Create NullConverter

Adds a new Converter type to handle null source or destination types.

Without this converter, the null logic was managed in a mix between
AbstractConverter and DefaultConverter - which means that all converters
had some null logic in them (potentially wrong for that given converter)
and could not run before DefaultConverter, without copying its null
logic.

Author: hinerm

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

@@ -0,0 +1,81 @@
+/*
+ * #%L
+ * SciJava Common shared library for SciJava software.
+ * %%
+ * Copyright (C) 2009 - 2014 Board of Regents of the University of
+ * Wisconsin-Madison, Broad Institute of MIT and Harvard, and Max Planck
+ * Institute of Molecular Cell Biology and Genetics.
+ * %%
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ * 
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ * #L%
+ */
+
+package org.scijava.convert;
+
+import org.scijava.Priority;
+import org.scijava.plugin.Plugin;
+import org.scijava.util.ConversionUtils;
+
+/**
+ * {@link Converter} implementation for handling {@code null} values. Performs
+ * basic casting when given a {@code null} source and returns {@code} null
+ * directly when given a {@code} null destination.
+ * <p>
+ * By running at {@link Priority#FIRST_PRIORITY}, other converters should
+ * not need to worry about {@code null} source or destination parameters.
+ * </p>
+ *
+ * @author Mark Hiner
+ */
+@Plugin(type = Converter.class, priority = Priority.FIRST_PRIORITY)
+public class NullConverter extends AbstractConverter<Object, Object> {
+
+	@Override
+	public boolean canConvert(final Object src, final Class<?> dest) {
+		return src == null || super.canConvert(src, dest);
+	}
+
+	@Override
+	public boolean canConvert(final Class<?> src, final Class<?> dest) {
+		return src == null || dest == null;
+	}
+
+	@Override
+	public <T> T convert(final Object src, final Class<T> dest) {
+		if (dest == null) return null;
+		if (src == null) return ConversionUtils.getNullValue(dest);
+
+		throw new IllegalArgumentException("Attempting non-null conversion: " +
+			src + " > " + dest + " using NullConverter.");
+	}
+
+	@Override
+	public Class<Object> getOutputType() {
+		return Object.class;
+	}
+
+	@Override
+	public Class<Object> getInputType() {
+		return Object.class;
+	}
+
+}