Java: Instanceof and Generics

Before I look through my generic data structure for a value's index, I'd like to see if it is even an instance of the type this has been parametrized to.

But Eclipse complains when I do this:

@Override
public int indexOf(Object arg0) {
	if (!(arg0 instanceof E)) {
		return -1;
	}

This is the error message:

> Cannot perform instanceof check against type parameter E. Use instead its erasure Object since generic type information will be erased at runtime

What is the better way to do it?

The error message says it all. At runtime, the type is gone, there is no way to check for it.

You could catch it by making a factory for your object like this:

 public static <T> MyObject<T> createMyObject(Class<T> type) {
    return new MyObject<T>(type);
 }

And then in the object's constructor store that type, so variable so that your method could look like this:

        if (arg0 != null && !(this.type.isAssignableFrom(arg0.getClass()))
        {
            return -1;
        }

Two options for runtime type checking with generics:

Option 1 - Corrupt your constructor

Let's assume you are overriding indexOf(...), and you want to check the type just for performance, to save yourself iterating the entire collection.

Make a filthy constructor like this:

public MyCollection<T>(Class<T> t) {

    this.t = t;
}

Then you can use isAssignableFrom to check the type.

public int indexOf(Object o) {
    
    if (
        o != null &&

        !t.isAssignableFrom(o.getClass())
    
    ) return -1;
    
//...

Each time you instantiate your object you would have to repeat yourself:

new MyCollection<Apples>(Apples.class);

You might decide it isn't worth it. In the implementation of ArrayList.indexOf(...), they do not check that the type matches.

Option 2 - Let it fail

If you need to use an abstract method that requires your unknown type, then all you really want is for the compiler to stop crying about instanceof. If you have a method like this:

protected abstract void abstractMethod(T element);

You can use it like this:

public int indexOf(Object o) {

    try {

        abstractMethod((T) o);

    } catch (ClassCastException e) {

//...

You are casting the object to T (your generic type), just to fool the compiler. Your cast does nothing at runtime, but you will still get a ClassCastException when you try to pass the wrong type of object into your abstract method.

NOTE 1: If you are doing additional unchecked casts in your abstract method, your ClassCastExceptions will get caught here. That could be good or bad, so think it through.

NOTE 2: You get a free null check when you use instanceof. Since you can't use it, you may need to check for null with your bare hands.

Old post, but a simple way to do generic instanceOf checking.

public static <T> boolean isInstanceOf(Class<T> clazz, Class<T> targetClass) {
    return clazz.isInstance(targetClass);
}

Provided your class extends a class with a generic parameter, you can also get this at runtime via reflection, and then use that for comparison, i.e.

class YourClass extends SomeOtherClass<String>
{

   private Class<?> clazz;

   public Class<?> getParameterizedClass()
   {
      if(clazz == null)
      {
         ParameterizedType pt = (ParameterizedType)this.getClass().getGenericSuperclass();
          clazz = (Class<?>)pt.getActualTypeArguments()[0];
       }
       return clazz;
    }
}

In the case above, at runtime you will get String.class from getParameterizedClass(), and it caches so you don't get any reflection overhead upon multiple checks. Note that you can get the other parameterized types by index from the ParameterizedType.getActualTypeArguments() method.

I had the same problem and here is my solution (very humble, @george: this time compiling AND working ...).

My probem was inside an abstract class that implements Observer. The Observable fires method update(...) with Object class that can be any kind of Object.

I only want to handler Objects of type T

The solution is to pass the class to the constructor in order to be able to compare types at runtime.

public abstract class AbstractOne<T> implements Observer {
    
  private Class<T> tClass;
    public AbstractOne(Class<T> clazz) {
    tClass = clazz;
  }

  @Override
  public void update(Observable o, Object arg) {
    if (tClass.isInstance(arg)) {
      // Here I am, arg has the type T
      foo((T) arg);
  	}
  }

  public abstract foo(T t);

}

For the implementation we just have to pass the Class to the constructor

public class OneImpl extends AbstractOne<Rule> {
  public OneImpl() {
    super(Rule.class);
  }

  @Override
  public void foo(Rule t){
  }
}

Or you could catch a failed attempt to cast into E eg.

public int indexOf(Object arg0){
  try{
    E test=(E)arg0;
    return doStuff(test);
  }catch(ClassCastException e){
    return -1;
  }
}

Technically you shouldn't have to, that's the point of generics, so you can do compile-type checking:

public int indexOf(E arg0) {
   ...
}

but then the @Override may be a problem if you have a class hierarchy. Otherwise see Yishai's answer.

The runtime type of the object is a relatively arbitrary condition to filter on. I suggest keeping such muckiness away from your collection. This is simply achieved by having your collection delegate to a filter passed in a construction.

public interface FilterObject {
     boolean isAllowed(Object obj);
}

public class FilterOptimizedList<E> implements List<E> {
     private final FilterObject filter;
     ...
     public FilterOptimizedList(FilterObject filter) {
         if (filter == null) {
             throw NullPointerException();
         }
         this.filter = filter;
     }
     ...
     public int indexOf(Object obj) {
         if (!filter.isAllows(obj)) {
              return -1;
         }
         ...
     }
     ...
}

     final List<String> longStrs = new FilterOptimizedList<String>(
         new FilterObject() { public boolean isAllowed(Object obj) {
             if (obj == null) {
                 return true;
             } else if (obj instanceof String) {
                 String str = (String)str;
                 return str.length() > = 4;
             } else {
                 return false;
             }
         }}
     );

Let Java determine it and catch the exception bottom line.

public class Behaviour<T> {
    public void behave(Object object) {
        T typedObject = null;
        
        try { typedObject = (T) object; }
        catch (ClassCastException ignored) {}
        
        if (null != typedObject) {
            // Do something type-safe with typedObject
        }
    }
}