Lazy List Utilities useful for lazy-loading of DAO collections.

Example.java

RepositoryInstance repositoryInstance; //some Repository which has findById.
List<T> lazyList = LazyListUtil.idListToLazyList(jdbcTemplate.queryForList("select foreign_key from join_table where primary_key = ?",new Object[]{id},String.class),repositoryInstance));
// The only query run so far "select foreign key from join_table where primary_key = ?" . the others aren't run until we use lazyList, which would be inside RepositoryInstance.

LazyListUtil.java

import java.util.List;

import com.google.common.base.Function;
import com.google.common.base.Supplier;
import com.google.common.base.Suppliers;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;

public class LazyListUtil {
	
	public static <T> Function<Supplier<T>, T> supplyFunction() {
		  return new Function<Supplier<T>, T>() {
		    public T apply(Supplier<T> supplier) {
		      return supplier.get();
		    }
		  };
		}
	
	public static <T> List<T> idListToLazyList(List<String> ids, final Repository<T> repo) {
		return Lists.transform((List<Supplier<T>>)
			    ImmutableList.copyOf(
			    	Lists.transform(
			    			ids,
			    			new Function<String, Supplier<T>>() {
			    				public Supplier<T> apply(String key) {
			    					return Suppliers.memoize(Suppliers.compose(
			    							new Function<String,T>() {
			    								@Override
			    								public T apply(String arg0) {
			    									return repo.findById(arg0);
			    								}
			    							},
			    							Suppliers.ofInstance(key)));
			    					}
			    				})),
			    	LazyListUtil.<T>supplyFunction());			    
	}

}