Wanted: Help serializing Generic Maps!

[Siegfried Heintze]

The previous version of this code works properly but does not use generics. I want to use generics.

I don't know how to deal with generic arrays like Item<K,V>[]. When I tried using ArrayList<Item><K,V>> instead of Item<K,V>[] the warning and the stack trace went away -- and so did the serialization!

Can someone kindly help me fix these problems so it will serialize properly?
Can someone kindly explain why I can say new ArrayList<Item><K,V>> but not new Item<K,V>[]. Thru google searching I've found such explanations as "K and V don't exist at run time so we cannot define an array of Item<K,V>, instead we can only define an array of Item." Is there basically just an inconsistency between arrays of Item<K,V> and Vectors, Sets and ArrayLists of Item<K,V>? It seems to me they should have been consistent instead of making arrays a special case. Perhaps someone can explain why sun was inconsistent here?

thanks,
Siegfried

/*
 * 
 * $Log: DemoMaps.java,v $
 * Revision 1.3  2009/08/29 01:58:46  Administrator
 * Try to add generics -- stack trace
 *
 * Revision 1.2  2009/08/29 01:35:44  Administrator
 * Works but no generics
 *
 *
 * Begin commands to execute this file using Java with bash
 * if [ `uname` == 'Linux' ]
 * then
 * export CLASSPATH=bin:.
 * else
 * export CLASSPATH=bin\;.
 * fi
 * pushd ../../
 * if [ -e "bin" ]
 * then
 * echo "already have build (bin) directory"
 * else
 * echo "mkdir bin"
 * mkdir bin
 * fi
 * javac -g com/heintze/DemoMaps.java -d bin -Xlint:unchecked
 * java  com.heintze.DemoMaps <<EOF 
 * ><?xml version="1.0" encoding="UTF-8" standalone="yes"?>
 *   <demoMaps><map/>
 *     <map2>
 *       <map>
 *         <item value="mumble" key="fratz"/>
 *       </map>
 *     </map2>
 * </demoMaps>
 * EOF
 * rm bin/com/heintze/DemoMaps.class
 * popd
 * End commands to execute this file using Java with bash
 *
 */

package com.heintze;
import java.io.FileReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;
import javax.xml.bind.annotation.adapters.XmlAdapter;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

// http://www.caucho.com/resin/doc/jaxb-annotations.xtp#@XmlJavaTypeAdapter
@XmlRootElement
class Bean<K,V> {
  @XmlJavaTypeAdapter(MyMapAdapter.class)
    public HashMap<K,V> map = new HashMap<K,V>();
}

class MyMapAdapter<K,V>
  extends XmlAdapter<Item><K,V>[],java.util.Map<K,V>> {
// http://weblogs.java.net/blog/kohsuke/archive/2005/04/xmladapter_in_j.html 
  @Override
  public Item<K,V>[] marshal(Map<K, V> map) throws Exception {
    Item[] result=new Item[map.size()];
    int ii=0;
    for(K key:map.keySet()){
      Item<K,V>x=new Item<K,V>();
      x.key=key;
      x.value=map.get(key);
      result[ii]=x;
      ii = ii + 1;
    }
    return (Item<K,V>[])result;
  }

  @Override
  public Map<K, V> unmarshal(Item<K,V>[] value) throws Exception {
      Map<K,V> r = new HashMap<K,V>();
      for( Item<K,V> c : value )
        r.put(c.key, c.value);
      return r;
  }
}

class Item<K,V> {
  @XmlAttribute public K key;
  @XmlAttribute public V value;
} 
class Temp<K,V> { @XmlElement private List<Item><K,V>> entry = new java.util.ArrayList<Item><K,V>>(); }
 
@XmlRootElement
public class DemoMaps {
  static java.io.PrintStream out = System.out;
  public static void main(String[] args) {
    DemoMaps data = new DemoMaps();
    try {
      JAXBContext context = JAXBContext.newInstance(DemoMaps.class);
      Marshaller m = context.createMarshaller();
      Unmarshaller um = context.createUnmarshaller();
      data = (DemoMaps)um.unmarshal(new FileReader(java.io.FileDescriptor.in) );
      data.map2.map.put("hello", "world");
      m.marshal(data, System.out);
    } catch (JAXBException e) {
      e.printStackTrace();
    }
  }
  @XmlElement()
  Bean<String,String> map2 = new Bean<String, String>();
 }

Here is the stack trace and comilation warning:

com\heintze\DemoMaps.java:84: warning: [unchecked] unchecked cast
found   : com.heintze.Item[]
required: com.heintze.Item<K,V>[]

		return (Item<K,V>[])result;

		                    ^

1 warning

Exception in thread "main" java.lang.ClassCastException: sun.reflect.generics.reflectiveObjects.GenericArrayTypeImpl cannot be cast to java.lang.Class
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.createArrayInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.createArrayInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ModelBuilder.getTypeInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.TypeRefImpl.calcRef(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.TypeRefImpl.getTarget(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeTypeRefImpl.getTarget(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeTypeRefImpl.getTarget(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ElementPropertyInfoImpl$1.get(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ElementPropertyInfoImpl$1.get(Unknown Source)
	at java.util.AbstractList$Itr.next(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ModelBuilder.getTypeInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.TypeRefImpl.calcRef(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.TypeRefImpl.getTarget(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeTypeRefImpl.getTarget(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeTypeRefImpl.getTarget(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ElementPropertyInfoImpl$1.get(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ElementPropertyInfoImpl$1.get(Unknown Source)
	at java.util.AbstractList$Itr.next(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.RuntimeModelBuilder.getClassInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ModelBuilder.getTypeInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.model.impl.ModelBuilder.getTypeInfo(Unknown Source)
	at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.getTypeInfoSet(Unknown Source)
	at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.<init>(Unknown Source)
	at com.sun.xml.internal.bind.v2.ContextFactory.createContext(Unknown Source)
	at com.sun.xml.internal.bind.v2.ContextFactory.createContext(Unknown Source)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at javax.xml.bind.ContextFinder.newInstance(Unknown Source)
	at javax.xml.bind.ContextFinder.find(Unknown Source)
	at javax.xml.bind.JAXBContext.newInstance(Unknown Source)
	at javax.xml.bind.JAXBContext.newInstance(Unknown Source)
	at com.heintze.DemoMaps.main(DemoMaps.java:108)
~/Documents and Settings/Siegfried/Eclipse-Workspaces/hello/SerializeMap/src/com/heintze
Compilation finished at Sat Aug 29 12:52:09

[Paul Clapham]

Siegfried Heintze wrote:I don't know how to deal with generic arrays like Item<K,V>[].

So read the original Generics tutorial and on page 15 it explains why you can't have generic arrays.

[Siegfried Heintze]

Thanks!

So I tried to apply page 15 and I'm still getting warnings on lines 27 and 28. Why cannot I cast without a warning. The examples on page 15 have an example of storing an element in an array, and examples of how not to extract an element in an array, and no examples of how to properly extract an element from an array!

Why cannot I cast on lines 27-28?

Thanks,
Siegfried

  @Override
    public Item<?,?>[] marshal(Map<K, V> map) throws Exception {
    Item<?,?>[] tmp=new Item<?,?>[map.size()];
    Object o =tmp;
    Object[] result = (Object[])o;
    int ii=0;
    for(K key:map.keySet()){
      Item<K,V>x=new Item<K,V>();
      x.key=key;
      x.value=map.get(key);
      result[ii]=x;
      ii = ii + 1;
    }
    return (Item<?,?>[])result;
  }

  @Override
    public Map<K, V> unmarshal(Item<?,?>[] tmp) throws Exception {
    Map<K,V> r = new HashMap<K,V>();
    Object ox =tmp;
    Object[] value = (Object[])ox;

    for( Object o : value ){
      Item x = (Item)o;
      Object ko = x.key;
      Object vo = x.value;
      K k = (K)ko;
      V v = (V)vo;
      r.put(k, v);
    }
    return r;
  }

[Paul Clapham]

Why can't you cast without a warning in that context? Because as that reference says, generic arrays are inherently not type-safe.

[Siegfried Heintze]

This code below works. However, it won't let me easily accommodate different types via generics. When I change the data members of Item2 to type Object, I get the following error:

@XmlAttribute/@XmlValue need to reference a Java type that maps to text in XML.

Well that kinda makes sense. It seems like an unnecessary constraint, however, since I'm writing the code to convert strings to/from objects using casts.

Hmmm....

When I change Item2 to be the following, I get the same error:

class Item<K,V> {
  @XmlAttribute public K key;
  @XmlAttribute public V value;
} 

I guess that makes sense too if I pass Item2<K,V> here:

class MyMapAdapter<K,V>
  extends XmlAdapter<Item[],java.util.Map><K,V>> {
...

Apparently the marshallar is does not know that K and V have been expanded to String and String (respectively) and is seeing K and V as objects.

So how do I get around this problem?

Thanks,
Siegfried

/*
 * 
 * $Log: MarshalBug.java,v $
 * Revision 1.2  2009/08/30 03:45:48  Administrator
 * works

 *
 * Revision 1.1  2009/08/30 03:29:07  Administrator
 * Initial revision
 *
 *
 *
 * Begin commands to execute this file using Java with bash
 * if [ `uname` == 'Linux' ]
 * then
 * export CLASSPATH=bin:.
 * else
 * export CLASSPATH=bin\;.
 * fi
 * pushd ../../
 * if [ -e "bin" ]
 * then
 * echo "already have build (bin) directory"
 * else
 * echo "mkdir bin"
 * mkdir bin
 * fi
 * javac -g com/heintze/MarshalBug.java -d bin
 * java  com.heintze.MarshalBug "/marshalBug/datum[1]/value/text()" '/marshalBug/datum[@name="abc"]' <<EOF 
 * ><?xml version="1.0" encoding="UTF-8" standalone="yes"?>
 *   <bean2>
 *     <map>
 *       <item value="mumble" key="fratz"/>
 *     </map>
 * </bean2>
 * EOF
 * rm bin/com/heintze/MarshalBug.class
 * popd
 * End commands to execute this file using Java with bash
 *
 */

package com.heintze;
import java.io.FileReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;
import javax.xml.bind.annotation.adapters.XmlAdapter;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlRootElement
class Bean2<K,V> {
  @XmlJavaTypeAdapter(MyMapAdapter2.class)
	  public HashMap<K,V> map = new HashMap<K,V>();
}

class MyMapAdapter2<K,V>
  extends XmlAdapter<Item2[],Map><K,V>> {
// http://weblogs.java.net/blog/kohsuke/archive/2005/04/xmladapter_in_j.html 
//	@Override public Item[] marshal(Map<V, V> map) throws Exception {	}

	@Override
	public Map<K,V> unmarshal(Item2[] value) throws Exception {
	    Map<K,V> r = new HashMap<K,V>();
	    for( Item2 c : value )
	      r.put((K)c.key, (V)c.value);
	    return r;
	}

	@Override
	public Item2[] marshal(Map<K, V> map) throws Exception {
		Item2[] result=new Item2[map.size()];
		int ii=0;
		for(K key:map.keySet()){
			result[ii]=new Item2();
			result[ii].key=(String) key;
			result[ii].value=(String)map.get((K)key);
			ii = ii + 1;
		}
		return result;

	}
}

class Item2 {
  @XmlAttribute
	  public String key;

  @XmlAttribute
	  public String value;
}

@XmlRootElement
public class MarshalBug {
  static java.io.PrintStream out = System.out;
  public static void main(String[] args) {
    Bean2<String,String> data = new Bean2<String,String>();
    try {
      JAXBContext context = JAXBContext.newInstance(Bean2.class);
      Marshaller m = context.createMarshaller();
      Unmarshaller um = context.createUnmarshaller();
      data = (Bean2)um.unmarshal(new FileReader(java.io.FileDescriptor.in) );
      for(String k : data.map.keySet())
        out.println("bean["+k+"]="+data.map.get(k));
      data.map.put("hello", "world");
      m.marshal(data, System.out);
    } catch (JAXBException e) {
      e.printStackTrace();
    }
  }
 }