Difficulties with Java

[Jon Harrop]

I am trying to write some software in Java and, amongst other problems, I'm
finding it very difficult to express simple concepts in Java.

For example, when writing RPC bindings I would like to write conversion
functions that translate a Java Set<T> into a format for the wire and back
again, where T is some type.

Ordinarily I would just pass the conversion function for the type T to the
conversion function for a polymorphic set but Java doesn't seem to support
higher-order functions. What hoops do I have to jump through to get this
done? I've considered wrapping every conversion function in a class in
order to mimic the passing of functions (as the passing of 1-method objects
instead). I've also considered using immediate objects to wrap functions at
each call site. Neither of these approaches seem reasonable, e.g. they are
both hugely verbose.

I also have some irritations regarding the stdlib. I had heard that it was
comprehensive but it appears to be largely superfluous. For example, I
believe the following is minimal Java code to load and save some XML:

import java.io.*;
import javax.xml.parsers.*;
import javax.xml.transform.*;
import javax.xml.transform.stream.*;
import javax.xml.transform.dom.*;
import org.w3c.dom.*;

public class dom {
public static void main(String[] argv)
throws javax.xml.transform.TransformerConfigurationException,
javax.xml.parsers.ParserConfigurationException,
javax.xml.transform.TransformerException,
java.io.FileNotFoundException,
java.io.IOException
{
DocumentBuilderFactory f = DocumentBuilderFactory.newInstance();
DOMImplementation i = (f.newDocumentBuilder()).getDOMImplementation();

Document doc = i.createDocument(null, null, null);

InputStream in = new FileInputStream("test.xml");
t.transform(new StreamSource(in), new DOMResult(doc));
in.close();

Transformer t = (TransformerFactory.newInstance()).newTransformer();
t.transform(new DOMSource(doc), new StreamResult(System.out));
}
}

Are these patterns not factored out into a library function somewhere? If
not, why not? Finally, why does that Java not work?

Here's the OCaml for comparison:

Xml.to_string (Xml.parse_file "test.xml")

Cheers,
Jon.

 

[Oliver Wong]

I am trying to write some software in Java and, amongst other problems,
I'm
finding it very difficult to express simple concepts in Java.

For example, when writing RPC bindings I would like to write conversion
functions that translate a Java Set<T> into a format for the wire and back
again, where T is some type.

Ordinarily I would just pass the conversion function for the type T to the
conversion function for a polymorphic set but Java doesn't seem to support
higher-order functions. What hoops do I have to jump through to get this
done? I've considered wrapping every conversion function in a class in
order to mimic the passing of functions (as the passing of 1-method
objects
instead). I've also considered using immediate objects to wrap functions
at
each call site. Neither of these approaches seem reasonable, e.g. they are
both hugely verbose.

"One method objects" is usually how "passing a function" works in Java,
aye.

I also have some irritations regarding the stdlib. I had heard that it was
comprehensive but it appears to be largely superfluous. For example, I
believe the following is minimal Java code to load and save some XML:

import java.io.*;
import javax.xml.parsers.*;
import javax.xml.transform.*;
import javax.xml.transform.stream.*;
import javax.xml.transform.dom.*;
import org.w3c.dom.*;

public class dom {
public static void main(String[] argv)
throws javax.xml.transform.TransformerConfigurationException,
javax.xml.parsers.ParserConfigurationException,
javax.xml.transform.TransformerException,
java.io.FileNotFoundException,
java.io.IOException
{
DocumentBuilderFactory f = DocumentBuilderFactory.newInstance();
DOMImplementation i =
(f.newDocumentBuilder()).getDOMImplementation();

Document doc = i.createDocument(null, null, null);

InputStream in = new FileInputStream("test.xml");
t.transform(new StreamSource(in), new DOMResult(doc));
in.close();

Transformer t =
(TransformerFactory.newInstance()).newTransformer();
t.transform(new DOMSource(doc), new StreamResult(System.out));
}
}

Are these patterns not factored out into a library function somewhere? If
not, why not? Finally, why does that Java not work?

Here's the OCaml for comparison:

Xml.to_string (Xml.parse_file "test.xml")

Yeah, Java's verbose.

- Oliver

 

[Jon Harrop]

"One method objects" is usually how "passing a function" works in
Java, aye.

Is there any stdlib support for these, or even a convention (e.g. is the
method always called "apply")? Presumably I have to start by declaring a
generic interface that is equivalent to the type 'a -> 'b?

 

[Jean-Francois Briere]

Consider creating your own utility/helper classes.
Then outputing an xml file to the console could *simply* be:

// DomTest.java
import java.io.*;
import javax.xml.parsers.*;
import org.w3c.dom.*;

public class DomTest
{
public static void main(String[] args)
throws Exception
{
DocumentBuilder db =
DocumentBuilderFactory.newInstance().newDocumentBuilder();
InputStream is = new FileInputStream("test.xml");
Document doc = db.parse(is);
is.close();
System.out.println(DomUtil.toString(doc));
}
}

This is the utility class:

// DomUtil.java
import java.io.*;
import javax.xml.transform.*;
import javax.xml.transform.stream.*;
import javax.xml.transform.dom.*;
import org.w3c.dom.*;

public class DomUtil
{
public static String toString(Document doc)
throws Exception
{
StringWriter sw = new StringWriter();
TransformerFactory tf = TransformerFactory.newInstance();
Transformer t = tf.newTransformer();
t.setOutputProperty(OutputKeys.METHOD, "xml");
if (doc.getXmlEncoding() != null)
t.setOutputProperty(OutputKeys.ENCODING,
doc.getXmlEncoding());
t.setOutputProperty(OutputKeys.INDENT, "yes");
t.transform(new DOMSource(doc), new StreamResult(sw));
return sw.toString();
}
}

Regards

 

[Oliver Wong]

Is there any stdlib support for these, or even a convention (e.g. is the
method always called "apply")?

No library to my knowledge. Closest to a convention is to name your
method "run", as that's what I usually see and I suspect it comes from
java.lang.Runnable.

Presumably I have to start by declaring a
generic interface that is equivalent to the type 'a -> 'b?

I've never done this, but after thinking about it for a bit, I came to
the same conclusion:

public interface Runnable2<A,B> {
public B run(A param);
}

- Oliver