Generics in Spring Controller/Service/Repository
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I would like to trim down my code to use only one function for filtering.
Filtering should work with 3 different classes: com.example.model.Category, com.example.model.Tag, java.util.Calendar.

Here's my Controller

    @Controller
    @RequestMapping( "/blog" )
    public class BlogController
    {
    	@Autowired
    	private ArticleService articleService;
    
    	@Autowired
    	private CategoryService categoryService;
    
    	@Autowired
    	private TagService tagService;
    
    	private static final int PER_PAGE = 5;
    
    	private static final int ARCHIVE_MONTHS = 12;
    
    	@ModelAttribute
    	public void addGlobalObjects( Map<String, Object> map )
    	{
    		map.put( "section", "blog" );
    
    		SortedMap<Category, Integer> categories = new TreeMap<Category, Integer>();
    		for ( Category category : categoryService.list() )
    		{
    			categories.put( category, articleService.size( category, Category.class ) );
    		}
    
    		Calendar cal = Calendar.getInstance();
    		cal.set( Calendar.DAY_OF_MONTH, 1 );
    
    		cal.add( Calendar.MONTH, ARCHIVE_MONTHS * -1 );
    
    		SortedMap<Date, Integer> archive = new TreeMap<Date, Integer>();
    		for ( int i = 0; i < ARCHIVE_MONTHS; ++i )
    		{
    			cal.add( Calendar.MONTH, 1 );
    			archive.put( cal.getTime(), articleService.size( cal, Calendar.class ) );
    		}
    
    		SortedMap<Tag, Integer> tags = new TreeMap<Tag, Integer>();
    		for ( Tag tag : tagService.list() )
    		{
    			tags.put( tag, articleService.size( tag, Tag.class ) );
    		}
    
    		map.put( "categories", categories );
    		map.put( "archive", archive );
    		map.put( "tags", tags );
    
    		map.put( "categoriesSize", categoryService.size() );
    		map.put( "tagsSize", tagService.size() );
    
    		map.put( "date", new Date() );
    	}
    
    	@RequestMapping( "/index.html" )
    	public String index( HttpServletRequest request, Map<String, Object> map )
    	{
    		return list( request, map, null, null );
    	}
    
    	@RequestMapping( "/archive/{date}.html" )
    	public String archive( @PathVariable( "date" ) @DateTimeFormat( iso = ISO.DATE, style = "yyyy/MM" ) Date date, HttpServletRequest request, Map<String, Object> map )
    	{
    		Calendar cal = Calendar.getInstance();
    		cal.setTime( date );
    
    		return list( request, map, cal, Calendar.class );
    	}
    
    	private <T> String list( HttpServletRequest request, Map<String, Object> map, Object filterObject, Class<T> clazz )
    	{
    		int page = ServletRequestUtils.getIntParameter( request, "page", 1 );
    		map.put( "articles", articleService.list( page * PER_PAGE - PER_PAGE, PER_PAGE, filterObject, clazz ) );
    		map.put( "numPages", Math.ceil( articleService.size() / PER_PAGE ) );
    		map.put( "currentPage", page );
    		return "articles";
    	}
    }

In my ArticleDAO now I need to implement the methods list/size:

    @Repository
    public class ArticleDAO extends BaseDAO<Article>
    {
    	public <T> List<Article> list( int offset, int limit, Object filterObject, Class<T> clazz )
    	{
    		Criteria c = doFilter( filterObject, clazz );
    
    		if ( limit > 0 )
    		{
    			c.setFirstResult( offset );
    			c.setMaxResults( limit );
    		}
    
    		return c.list();
    	}
    
    	public <T> Integer size( Object filterObject, Class<T> clazz )
    	{
    		Number ret = ( Number ) doFilter( filterObject, clazz ).setProjection( Projections.rowCount() ).uniqueResult();
    		return ret.intValue();
    	}
    
    	private <T> Criteria doFilter( Object filterObject, Class<T> clazz )
    	{
    		Criteria criteria = sessionFactory.getCurrentSession().createCriteria( Article.class );
    
    		if ( filterObject != null && clazz != null )
    		{
    			T filter = clazz.cast( filterObject );
    
    			if ( filter instanceof Calendar )
    			{
    				// The method set(int, int) is undefined for the type T
    				filter.set( Calendar.DAY_OF_MONTH, 1 );
    				filter.set( Calendar.HOUR_OF_DAY, 0 );
    				filter.set( Calendar.MINUTE, 0 );
    				filter.set( Calendar.SECOND, 0 );
    				// The method getTime() is undefined for the type T
    				Date d1 = filter.getTime();
    
    				// The method getActualMaximum(int) is undefined for the type T
    				filter.set( Calendar.DAY_OF_MONTH, filter.getActualMaximum( Calendar.DAY_OF_MONTH ) );
    				filter.set( Calendar.HOUR_OF_DAY, 23 );
    				filter.set( Calendar.MINUTE, 59 );
    				filter.set( Calendar.SECOND, 59 );
    				Date d2 = filter.getTime();
    
    				criteria.add( Restrictions.between( "creationDate", d1, d2 ) );
    			}
    
    			if ( filter instanceof Category )
    			{
    				// The method getId() is undefined for the type T
    				criteria.createCriteria( "categories" ).add( Restrictions.eq( "id", filter.getId() ) );
    			}
    
    			if ( filter instanceof Tag )
    			{
    				// The method getId() is undefined for the type T
    				criteria.createCriteria( "tags" ).add( Restrictions.eq( "id", filter.getId() ) );
    			}
    		}
    
    		return criteria;
    	}
    }

How ArticleDAO.doFilter should look like? I guess I don't understand the Generics stuff.