CommandInfo: flag abstract classes as invalid

An abstract class cannot be instantiated, even by reflection.
So there will be grievous problems when it comes time to instantiate the
command, which will manifest with the following cryptic stack trace:

  [ERROR] Cannot create module: foo.Bar
  org.scijava.module.ModuleException: org.scijava.InstantiableException: java.lang.InstantiationException
          at org.scijava.command.CommandModule.instantiateCommand(CommandModule.java:250)
          at org.scijava.command.CommandModule.<init>(CommandModule.java:96)
          at org.scijava.command.CommandInfo.createModule(CommandInfo.java:330)
          at org.scijava.module.DefaultModuleService.createModule(DefaultModuleService.java:156)
          ...
  Caused by: org.scijava.InstantiableException: java.lang.InstantiationException
          at org.scijava.plugin.PluginInfo.createInstance(PluginInfo.java:307)
          at org.scijava.command.CommandInfo.createInstance(CommandInfo.java:250)
          at org.scijava.command.CommandModule.instantiateCommand(CommandModule.java:247)
          ... 17 more
  Caused by: java.lang.InstantiationException
          at sun.reflect.InstantiationExceptionConstructorAccessorImpl.newInstance(InstantiationExceptionConstructorAccessorImpl.java:48)
          at java.lang.reflect.Constructor.newInstance(Constructor.java:408)
          at java.lang.Class.newInstance(Class.java:433)
          at org.scijava.plugin.PluginInfo.createInstance(PluginInfo.java:302)
          ... 19 more

Let's head off future confusion by marking abstract classes as invalid.

Author: ctrueden

src/main/java/org/scijava/command/CommandInfo.java

@@ -439,6 +439,12 @@ private synchronized void initParams() {
 	/** Processes the given class's @{@link Parameter}-annotated fields. */
 	private void checkFields(final Class<?> type) {
 		if (type == null) return;
+
+		// NB: Reject abstract classes.
+		if (Modifier.isAbstract(type.getModifiers())) {
+			problems.add(new ValidityProblem("Delegate class is abstract"));
+		}
+
 		final List<Field> fields =
 			ClassUtils.getAnnotatedFields(type, Parameter.class);
 

src/test/java/org/scijava/command/InvalidCommandTest.java

@@ -81,15 +81,18 @@ public void testInvalid() {
 
 		final List<ValidityProblem> problems = info.getProblems();
 		assertNotNull(problems);
-		assertEquals(2, problems.size());
+		assertEquals(3, problems.size());
 
 		final String p0 = problems.get(0).getMessage();
-		assertEquals("Invalid duplicate parameter: private int "
-			+ "org.scijava.command.InvalidCommandTest$InvalidCommand.x", p0);
+		assertEquals("Delegate class is abstract", p0);
 
 		final String p1 = problems.get(1).getMessage();
+		assertEquals("Invalid duplicate parameter: private int "
+			+ "org.scijava.command.InvalidCommandTest$InvalidCommand.x", p1);
+
+		final String p2 = problems.get(2).getMessage();
 		assertEquals("Invalid final parameter: private final float "
-			+ "org.scijava.command.InvalidCommandTest$InvalidCommand.y", p1);
+			+ "org.scijava.command.InvalidCommandTest$InvalidCommand.y", p2);
 	}
 
 	// -- Helper classes --