Java Generics: Comparing the class of Object o to <E>

Question

Let's say I have the following class:

public class Test<E> {     public boolean sameClassAs(Object o) {         // TODO help!     } } 

How would I check that o is the same class as E?

Test<String> test = new Test<String>(); test.sameClassAs("a string"); // returns true; test.sameClassAs(4); // returns false; 

I can't change the method signature from (Object o) as I'm overridding a superclass and so don't get to choose my method signature.

I would also rather not go down the road of attempting a cast and then catching the resulting exception if it fails.

Answer 1

An instance of Test has no information as to what E is at runtime. So, you need to pass a Class<E> to the constructor of Test.

public class Test<E> {     private final Class<E> clazz;     public Test(Class<E> clazz) {         if (clazz == null) {             throw new NullPointerException();         }         this.clazz = clazz;     }     // To make things easier on clients:     public static <T> Test<T> create(Class<T> clazz) {         return new Test<T>(clazz);     }     public boolean sameClassAs(Object o) {         return o != null && o.getClass() == clazz;     } } 

If you want an "instanceof" relationship, use Class.isAssignableFrom instead of the Class comparison. Note, E will need to be a non-generic type, for the same reason Test needs the Class object.

For examples in the Java API, see java.util.Collections.checkedSet and similar.

Answer 2

The method I've always used is below. It is a pain and a bit ugly, but I haven't found a better one. You have to pass the class type through on construction, as when Generics are compiled class information is lost.

public class Test<E> {     private Class<E> clazz;     public Test(Class<E> clazz) {        this.clazz = clazz;     }     public boolean sameClassAs(Object o) {         return this.clazz.isInstance(o);     } }