Add capacity for type aliasing module inputs

Added methods ot the Converter class for identifying the Input and
Output classes that can be converted between, and the populateInputs
method - allowing Converters the potential to point consumers to
potential input objects that can certainly be converted to appropriate
output objects.

Added API to the ConvertService to take advantage of the new Converter methods
- aggregating the potential input objects from converters matching a queried
output type.

Author: hinerm

src/main/java/org/scijava/convert/AbstractConverter.java

@@ -31,24 +31,44 @@
 
 package org.scijava.convert;
 
+import java.util.Set;
+
 import org.scijava.plugin.AbstractHandlerPlugin;
 
 /**
- * Abstract superclass for {@link Converter} plugins. Performs
- * appropriate dispatching of {@link #canConvert(ConversionRequest)} and
+ * Abstract superclass for {@link Converter} plugins. Performs appropriate
+ * dispatching of {@link #canConvert(ConversionRequest)} and
  * {@link #convert(ConversionRequest)} calls based on the actual state of the
  * given {@link ConversionRequest}.
  * <p>
  * Note that the {@link #supports(ConversionRequest)} method is overridden as
  * well, to delegate to the appropriate {@link #canConvert}.
  * </p>
+ * <p>
+ * NB: by default, the {@link #populateInputs(Set)} method has a dummy
+ * implementation. Effectively, this is opt-in behavior. If a subclass of this
+ * would like to declare automatic mappings between input and output conversion
+ * types, this method can be overridden.
+ * </p>
  *
  * @author Mark Hiner
  */
 public abstract class AbstractConverter extends
 	AbstractHandlerPlugin<ConversionRequest> implements Converter
 {
 
+	// -- Fields --
+
+	private final Class<?> inClass;
+	private final Class<?> outClass;
+
+	// -- Constructor --
+
+	public AbstractConverter(final Class<?> inClass, final Class<?> outClass) {
+		this.inClass = inClass;
+		this.outClass = outClass;
+	}
+
 	// -- ConversionHandler methods --
 
 	@Override
@@ -74,6 +94,21 @@ public Object convert(final ConversionRequest request) {
 		return null;
 	}
 
+	@Override
+	public void populateInputs(Set<Object> objects) {
+		// No-op
+	}
+
+	@Override
+	public Class<?> getOutputType() {
+		return outClass;
+	}
+
+	@Override
+	public Class<?> getInputType() {
+		return inClass;
+	}
+
 	// -- Typed methods --
 
 	@Override

src/main/java/org/scijava/convert/ConvertService.java

@@ -32,6 +32,7 @@
 package org.scijava.convert;
 
 import java.lang.reflect.Type;
+import java.util.List;
 
 import org.scijava.plugin.HandlerService;
 
@@ -101,4 +102,6 @@ public interface ConvertService extends
 	 * @see #supports(ConversionRequest)
 	 */
 	boolean supports(Class<?> src, Type dest);
+
+	List<Object> getCompatibleInputs(Class<?> dest);
 }

src/main/java/org/scijava/convert/Converter.java

@@ -46,7 +46,8 @@
  * @see ConversionRequest
  * @author Mark Hiner
  */
-public interface Converter extends HandlerPlugin<ConversionRequest> {
+public interface Converter extends HandlerPlugin<ConversionRequest>
+{
 
 	/**
 	 * Checks whether a given {@ConversionRequest} can be
@@ -155,4 +156,24 @@ public interface Converter extends HandlerPlugin<ConversionRequest> {
 	 * @return The conversion output
 	 */
 	Object convert(ConversionRequest request);
+
+	/**
+	 * Fills the given {@code Object} set with the potential input types known to
+	 * this {@code Converter}.
+	 *
+	 * @param objects A pre-allocated {@link Set} of potential input objects.
+	 *          Input objects known to this {@code Converter} will be added to
+	 *          this set.
+	 */
+	void populateInputs(Set<Object> objects);
+
+	/**
+	 * @return The base {@code Class} this {@code Converter} produces as output.
+	 */
+	Class<?> getOutputType();
+
+	/**
+	 * @return The base {@code Class} this {@code Converter} accepts as input.
+	 */
+	Class<?> getInputType();
 }

src/main/java/org/scijava/convert/DefaultConvertService.java

@@ -32,6 +32,10 @@
 package org.scijava.convert;
 
 import java.lang.reflect.Type;
+import java.util.ArrayList;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
 
 import org.scijava.plugin.Plugin;
 import org.scijava.service.Service;
@@ -96,4 +100,17 @@ public boolean supports(final Object src, final Type dest) {
 	public boolean supports(final Class<?> src, final Type dest) {
 		return supports(new ConversionRequest(src, dest));
 	}
+
+	@Override
+	public List<Object> getCompatibleInputs(Class<?> dest) {
+		Set<Object> objects = new HashSet<Object>();
+
+		for (final Converter c : getInstances()) {
+			if (dest.isAssignableFrom(c.getOutputType())) {
+				c.populateInputs(objects);
+			}
+		}
+
+		return new ArrayList<Object>(objects);
+	}
 }

src/main/java/org/scijava/convert/DefaultConverter.java

@@ -57,6 +57,12 @@
 @Plugin(type = Converter.class)
 public class DefaultConverter extends AbstractConverter {
 
+	// -- Constructor --
+
+	public DefaultConverter() {
+		super(Object.class, Object.class);
+	}
+
 	// -- ConversionHandler methods --
 
 	@Override