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Jacob Macallan
Jacob Macallan

Reputation: 979

How to get the type of a value (Java)

The solutions I've seen online make sense; if you know the type of the variable, then you know the type of its value. Java makes it that way; however, if I have a system of inherited classes such as this ...

DynastyPQ (base class)
FirstPQ (inherited class)

And create the objects in this manner ...

DynastyPQ pq = new FirstPQ();

Is there a way to get the type of FirstPQ so that I can use it in a cast so that I can access the class's exclusive methods? Maybe something akin to this?

(typeof(pq's value)pq).exclusiveMethod()

Upvotes: 3

Views: 320

Answers (3)

CosmicGiant
CosmicGiant

Reputation: 6441

There are a few options:

  1. With the instanceof operator:

    • if (pq instanceof FirstPQ) {((FirstPQ)pq).exclusiveMethod();}

  2. With the Class.isInstance(Object obj) instance method:

    • FirstPQ firstPQ; if(pq.getClass().isInstance(firstPQ)) { firstPQ = (FirstPQ)pq; }
      (Note: Not yet tested. Confirmed.)
  3. With a visitor pattern:

Upvotes: 0

Vishal Vijayan
Vishal Vijayan

Reputation: 318

Try the getClass() method. This will return the run time class of the particular object. http://www.tutorialspoint.com/java/lang/object_getclass.htm

Upvotes: 1

Vince
Vince

Reputation: 15146

You have a few options.

  1. You could use reflection
  2. You could use instanceof
  3. You could use the visitor pattern

For these examples, we will attempt to find the type of this variable:

Object obj = new TargetType();

We want to see if the object referenced by obj is of type TargetType.

Reflection

There are a couple ways you could do this:

if(obj == TargetType.class) {
    //do something
}

The idea behind the code above is that getClass() returns a reference to the Class object used to instantiate that object. You can compare the reference.

if(TargetType.class.isInstance(obj)) {
    //do something
}

Class#isInstance checks to see if the object value passed to method is an instance of the class we are calling isInstance on. It will return false if obj null, so no null check is needed. This requires casting to perform operations on the object.

instanceof

This one is simple:

if(obj instanceof TargetType) {
    //do something
}

instanceof is part of the language specification. This returns false if obj is null. This requires casting to perform operations on the object.

Visitor Pattern

I have explained this in detail in one of my other answers. You would be in charge of handling null. You should look deeper into the pattern to see if it's right for your situation, as it could be an overkill. This does not require casting.

Upvotes: 1

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