Need to find Max Value in Liferay table using Service Builder

I have built a Liferay portlet using Service Builder and it has one table. One of the fields holds a double value called 'ZValue'. I need to add to my -LocalServiceImpl.java file a public method that will return the maximum value currently found in the field 'ZValue'. I was hoping there was a Liferay class similar to DynamicQuery that instead returns a single value. I know I can return all the records and cycle through them myself to get the maximum value, but I'm sure there is a simpler way to get the max value.

What I have found in my search of stackoverflow is:

DynamicQuery query = DynamicQueryFactoryUtil.forClass(classname);
query.setProjection(ProjectionFactoryUtil.max("ZValue"));

but I didn't understand how to actually return the value as a DynamicQuery returns a list and not a single value.

Solution

In the following example, we query the JournalArticle table to find all the articles that match a certain criteria, and then only get the newest version of each of those (the max).

As Pankaj said, you need to create two dynamic queries. The first one is used to specify that you need the max to be returned:

DynamicQuery subQuery = DynamicQueryFactoryUtil.forClass(JournalArticle.class, "articleSub", PortalClassLoaderUtil.getClassLoader())
            .add(PropertyFactoryUtil.forName("articleId").eqProperty("articleParent.articleId"))
            .setProjection(ProjectionFactoryUtil.max("id"));

articleSub is just an alias you assign to the query. ProjectionFactoryUtil.max will return the max.

The second one is the actual query. It will take the value returned from the first query:

DynamicQuery query = DynamicQueryFactoryUtil.forClass(JournalArticle.class, "articleParent", PortalClassLoaderUtil.getClassLoader())
            .add(PropertyFactoryUtil.forName("id").eq(subQuery))
            .add(PropertyFactoryUtil.forName("type").eq("Slider"));

The execution of this DynamicQuery will return a List<JournalArticle> with all the matches returned by the generated SQL sentence.

List<JournalArticle> myList = JournalArticleLocalServiceUtil.dynamicQuery(query);