simple Java question

[Bluga]

Given:

Integer i = new Integer (42);
Long 1 = new Long (42);
Double d = new Double (42.0);
Which two expressions evaluate to True? (Choose Two)
A. (i ==1)
B. (i == d)
C. (d == 1)
D. (i.equals (d))
E. (d.equals (i))
F. (i.equals (42))

Answer: D, E??

can anyone explanis this? Thanks!

 

[m0ti]

Ok, here goes:

The first 3 (A,B, and C) most definitely aren't correct.

Object x == Object y iff x IS y! (can't think of any exceptions to this). It doesn't matter if x has the same "value" as y.

Note that for primitive types (int, long, double, etc.) int i = 10, long j = 10 and i==j is true.

F is an illegal statement since .equals() receives an object, and 42 is a primitive type.

D and E are correct!

Use .equals() whenever you want to compare between the values of two objects!

Note, that there are some cases where .equals() is the same as ==, and you might want to override .equals() (or look for alternative methods). For example if you have an array int a[], and another array (remember, arrays ARE objects) int b[], which are of the same length and have the same contents, a.equals(b) will still return false since a and b aren't the same object. In general, this is true of container classes (objects which you use to store other objects). For arrays there exists java.util.Arrays which provides methods to compare equality between two arrays, to sort them, and more.

 

[Bluga]

Thanks a lot m0ti!

 

[MSantiago]

m0ti did a great job of explaining things. 🙂 There's just one thing I thought I'd point out in the example you posted:

Originally posted by: Bluga

Long 1 = new Long (42);

I think you meant to type in "Long l" and not "Long 1". Variables names can't be a number (or start with a number for that matter).