ConversionUtils: avoid NPEs in canCast methods

The following calls now return true:
* canCast((Class) null, Foo.class) -- previously threw NPE
* canCast((Object) null, Foo.class) -- already did

The following calls now return false:
* canCast((Class) null, null) -- previously threw NPE
* canCast((Object) null, null) -- previously threw NPE
* canCast(new Foo(), null) -- previously threw NPE
* canCast(Foo.class, null) -- previously threw NPE

This commit also adds tests to ensure these calls continue to work.

Author: ctrueden

src/main/java/org/scijava/util/ConversionUtils.java

@@ -100,18 +100,25 @@ public static <T> T cast(final Object src, final Class<T> dest) {
 	 * Checks whether objects of the given class can be cast to the specified
 	 * type.
 	 *
+	 * @return true If the destination class is assignable from the source one, or
+	 *         if the source class is null and destination class is non-null.
 	 * @see #cast(Object, Class)
 	 */
 	public static boolean canCast(final Class<?> src, final Class<?> dest) {
-		return dest.isAssignableFrom(src);
+		if (dest == null) return false;
+		return src == null || dest.isAssignableFrom(src);
 	}
 
 	/**
 	 * Checks whether the given object can be cast to the specified type.
 	 *
+	 * @return true If the destination class is assignable from the source
+	 *         object's class, or if the source object is null and destionation
+	 *         class is non-null.
 	 * @see #cast(Object, Class)
 	 */
 	public static boolean canCast(final Object src, final Class<?> dest) {
+		if (dest == null) return false;
 		return src == null || canCast(src.getClass(), dest);
 	}
 

src/test/java/org/scijava/util/ConversionUtilsTest.java

@@ -78,9 +78,18 @@ public void testCanCast() {
 		assertFalse(ConversionUtils.canCast(int.class, Number.class));
 
 		// casting from null always works
+		final Class<?> nullClass = null;
+		assertTrue(ConversionUtils.canCast(nullClass, Object.class));
+		assertTrue(ConversionUtils.canCast(nullClass, int[].class));
 		final Object nullObject = null;
 		assertTrue(ConversionUtils.canCast(nullObject, Object.class));
 		assertTrue(ConversionUtils.canCast(nullObject, int[].class));
+
+		// casting to null is not allowed
+		assertFalse(ConversionUtils.canCast(nullClass, null));
+		assertFalse(ConversionUtils.canCast(Object.class, null));
+		assertFalse(ConversionUtils.canCast(nullObject, null));
+		assertFalse(ConversionUtils.canCast(new Object(), null));
 	}
 
 	/** Tests {@link ConversionUtils#cast(Object, Class)}. */