import org.eclipse.jface.viewers.Viewer;

/**
 * An element which can be used in JFace {@link Viewer}s to represent a
 * <code>null</code> value in the model.
 * 
 * @see NullElementIdentityConverter
 * @see NullElementStructuredContentProviderDecorator
 * 
 * @author Henno Vermeulen
 */
public final class NullElement {
	private static final NullElement instance = new NullElement();
	private final String name;

	private NullElement() {
		this("");
	}

	public NullElement(String name) {
		this.name = name;
	}

	public static NullElement getInstance() {
		return instance;
	}

	@Override
	public String toString() {
		return name;
	}

}

import org.eclipse.core.databinding.conversion.IConverter;

/**
 * A converter which simply returns the object itself, but converts
 * <code>null</code> into a {@link NullElement} and vice versa.
 * 
 * <p>
 * Possible refactoring: make this into a decorator for an existing converter.
 * 
 * @author Henno Vermeulen
 */
public final class NullElementIdentityConverter implements IConverter {

	@Override
	public Object convert(Object fromObject) {
		if (fromObject == null) {
			return NullElement.getInstance();
		}
		if (fromObject instanceof NullElement) {
			return null;
		}
		return fromObject;
	}

	@Override
	public Object getFromType() {
		return null;
	}

	@Override
	public Object getToType() {
		return null;
	}

}

import java.util.ArrayList;
import java.util.Arrays;

import org.eclipse.jface.viewers.IStructuredContentProvider;
import org.eclipse.jface.viewers.Viewer;

/**
 * A decorator for an <code>IStructuredContentProvider</code> which adds a
 * {@link NullElement} as the first element. This is useful because it does not
 * require the input to contain a <code>NullElement</code>.
 */
public class NullElementStructuredContentProviderDecorator implements
		IStructuredContentProvider {

	private final IStructuredContentProvider decorated;
	private final NullElement nullElement;

	public NullElementStructuredContentProviderDecorator(
			IStructuredContentProvider decorated) {
		this(decorated, NullElement.getInstance());
	}

	public NullElementStructuredContentProviderDecorator(
			IStructuredContentProvider decorated, NullElement nullElement) {
		this.decorated = decorated;
		this.nullElement = nullElement;
	}

	/**
	 * Returns the elements in the input, which must be either an array or a
	 * <code>Collection</code>.
	 */
	public Object[] getElements(Object inputElement) {
		Object[] elements = decorated.getElements(inputElement);
		ArrayList<Object> l = new ArrayList<Object>();
		l.add(nullElement);
		l.addAll(Arrays.asList(elements));
		return l.toArray();
	}

	public void inputChanged(Viewer viewer, Object oldInput, Object newInput) {
		decorated.inputChanged(viewer, oldInput, newInput);
	}

	public void dispose() {
		decorated.dispose();
	}
}