将对象强制转换为ArrayList <String>

Question

is it possible to cast an Object to eg ArrayList<String>

the code below gives an example of the problem. The Problem is in the last row

setDocs((ArrayList<Document>)obj);

where I want to cast an Object obj to ArrayList<String>

public void setValue(Object obj)
    {
        if(obj instanceof TFile)
            setTFile((TFile)obj);
        else
            if(obj instanceof File)
                setFile((File)obj));
            else
                if(obj instanceof Document)
                    setDoc((Document)obj);
                else
                    if(obj instanceof ArrayList)
                        setDocs((ArrayList<Document>)obj);

    }

Answer 1

In Java generics are not reified, ie their generic type is not used when casting.

So this code

setDocs((ArrayList<Document>)obj);

will be executed as

setDocs((ArrayList)obj);

As that runtime cast won't check your ArrayList contains Document objects, the compiler raises a warning.

Answer 2

No, that is not possible due to how generics are implemented in Java.

The type information is not available at runtime, so it cannot be checked by instanceof .

What you can do is cast to List and then check each element if it is a Document or not.