The name reflection is used to describe code which is able to inspect other code in the same system (or itself).
For example, say you have an object of an unknown type in Java, and you would like to call a 'doSomething' method on it if one exists. Java's static typing system isn't really designed to support this unless the object conforms to a known interface, but using reflection, your code can look at the object and find out if it has a method called 'doSomething' and then call it if you want to.
So, to give you a code example of this in Java (imagine the object in question is foo) :
Method method = foo.getClass().getMethod("doSomething", null);
method.invoke(foo, null);
One very common use case in Java is the usage with annotations. JUnit 4, for example, will use reflection to look through your classes for methods tagged with the @Test annotation, and will then call them when running the unit test.
There are some good reflection examples to get you started at http://docs.oracle.com/javase/tutorial/reflect/index.html
And finally, yes, the concepts are pretty much similar in other statically typed languages which support reflection (like C#). In dynamically typed languages, the use case described above is less necessary (since the compiler will allow any method to be called on any object, failing at runtime if it does not exist), but the second case of looking for methods which are marked or work in a certain way is still common.
Update from a comment:
The ability to inspect the code in the system and see object types is
not reflection, but rather Type Introspection. Reflection is then the
ability to make modifications at runtime by making use of
introspection. The distinction is necessary here as some languages
support introspection, but do not support reflection. One such example
is C++
There are several differences between HashMap
and Hashtable
in Java:
Hashtable
is synchronized, whereas HashMap
is not. This makes HashMap
better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
Hashtable
does not allow null
keys or values. HashMap
allows one null
key and any number of null
values.
One of HashMap's subclasses is LinkedHashMap
, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap
for a LinkedHashMap
. This wouldn't be as easy if you were using Hashtable
.
Since synchronization is not an issue for you, I'd recommend HashMap
. If synchronization becomes an issue, you may also look at ConcurrentHashMap
.
Best Answer
tl;dr: "PECS" is from the collection's point of view. If you are only pulling items from a generic collection, it is a producer and you should use
extends
; if you are only stuffing items in, it is a consumer and you should usesuper
. If you do both with the same collection, you shouldn't use eitherextends
orsuper
.Suppose you have a method that takes as its parameter a collection of things, but you want it to be more flexible than just accepting a
Collection<Thing>
.Case 1: You want to go through the collection and do things with each item.
Then the list is a producer, so you should use a
Collection<? extends Thing>
.The reasoning is that a
Collection<? extends Thing>
could hold any subtype ofThing
, and thus each element will behave as aThing
when you perform your operation. (You actually cannot add anything (except null) to aCollection<? extends Thing>
, because you cannot know at runtime which specific subtype ofThing
the collection holds.)Case 2: You want to add things to the collection.
Then the list is a consumer, so you should use a
Collection<? super Thing>
.The reasoning here is that unlike
Collection<? extends Thing>
,Collection<? super Thing>
can always hold aThing
no matter what the actual parameterized type is. Here you don't care what is already in the list as long as it will allow aThing
to be added; this is what? super Thing
guarantees.