Effective Java

with an 
@SuppressWarnings("unchecked")
 annotation.
If you suppress warn-
ings without first proving that the code is typesafe, you are giving yourself a false
sense of security. The code may compile without emitting any warnings, but it can
still throw a 
ClassCastException
at runtime. If, however, you ignore unchecked
warnings that you know to be safe (instead of suppressing them), you won’t notice
when a new warning crops up that represents a real problem. The new warning
will get lost amidst all the false alarms that you didn’t silence.

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CHAPTER 5
GENERICS
124
The 
SuppressWarnings
annotation can be used on any declaration, from an
individual local variable declaration to an entire class. 
Always use the
SuppressWarnings
 annotation on the smallest scope possible.
Typically this
will be a variable declaration or a very short method or constructor. Never use
SuppressWarnings
on an entire class. Doing so could mask critical warnings.
If you find yourself using the 
SuppressWarnings
annotation on a method or
constructor that’s more than one line long, you may be able to move it onto a local
variable declaration. You may have to declare a new local variable, but it’s worth
it. For example, consider this 
toArray
method, which comes from 
ArrayList
:
public  T[] toArray(T[] a) {
if (a.length < size)
return (T[]) Arrays.copyOf(elements, size, a.getClass());
System.arraycopy(elements, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
If you compile 
ArrayList
, the method generates this warning:
ArrayList.java:305: warning: [unchecked] unchecked cast
return (T[]) Arrays.copyOf(elements, size, a.getClass());
^
required: T[]
found: Object[]
It is illegal to put a 
SuppressWarnings
annotation on the return statement,
because it isn’t a declaration [JLS, 9.7]. You might be tempted to put the annota-
tion on the entire method, but don’t. Instead, declare a local variable to hold the
return value and annotate its declaration, like so: