ClassCastException

[ankur rathi]

public class Test
{
 
    public static void main( String[] args ) throws Exception
    {
        Object a = new My();
        Object b = new Object();
        System.out.println( "equal? " + a.equals( b ) );
    }
 
}
 
 
class My
{
    transient int x;
 
    public boolean equals( Object o )
    {
        My m = (My)o;
        if ( m.x == x )
        {
            return true;
        }
        return false;
    }
}

why this program is giving ClassCastException ...
thanks .

[Jim Yingst]

Look carefully at the ClassCastException. It tells you the line numbers in the file which are generating the exception. What is the first line in the stack trace? Which part of the program does it refer to?

[Edwin Dalorzo]

You receive a ClassCastException because you are trying to cast a Object to a "My" object reference.

Remember, you can do upcasting implicitly. That is to say, you can assign any object of a subclass to an object of its parent class. The contrary is not always true.

You can do downcasting explicitly by means of the cast operator, however the object has to be assignable to the reference variable receiving the casted reference.

By assignable I mean this casted object has to be of the same class or a subclass of the variable receiving the casted reference.

That is why your code failing at runtime:

public class Test
{
 
    public static void main( String[] args ) throws Exception
    {
        Object a = new My();
        Object b = new Object();
        //You are passing a Object as parameter
        System.out.println( "equal? " + a.equals( b ) );
    }
 
}
 
 
class My
{
    transient int x;
 
    public boolean equals( Object o )
    {
        //You are trying to downcast a Object to a My reference
        //That is no possible, "o" must be of type My or a subclass of My.
        My m = (My)o;
        if ( m.x == x )
        {
            return true;
        }
        return false;
    }
}

Hope it helps.