Java 7 for Absolute Beginners

CHAPTER 15 ■ GENERICS AND REGULAR EXPRESSIONS

Listing 15-2. A Simplified Generic

LinkedList<JPanel> panelList = new LinkedList<>();

Notice that the constructor for our LinkedList object indicates that it's a generic but doesn't provide

the type of the objects that can go in the list. We can leave that out because the compiler can infer a type

of JPanel from the type declaration portion of this variable declaration. By the way, the <> expression is

often called “the diamond.”

An object can have multiple parameters, provided a matching class exists to define that object.

Listing 15-3 shows an example.

Listing 15-3. A Generic with Multiple Parameters

package com.bryantcs.examples;

public class GenericRole<Actor, Role> {

private Actor actor;

private Role role;

public GenericRole(Actor p, Role a) {

actor = p;

role = a;

}

public Actor getActor() {

return actor;

}

public Role getRole() {

return role;

}

}

A significant feature of this class is that you do not have to create an Actor class or a Role class.

Because it uses generics, the declarations that create instances of the GenericRole object must specify

the types of the Actor and Role objects.

As I mentioned at the beginning of this section, this arrangement is where the word “generic” comes

from. These objects can be undeclared at this point, so they are, in a sense, generic.

So let's look at a class that does something with our GenericRole class. Consider Listing 15-4.

Listing 15-4. Using a Multiple-Parameter Generic

package com.bryantcs.examples;

import java.util.LinkedList;

public class GenericRoleProgram {