Johnson <nospam@rabbitbrush.frazmtn.com> writes:
>> Adding the .class files to a .jar and running it produces the same:
>
>    To reproduce what you are doing, it might help, when you
>    give /every/ command used in the sequence used in addition
>    to the source code, if the problem should still persist.
>
>> The second question is, what type is test.main()?
>> I want to be able to put it into a Map.
>
> public final class Main
> { public static void main( final java.lang.String[] args )
>    { final java.util.Map
>      < java.util.function.Consumer< java.lang.String[] >,
>        java.util.function.Consumer< java.lang.String[] >> map =
>      new java.util.HashMap<>();
>      map.put( Main::main, Main::main ); }}
>

What is Main::main, is it a Method?  You can't call Main::main.invoke(). 
  I'd like to figure out a way to store some sort of a reference to 
Main.main() so that I could execute it.

Maybe I'm barking up a non-existent tree?

Thanks,

knute...

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#29975

On 9/12/14 6:18 PM, Knute Johnson wrote:
>   I'd like to figure out a way to store some sort of a reference to
> Main.main() so that I could execute it.

Any reason you don't want to use plain old reflection?

import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;
import java.util.HashMap;
...

Method callme;
try {
     callme=test1.class.getDeclaredMethod("main",
      new Class[]{new String[0].getClass()});
} catch (final NoSuchMethodException n) {
     return;
}
HashMap<String, Method> store=new HashMap<String, Method>();
store.put("main", callme);


try {
     store.get("main").invoke(null, new Object[]{new String[0]});
} catch (final IllegalAccessException a) {
} catch (final InvocationTargetException t) {
}

--Mike Amling
Tm8gbmVlZCBmb3IgMS44LCBqdXN0IDEuMg==

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#29976

On 9/14/2014 06:52, Mike Amling wrote:
> On 9/12/14 6:18 PM, Knute Johnson wrote:
>>   I'd like to figure out a way to store some sort of a reference to
>> Main.main() so that I could execute it.
>
> Any reason you don't want to use plain old reflection?
>
> import java.lang.reflect.Method;
> import java.lang.reflect.InvocationTargetException;
> import java.util.HashMap;
> ...
>
> Method callme;
> try {
>      callme=test1.class.getDeclaredMethod("main",
>       new Class[]{new String[0].getClass()});
> } catch (final NoSuchMethodException n) {
>      return;
> }
> HashMap<String, Method> store=new HashMap<String, Method>();
> store.put("main", callme);
>
>
> try {
>      store.get("main").invoke(null, new Object[]{new String[0]});
> } catch (final IllegalAccessException a) {
> } catch (final InvocationTargetException t) {
> }
>
> --Mike Amling
> Tm8gbmVlZCBmb3IgMS44LCBqdXN0IDEuMg==

That works and I guess that's what I'm going to end up with.  I just 
thought that since you can put test1.main() in a program that you could 
get there from a String without quite so much code.

Thanks everybody for all the answers!

knute...

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#29961

On Fri, 12 Sep 2014 08:11:39 -0700, Knute Johnson
<nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted
someone who said :

>When all the .class files are in the 
>working directory they are in the CLASSPATH.  Are all of the classes in 
>a .jar in the CLASSPATH too?

When you have a jar, the classpath is ignored, unless you have one
inside the jar.  All the classes in the jar are potentially loadable.
-- 
Roedy Green Canadian Mind Products http://mindprod.com
There are only two industries that refer to their customers as users.
~ Edward Tufte (born: 1942 age: 72)

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#29967

On Fri, 12 Sep 2014 18:11:05 -0700, Roedy Green wrote:

> On Fri, 12 Sep 2014 08:11:39 -0700, Knute Johnson
> <nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted
> someone who said :
>>When all the .class files are in the 
>>working directory they are in the CLASSPATH.  Are all of the classes in 
>>a .jar in the CLASSPATH too?
> When you have a jar, the classpath is ignored, unless you have one
> inside the jar.  All the classes in the jar are potentially loadable.

That is true only if you use the jar together with the -jar parameter. If
you instead add a jar to the -classpath, you can have multiple jars (or
directories) on the classpath.

Liebe Gruesse,
		Joerg

-- 
Ich lese meine Emails nicht, replies to Email bleiben also leider
ungelesen.

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#29978

On 9/13/2014 02:59, Joerg Meier wrote:
> On Fri, 12 Sep 2014 18:11:05 -0700, Roedy Green wrote:
>
>> On Fri, 12 Sep 2014 08:11:39 -0700, Knute Johnson
>> <nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted
>> someone who said :
>>> When all the .class files are in the
>>> working directory they are in the CLASSPATH.  Are all of the classes in
>>> a .jar in the CLASSPATH too?
>> When you have a jar, the classpath is ignored, unless you have one
>> inside the jar.  All the classes in the jar are potentially loadable.
>
> That is true only if you use the jar together with the -jar parameter. If
> you instead add a jar to the -classpath, you can have multiple jars (or
> directories) on the classpath.
>
> Liebe Gruesse,
> 		Joerg
>

So any classes on the CLASSPATH are potentially loadable?

knute...

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#29983

On 9/14/2014 10:51 AM, Knute Johnson wrote:
> On 9/13/2014 02:59, Joerg Meier wrote:
>> On Fri, 12 Sep 2014 18:11:05 -0700, Roedy Green wrote:
>>> On Fri, 12 Sep 2014 08:11:39 -0700, Knute Johnson
>>> <nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted
>>> someone who said :
>>>> When all the .class files are in the
>>>> working directory they are in the CLASSPATH.  Are all of the classes in
>>>> a .jar in the CLASSPATH too?
>>> When you have a jar, the classpath is ignored, unless you have one
>>> inside the jar.  All the classes in the jar are potentially loadable.
>>
>> That is true only if you use the jar together with the -jar parameter. If
>> you instead add a jar to the -classpath, you can have multiple jars (or
>> directories) on the classpath.
>
> So any classes on the CLASSPATH are potentially loadable?

Since classpath = path where classes can be laoded from then ...

:-)

Arne

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#29977

On 9/12/2014 18:11, Roedy Green wrote:
> On Fri, 12 Sep 2014 08:11:39 -0700, Knute Johnson
> <nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted
> someone who said :
>
>> When all the .class files are in the
>> working directory they are in the CLASSPATH.  Are all of the classes in
>> a .jar in the CLASSPATH too?
>
> When you have a jar, the classpath is ignored, unless you have one
> inside the jar.  All the classes in the jar are potentially loadable.

That was my question.  So any class in a jar can be loaded and a static 
method called on it by using its classname.methodname().  I assume then 
if the jar manifest has a Class-Path: statement pointing to another jar, 
all those classes are potentially loadable too?

knute...

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#29984

On 9/14/2014 10:50 AM, Knute Johnson wrote:
> On 9/12/2014 18:11, Roedy Green wrote:
>> On Fri, 12 Sep 2014 08:11:39 -0700, Knute Johnson
>> <nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted
>> someone who said :
>>
>>> When all the .class files are in the
>>> working directory they are in the CLASSPATH.  Are all of the classes in
>>> a .jar in the CLASSPATH too?
>>
>> When you have a jar, the classpath is ignored, unless you have one
>> inside the jar.  All the classes in the jar are potentially loadable.
>
> That was my question.  So any class in a jar can be loaded and a static
> method called on it by using its classname.methodname().  I assume then
> if the jar manifest has a Class-Path: statement pointing to another jar,
> all those classes are potentially loadable too?

Yes.

If it is in the classpath then it can be loaded - how the path
got in the classpath should not matter.

Arne

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#30014

On 9/14/2014 19:26, Arne Vajhøj wrote:
> On 9/14/2014 10:50 AM, Knute Johnson wrote:
>> On 9/12/2014 18:11, Roedy Green wrote:
>>> On Fri, 12 Sep 2014 08:11:39 -0700, Knute Johnson
>>> <nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted
>>> someone who said :
>>>
>>>> When all the .class files are in the
>>>> working directory they are in the CLASSPATH.  Are all of the classes in
>>>> a .jar in the CLASSPATH too?
>>>
>>> When you have a jar, the classpath is ignored, unless you have one
>>> inside the jar.  All the classes in the jar are potentially loadable.
>>
>> That was my question.  So any class in a jar can be loaded and a static
>> method called on it by using its classname.methodname().  I assume then
>> if the jar manifest has a Class-Path: statement pointing to another jar,
>> all those classes are potentially loadable too?
>
> Yes.
>
> If it is in the classpath then it can be loaded - how the path
> got in the classpath should not matter.
>
> Arne
>
>

Thanks!

knute...

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#29962

On Fri, 12 Sep 2014 08:11:39 -0700, Knute Johnson
<nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted
someone who said :

>
>I have two questions, the first is how does the test1 class get loaded 
>when test is run from the .jar?

It is the perfectly normal class load mechanism.  See
http://mindprod.com/jgloss/classpath.html for various explanations how
it works.

main is an ordinary static method.  It can be called from other
classes just like any other public static method.
-- 
Roedy Green Canadian Mind Products http://mindprod.com
There are only two industries that refer to their customers as users.
~ Edward Tufte (born: 1942 age: 72)

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#29974

On 9/12/14 10:11 AM, Knute Johnson wrote:
> Given two classes:
>
> public class test {
>      public static void main(String... args) {
>          System.out.println("calling test1.main()");
>          test1.main();
>      }
> }
>
> public class test1 {
>      public static void main(String... args) {
>          System.out.println("test1");
>      }
> }

How did you get this to compile? Does the source actually call 
test1.main(args)?

--Mike Amling
bWFpbihqYXZhLmxhbmcuU3RyaW5nW10pIGluIHRlc3QxIGNhbm5vdCBiZSBhcHBsaWVkIHRvICgp

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#29979

On 9/14/2014 06:29, Mike Amling wrote:
> On 9/12/14 10:11 AM, Knute Johnson wrote:
>> Given two classes:
>>
>> public class test {
>>      public static void main(String... args) {
>>          System.out.println("calling test1.main()");
>>          test1.main();
>>      }
>> }
>>
>> public class test1 {
>>      public static void main(String... args) {
>>          System.out.println("test1");
>>      }
>> }
>
> How did you get this to compile? Does the source actually call
> test1.main(args)?
>

Yes and it works fine too.  That is why I am curious to the mechanism 
and wondering if there is a way to get from a String that contains the 
class name to executing the class' main method.

knute...

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#30002

On Sun, 14 Sep 2014 07:53:48 -0700, Knute Johnson
<nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted
someone who said :

>there is a way to get from a String that contains the 
>class name to executing the class' main method.


see http://mindprod.com/classforname.html

The tricky part is you must make sure all the classes you load this
way are available in the jar or on the classpath.

The dynamically loaded class implements an Interface or an abstract
class.  That gives you a handle to call its various instance methods.

The problem is, interfaces don't have static methods, so there is no
way to refer to "main".  To get at it, you would need to use
reflection.  A simpler technique would be to  invent an instance main
method.

Another technique is to have a lookup by string to class. Then you can
call static methods too.
-- 
Roedy Green Canadian Mind Products http://mindprod.com
There are only two industries that refer to their customers as users.
~ Edward Tufte (born: 1942 age: 72)

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#30015

On Wed, 17 Sep 2014 00:40:02 -0700, Roedy Green wrote:

> The problem is, interfaces don't have static methods, so there is no
> way to refer to "main".

As of Java 8, that is no longer correct. This will compile:

public interface App {
	public static void main(String[] args) {}
}

as will:

public class Main implements App {
	public static void main(String[] args) {}
}

Although I am uncertain of how much help that would actually be in this
situation.

Liebe Gruesse,
		Joerg

-- 
Ich lese meine Emails nicht, replies to Email bleiben also leider
ungelesen.

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