Head First Java a brain-Friendly Guide Kathy Sierra and Bert Bates

this won’t work:
String num = “2”;
int x = 2;
if (x == num) // horrible explosion!
Trying to compile that makes the compiler laugh and mock you:
operator == cannot be applied to
int,java.lang.String 
if (x == num) { }
^
So to get around the whole apples and oranges thing, we have to make
the 
String
“2” into the 
int
2. Built into the Java class library is a class
called Integer (that’s right, an Integer 
class
, not the int 
primitive
), and
one of its jobs is to take Strings that 
represent
numbers and convert
them into 
actual
numbers.

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Casting primitives

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In 
Chapter 3
 we talked about the sizes of the various primitives, and how
you can’t shove a big thing directly into a small thing:
long y = 42;
int x = y; // won’t compile
A 
long
is bigger than an 
int
and the compiler can’t be sure where that 
long
has been. It might have been out drinking with the other longs, and taking
on really big values. To force the compiler to jam the value of a bigger
primitive variable into a smaller one, you can use the 
cast
operator. It looks
like this:
long y = 42; // so far so good
int x = (int) y; // x = 42 cool!
Putting in the cast tells the compiler to take the value of y, chop it down to
int size, and set x equal to whatever is left. If the value of y was bigger than
the maximum value of x, then what’s left will be a weird (but calculable*)
number:
long y = 40002;