Effective Java

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CHAPTER 8
METHODS
250
This method throws an 
IllegalArgumentException
if the given collection is
empty. We mentioned in Item 30 that a better alternative would be to return
Optional
. Here’s how the method looks when it is modified to do so:
// Returns maximum value in collection as an Optional
public static >
Optional
max(Collection c) {
if (c.isEmpty())
return 
Optional.empty()
;
E result = null;
for (E e : c)
if (result == null || e.compareTo(result) > 0)
result = Objects.requireNonNull(e);
return 
Optional.of(result)
;
} 
As you can see, it is straightforward to return an optional. All you have to do
is to create the optional with the appropriate static factory. In this program, we use
two: 
Optional.empty()
returns an empty optional, and 
Optional.of(value)
returns an optional containing the given non-null value. It is a programming error
to pass 
null
to 
Optional.of(value)
. If you do this, the method responds by
throwing a 
NullPointerException
. The 
Optional.ofNullable(value)
method
accepts a possibly null value and returns an empty optional if 
null
is passed in.
Never return a null value from an 
Optional
-returning method:
it defeats the
entire purpose of the facility.
Many terminal operations on streams return optionals. If we rewrite the 
max
method to use a stream, 
Stream
’s 
max
operation does the work of generating an
optional for us (though we do have to pass in an explicit comparator):