Generic Wildcard (Covariance)

Allowed Type Conversion

In coding, we are often concerned with how types translate. For example, in this example we assign a String to a variable of type Object. In other words we assign a more specific object to a more general variable. It is allowed.

Object o;
o = "I am a String";

Not-Allowed Type Conversion

From our previous successful snippet, we might assume we can make the assignment shown here (to the "objects" var). However, Java does not allow it, giving us an error such as:

incompatible types: List<String> cannot be converted to List<Object>.

Why?
The following snippet will show us.

List<Object> objects;
List<String> strings = new ArrayList<>();
objects = strings;

Mixed Types

The list is read-write meaning elements can be added to it. So Java thinks that if we're allowed to assign "strings" to "objects", we may try to add an improper type as shown (e.g. an Integer into a List of String subtypes).

This is the reason for the "incompatible types..." error.

List<Object> objects;
List<String> strings = new ArrayList<>();
objects = strings;
objects.add(10);

Read-Only Case

But what if we have a method that uses the list in a "read-only" way (like "preintList" here). This looks like it should work.

Java does not allow it (for reasons we just discussed). The method parm is "invariant" (restricted).

The next snippet will show us how we can widen the type restriction.

public void printList(List<Object> list) {
	for (Object o: list)
		prn(o.toString());
}

public void caller() {
	List<String> names =
		Arrays.asList("A", "B", "C", "D");
	printList(names);
}

Widening Type Restriction (Covariant)

By adding the generic wildcard "? extends Object", Java allows it. Any subtype that extends Object is allowed. This parameter is now "covariant" per the wildcard (any list subtype that extends "Object" is allowed).

public void printList(List<? extends Object> list) {
	for (Object o: list)
		prn(o.toString());
}
public void caller() {
	List<String> names =
		Arrays.asList("A", "B", "C", "D");
	printList(names);
}