Usage of instanceof in a generic method

Question

I started to learn generics today, but this is somelike weird for me:

I have a generic method:

  public<T> HashMap<String, T> getAllEntitySameType(T type) {

        System.out.println(type.getClass());
        HashMap<String, T> result = null;

        if(type instanceof Project)
        {
            System.out.println(type.toString());
            System.out.println("Yes, instance of Project;");
        }

        if(type instanceof String)
        {
            System.out.println(type.toString());
            System.out.println("Yes, instance of String;");
        }
        this.getProjects();
        return result;
    }

And i can easily determinate the class of the T type

    Project<Double> project = new Project<Double>();
    company2.getAllEntitySameType(project);
    company2.getAllEntitySameType("TestString");

The output will be:

class Project
Yes, instance of Project;
class java.lang.String
TestString
Yes, instance of String;

I thought in generics we can't use instance of. Something is not complete in my knowledge. Thanks...

@PeterLawrey: That should be an answer, IMO, because that's the core of the OP's confusion. — Oliver Charlesworth
– Oliver Charlesworth, Commented Jan 8, 2013 at 16:52
so as it seems, i can not use exactly the opposite of what i've used? — czupe
– czupe, Commented Jan 8, 2013 at 16:52
You can do System.out.println(type); so you don't even know if it is null or not. — Peter Lawrey
– Peter Lawrey, Commented Jan 8, 2013 at 16:54
thanks, i can really use everything. So the thing is i can determine the arguments and i can't determine the parameters... Thanks! Wold you mind doing an answer? and i can accept it! Thanks! — czupe
– czupe, Commented Jan 8, 2013 at 17:05

Community · Accepted Answer · 2017-05-23 10:29:39Z

6

You can use instanceof to check the raw type of an object, for example Project:

if (type instanceof Project)

Or with proper generics syntax for a Project of some unknown type:

if (type instanceof Project<?>)

But you can't reify a parameterized type like Project<Double> with instanceof, due to type erasure:

if (type instanceof Project<Double>) //compile error

As Peter Lawrey pointed out, you also can't check against type variables:

if (type instanceof T) //compile error

edited May 23, 2017 at 10:29

CommunityBot

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answered Jan 8, 2013 at 16:42

Paul Bellora

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3 Comments

czupe Over a year ago

Anyway i make a mistake, because Project is a parameterized type ---> Project<Double> ... So now?:)

Paul Bellora Over a year ago

@czupe Edited to address that.

czupe Over a year ago

Thanks, now easier to understand. Voted up!

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