Why class implementing NamespaceContext compiles in Java 6, but not Java 5?

I have a class that implements javax.xml.namespace.NamespaceContext.
As far as I can tell, this interface has not changed between Java 5
and Java 6 (specifically 1.5.0_11 and 1.6.0_07). My class implements
the three methods of the interface, and puts the "@override"
annotation on each one of them.

When I compile this class with Java 6, either from within eclipse or
standalone, it compiles and runs fine. When I compile it with Java 5
(eclipse and standalone), it complains "method does not override a
method from its superclass", for all three interface methods. What am
I missing here?

Here's the class:
-----------------------
package xpathfind;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

import javax.xml.namespace.NamespaceContext;

public class MapNamespaceContext implements NamespaceContext
{
    private Map<String, String> uriMap = new HashMap<String,
String>();
    private Map<String, String> prefixMap = new HashMap<String,
String>();

    public MapNamespaceContext(Map<String, String> uriMap)
    {
        this.uriMap = uriMap;

        for (String key: uriMap.keySet())
            prefixMap.put(uriMap.get(key), key);
    }

    public MapNamespaceContext(String[] colonPairs)
    {
        uriMap = new HashMap<String, String>();

        for (String colonPair: colonPairs)
        {
            int colonIndex = colonPair.indexOf(':');
            uriMap.put(colonPair.substring(0, colonIndex).trim(),
                       colonPair.substring(colonIndex + 1));
        }

        for (String key: uriMap.keySet())
            prefixMap.put(uriMap.get(key), key);
    }

    @Override
    public String getNamespaceURI(String prefix)
    {
        return (uriMap.get(prefix));
    }

    @Override
    public String getPrefix(String namespaceURI)
    {
        return (prefixMap.get(namespaceURI));
    }

    @Override
    public Iterator getPrefixes(String namespaceURI)
    {
        return (null);
    }
}
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