Thrown when an object is an instance of an unexpected type (a class or interface). This exception supplements the standard IllegalArgumentException by providing a more semantically rich description of the problem.

IllegalClassException represents the case where a method takes in a genericly typed parameter like Object (typically because it has to due to some other interface it implements), but this implementation only actually accepts a specific type, for example String. This exception would be used in place of IllegalArgumentException, yet it still extends it.

 public void foo(Object obj) {
   if (obj instanceof String == false) {
     throw new IllegalClassException(String.class, obj);
   }
   // do something with the string
 }