cast object to ArrayList<String>

Question

is it possible to cast an Object to e.g. 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);

    }

you say that, want to cast an Object obj to ArrayList<String> but in the last line u seem to cast to ArrayList<Document> — codeMan
– codeMan, Commented Oct 9, 2013 at 10:22
I wrote e.g. ArrayList<String>, but you are right, the type should be Document — padre
– padre, Commented Oct 9, 2013 at 10:24
Note that there will be a lot of "else if" and not else{ if(){}else{} } like it would be if you wrote this in python or something that cared about indentation and lines. I don't think the actual outcome will differ though. — spydon
– spydon, Commented Oct 9, 2013 at 10:37

Guillaume · Accepted Answer · 2014-04-22 11:45:28Z

10

In Java generics are not reified, i.e. 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.

edited Apr 22, 2014 at 11:45

answered Oct 9, 2013 at 10:25

Guillaume

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2 Comments

padre Over a year ago

I see ... so any suggetions how to re-write the method? Should I make the method generic as well, will then work?

Guillaume Over a year ago

As long as you have to cast you will have the issue. You can either ignore the compiler warning or directly call setDocs(ArrayList<Document>) when you have the correct type.

Thilo · Accepted Answer · 2013-10-09 10:22:17Z

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.

answered Oct 9, 2013 at 10:22

Thilo

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3 Comments

padre Over a year ago

but I know, if its from type ArrayList, then it will be ArrayList<Document>. The compiler is giving me warning on the next row, where I cast to ArrayList<Document>

Thilo Over a year ago

Best you can do is suppress the warning. And re-think if you really need this instanceof monster. In most programs, types should be known at compile time, without the need for this dynamic inspection.

padre Over a year ago

yes I know, but the code is very old and compiles with Java 1.4, so I am trying to update it to Java 1.7 and wanted to clean up some basic warning like this. Thanks any way

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