Java泛型和原始类型

Question

When I write the method this way. I get this warning:

BaseEvent is a raw type. References to generic type BaseEvent should be parameterized

@Override
public <T extends BaseEvent> void actionPerformed(T event) { ... }

The code still runs fine, although the warning sign is annoying. When I write the code this way the warning goes away.

@Override
public <T> void actionPerformed(BaseEvent<T> event) { ... }

With the previous message, It doesn't guarantee that is a subClass of BaseEvent. So I changed it again:

@Override
public <T extends EventObject> void actionPerformed(BaseEvent<T> event) { ... }


@Override
public <T extends BaseEvent<T>> void actionPerformed(BaseEvent<T> event) { ... }

BaseEvent class is a class I made that extends EventOBject

public abstract class BaseEvent<T> extends EventObject
{
    private String eventType;

    // Constructor
    public BaseEvent(Object source, String type) 
    {
        super(source);
        eventType = type;
    }


    public String getEventType()    { return eventType; }   
}

All the methods seem to work fine. But I was wondering which is the better solution.

Answer 1

Where do you use T in BaseEvent definition? Define it in the following way

public abstract class BaseEvent extends EventObject

then you won't get a warning with

@Override
public void actionPerformed(BaseEvent event) { ... }

UPDATE

Suppose your BaseEvent really required to be parametrized. Then write following

@Override
public <T> void actionPerformed(BaseEvent<T> event) { ... }

This will give you a parametrized method.

UPDATE 1

It doesn't guarantee that is a subClass of BaseEvent.

It does. <T> is a parameter for method template. This parameter goes to BaseEvent<T> which is subclass of EventObject by definition.

UPDATE 2

Do not use generics at the beginning of your learning. Generics are just for additional self testing. Use raw types. Then when you start to feel them, you will parametrize them correctly.

Answer 2

The best solution is the one that avoids the warnings and guarantees your type safety at compile time.

If the type of T in BaseEvent doesn't matter, couldn't you just use your first one and parameterize BaseEvent? Do something like:

@Override
public <T extends BaseEvent<?>> void actionPerformed(T event) { ... }

Alternatively, it looks like your BaseEvent class does not actually use T for anything - why is it there?

Answer 3

The type parameter T is never used in the class definition. You might be able to remove the type parameter from BaseEvent :

public abstract class BaseEvent extends EventObject { ... }

and just define your method without a type parameter:

@Override
public void actionPerformed(BaseEvent event) { ... }