equals and ==

Question 1

Consider the following lines of code:

Integer i = new Integer(42); 
Long ln = new Long(42); 
Double d = new Double(42.0);  

Which of the following options are valid code fragments?

i == ln;
ln == d;
i.equals(d);
d.equals(ln);
ln.equals(42);

Answer 1

i. equals(d);
d. equals(ln);
ln. equals(42);

Due to auto-boxing int 42 is converted into an Integer object containing 42. So this is valid. It will return false though because ln is a Long and 42 is boxed into an Integer.

The concept to understand here is as follows -
If the compiler can figure out that something can NEVER happen, then it flags an error.

In this question, the compiler knows that ln, i or d can never point to the same object in any case because they are references to different classes of objects that have no relation ( superclass/subclass ) between themselves.