instanceof confusion

[Mark Henryson]

clas  A {}
class B extends A {}
class C extends B {}
class D extends A {}
public class Inst
{
  static A a1,a2,a3;
  public static void main(String args[])
  {
    if((a1 = new D()) instanceOf C && (a2 = new B()) instanceof A)
      System.out.print("1");           
    if(a2 instanceof B & (a3 = new C()) instanceof A)
      System.out.print("2"); 
  }
} 

The above coding is very confusing to me. No problem in the operators used in the above program. I know the precedence and how the &&,&,|| and | will work.

All my confusion is in instanceof thing!!
What the below things are exactly mean?

1. (a1 = new D()) instanceOf C
2. a2 = new B()) instanceof A
3. a2 instanceof B
4. a3 = new C()) instanceof A

[marc weber]

    A
   / \
  B   D
 /
C

So,
any instance of B is also an instance of A, and
any instance of D is also an instance of A, and
any instance of C is also an instance of B, and
any instance of C is also an instance of A.

According to the Java Language Specification, "the result of the instanceof operator is true if the value of the ... [first operand, which must be a reference] is not null and the reference could be cast (�15.16) to the ... [second operand, which must be a type] without raising a ClassCastException. Otherwise the result is false." (The bold is mine.)

"a1 = new D()" creates a new instance of type D, and assigns it to a variable of type A (an upcast). So, "(a1 = new D()) instanceof C" returns false, because the true runtime type of a1 is D, and an instance of D is not an instance of C. In particular, a reference to an instance of D cannot be cast to type C without a ClassCastException.

Now, here's the trick: Because this evaluates to false, the second boolean is not evaluated due to short-circuiting of the && operator. In particular, "a2 = new B()" does not execute, and a2 remains a null reference. Therefore, "a2 instance of B" actually returns false.


Ref: http://java.sun.com/docs/books/jls/second_edition/html/expressions.doc.html#80289
[ August 23, 2005: Message edited by: marc weber ]

[Rick O'Shay]

There are situations where you have a general type and need something more specific. Consider a method that counts ten-speed bikes but it gets a list containing all different makes and models.

int count = 0;
for( Bike bike : bikes ) {
   if( bike instanceof com.acme.TenSpeed ) {
      count++; 
   }
 }

There is no "type" attribute in Bike so you are stuck testing the type using instanceof. Any TenSpeed or any bike that inherits TenSpeed will be counted. If a Buzzard inherits from Bird then Buzzard IS-A bird.

http://java.sun.com/docs/books/tutorial/java/concepts/inheritance.html]http://java.sun.com/docs/books/tutorial/java/concepts/inheritance.html]http://java.sun.com/docs/books/tutorial/java/concepts/inheritance.html
[ August 22, 2005: Message edited by: Rick O'Shay ]