CODEWITHSUNDEEP
javagenerics
Rory G
Rory G

Reputation: 172

Generic Method, generic return type mismatch

The following Java code will not compile. 

public <DO extends ServerDataObject> ProxyDataObject<DO> convertToProxy(Class<DO>   doClass) throws Exception{
    if(getId()==0)return null;
    ProxyDataObject<DO> proxy = (ProxyDataObject<DO>) doClass.newInstance();
    proxy.setID(getId());
    return proxy;
}

public interface ProxyDataObject<DO extends ServerDataObject> extends ServerDataObject, DataTransferInterface {

public void setID(int id);

@Ignore
public String getIDName();
}

The following message is given:

Bound mismatch: The type DO is not a valid substitute for the bounded parameter DO extends ServerDataObject of the type ProxyDataObject

This doesn't make sense to me.

Upvotes: 0

Views: 207

Answers (2)

Simon G.
Simon G.

Reputation: 6707

Strange. It compiles fine for me with Java 1.6.0_26, albeit with a warning.

The first problem is that your method signature should read either:

public <DO extends ServerDataObject> ProxyDataObject<DO> convertToProxy(
        Class<? extends ProxyDataObject<DO>> doClass
    ) throws Exception {
    if(getId()==0)return null;
    ProxyDataObject<DO> proxy = doClass.newInstance();
    proxy.setID(getId());
    return proxy;
}

or:

public <DO extends ProxyDataObject<?>> DO convertToProxy(Class<DO> doClass) throws Exception{
    if(getId()==0)return null;
    DO proxy = doClass.newInstance();
    proxy.setID(getId());
    return proxy;
}

depending on what you actually meant.

Second, DO extends ServerDataObject, not ProxyDataObject. With the inheritance you have defined it is possible for an instance of ServerDataObject to not be a ProxyDataObject. Therefore you get the error you are seeing.

Upvotes: 0

newacct
newacct

Reputation: 122429

doClass is type Class<DO>, so doClass.newInstance() has type DO. How can you possibly cast this to ProxyDataObject<DO>?

Upvotes: 1

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