Improve error message for missing required inputs

src/main/java/org/scijava/object/DefaultObjectService.java

@@ -29,6 +29,9 @@
 
 package org.scijava.object;
 
+import java.util.HashMap;
+import java.util.Map;
+
 import org.scijava.event.EventHandler;
 import org.scijava.event.EventService;
 import org.scijava.object.event.ObjectCreatedEvent;
@@ -56,13 +59,17 @@
 public final class DefaultObjectService extends AbstractService implements
  ObjectService
 {
+ private static String DEFAULT_OBJECT_NAME = "an object";
 
  @Parameter
  private EventService eventService;
 
  /** Index of registered objects. */
  private NamedObjectIndex<Object> objectIndex;
 
+ /** Map of human-friendly names */
+ private Map<Class<?>, String> aliasMap = new HashMap<>();
+
  // -- ObjectService methods --
 
  @Override
@@ -75,6 +82,16 @@ public NamedObjectIndex<Object> getIndex() {
  return objectIndex;
  }
 
+ @Override
+ public String getHumanFriendlyName(Class<?> c) {
+ return aliasMap.getOrDefault(c, DEFAULT_OBJECT_NAME);
+ }
+
+ @Override
+ public void addHumanFriendlyName(Class<?> c, String name) {
+ aliasMap.put(c, name);
+ }
+
  // -- Service methods --
 
  @Override

src/main/java/org/scijava/object/ObjectService.java

@@ -98,6 +98,10 @@ default void removeObject(final Object obj) {
  eventService().publish(new ObjectsRemovedEvent(obj));
  }
 
+ String getHumanFriendlyName(Class<?> c);
+
+ void addHumanFriendlyName(Class<?> c, String name);
+
  // -- Deprecated methods --
 
  /** @deprecated Use {@link #eventService()} instead. */

src/main/java/org/scijava/widget/AbstractInputHarvester.java

@@ -119,8 +119,8 @@ private <T> WidgetModel addInput(final InputPanel<P, W> inputPanel,
  }
 
  if (item.isRequired()) {
- throw new ModuleException("A " + type.getSimpleName() +
- " is required but none exist.");
+ throw new ModuleException("This operation requires " + objectService.getHumanFriendlyName(type)
+  + " of type '" + type.getSimpleName() + "', but none was found.");
  }
 
  // item is not required; we can skip it