instanceof

[Mark Henryson]

Can anyone explain this program, especially the instanceof thing in the program.

class A
{
 
}
 
class B extends A
{
 
}
 
class C extends B
{
 
}
 
class D extends A
{
 
}
 
public class Q19
{
  static A a1;
  static A a2;
  static A a3;
 
  public static void main(String args[])
  {
     if((a1 = new D()) instanceof C && (a2 = new B()) instanceof A)
         System.out.println("1 ");
     if(a2 instanceof B & (a3 = new C()) instanceof A)
         System.out.println("2 ");
     if(a3 instanceof B | (a2=new D()) instanceof A)
        System.out.println("3 ");
     if(a2 instanceof C || (a2 = new D()) instanceof C)
        System.out.println("4 ");
   }
}  
       

a. 1 2
b. 3 4
c. 1 3 4
d. 3
e. Compilation error.

The answer is 3 (d). Please explain how???

[Stephen O'Kane]

the first if statement is false, as al is an object of class D, which is not an instance of C. The part after && is not evaluated

the second if statment is false, the first part gives a value of first, but since & is used, the second part of the if condition is evaluated, which means a3 is an object of class C

the third if is true, as the first part (before |) is true, but the second part is also evaluated (a2 = new D()) again because it is a single |.

the fourth if is false, as a2 is not an instance of C (from third if construct), and class D does noot extend C

Sok

[sameer inamdar]

what is difference betwee & and &&
| and || can you tell some details.

[Stephen O'Kane]

&& and || are known as short circuit expressions. using the example above

if((a1 = new D()) instanceof C && (a2 = new B()) instanceof A)

if the first part before && evaluates to false, then the second is not executed. whereas for

if(a2 instanceof B & (a3 = new C()) instanceof A)

it doesn't matter what the first part evaluates to, the second part must also be evaluated, and then the result of both expressions are used with &

the same goes for | and ||, except that if the first part of a || expression must be true so that the second part is not evaluated.

Sok

[Edwin Dalorzo]

The operator && evaluates the first operand, if it is false does not evaluate the second operand. On the other hand & evaluates both operands whether the first one is false or not.

The operator || evaluates the first operand, if it is true does not evaluate the second operand. On the other hand | evaluates both operands whether the first one is true or not.