Wildcards in "indirect" or "second level" generics.

CovarianceTest.java

package x;

/**
 * <p>
 * Wildcards in "indirect" or "second level" generics.
 * <p>
 * {@link #list} contains generic items.
 * {@link #testWildcard()} and {@link #testGeneric()} wants to work with that items
 * and shouldn't care about it's generic type <i>as long as it's the same type through the
 * whole method<i>. How to express that correctly (if it's possible at all)?
 * 
 * Solution: {@link #testWithHelper()}
 */
public class CovarianceTest {
	private List<ListItem<? extends Number>> list = Arrays.asList(new ListItem<>(1l), new ListItem<>(1d));

	@Test
	public void testWildcard() {
		ListItem<?> listItem = list.get(0);
		ListItemService<?> service = new ListItemService<>(listItem.getData());
		// compile error
		// listItem.setService(service);
	}

	@Test
	public <T extends Number> void testGeneric() {
		// ugly with unchecked warning
		ListItem<T> listItem = (ListItem<T>) list.get(0);
		ListItemService<T> service = new ListItemService<>(listItem.getData());
		listItem.setService(service);
	}
	
	@Test
	public void testWithHelper() {
		initHelper(list.get(0));
	}
	
	private <T extends Number> void initHelper(ListItem<T> listItem) {
		ListItemService<T> service = new ListItemService<>(listItem.getData());
		listItem.setService(service);
	}
}

ListItem.java

package x;

public class ListItem<T extends Number> {
	private T data;
	private ListItemService<T> service;
	
	public ListItem(T data) {
		this.data = data;
	}

	public T getData() {
		return data;
	}
	
	public void setService(ListItemService<T> service) {
		this.service = service;
	}
}

ListItemService.java

package x;

public class ListItemService<T extends Number> {
	private T data;

	public ListItemService(T data) {
		this.data = data;
	}
	
}