I couldn't find documentation on an equivalent of Java's final in Python, is there such a thing?
I'm creating a snapshot of an object (used for restoration if anything fails); once this backup variable is assigned, it should not be modified -- a final-like feature in Python would be nice for this.
There is no final equivalent in Python. To create read-only fields of class instances, you can use the property function, or you could do something like this:
class WriteOnceReadWhenever:
def __setattr__(self, attr, value):
if hasattr(self, attr):
raise Exception("Attempting to alter read-only value")
self.__dict__[attr] = value
Also note that while there's #typing.final as of Python 3.8 (as Cerno mentions), that will not actually make values final at runtime.
Having a variable in Java be final basically means that once you assign to a variable, you may not reassign that variable to point to another object. It actually doesn't mean that the object can't be modified. For example, the following Java code works perfectly well:
public final List<String> messages = new LinkedList<String>();
public void addMessage()
{
messages.add("Hello World!"); // this mutates the messages list
}
but the following wouldn't even compile:
public final List<String> messages = new LinkedList<String>();
public void changeMessages()
{
messages = new ArrayList<String>(); // can't change a final variable
}
So your question is about whether final exists in Python. It does not.
However, Python does have immutable data structures. For example, while you can mutate a list, you can't mutate a tuple. You can mutate a set but not a frozenset, etc.
My advice would be to just not worry about enforcing non-mutation at the language level and simply concentrate on making sure that you don't write any code which mutates these objects after they're assigned.
Python 3.8 (via PEP 591) adds Final variables, functions, methods and classes. Here are some ways to use it:
#final Decorator (classes, methods)
from typing import final
#final
class Base:
# Cannot inherit from Base
class Base:
#final
def foo(self):
# Cannot override foo in subclass
Final annotation
from typing import Final
PI: Final[float] = 3.14159 # Cannot set PI to another value
KM_IN_MILES: Final = 0.621371 # Type annotation is optional
class Foo:
def __init__(self):
self.bar: Final = "baz" # Final instance attributes only allowed in __init__
Please note that like other typing hints, these do not prevent you from overriding the types, but they do help linters or IDEs warn you about incorrect type usage.
An assign-once variable is a design issue. You design your application in a way that the variable is set once and once only.
However, if you want run-time checking of your design, you can do it with a wrapper around the object.
class OnePingOnlyPleaseVassily(object):
def __init__(self):
self.value = None
def set(self, value):
if self.value is not None:
raise Exception("Already set.")
self.value = value
someStateMemo = OnePingOnlyPleaseVassily()
someStateMemo.set(aValue) # works
someStateMemo.set(aValue) # fails
That's clunky, but it will detect design problems at run time.
There is no such thing. In general, the Python attitude is "if you don't want this modified, just don't modify it". Clients of an API are unlikely to just poke around your undocumented internals anyway.
You could, I suppose, work around this by using tuples or namedtuples for the relevant bits of your model, which are inherently immutable. That still doesn't help with any part of your model that has to be mutable of course.
you can simulate something like that through the descriptor protocol, since it allows to define reading and setting a variable the way you wish.
class Foo(object):
#property
def myvar(self):
# return value here
#myvar.setter
def myvar(self, newvalue):
# do nothing if some condition is met
a = Foo()
print a.myvar
a.myvar = 5 # does nothing if you don't want to
As of 2019 and PEP 591, Python has a Final type. It won't be available in the standard library until the release of Python 3.8, but until then you can use it via the typing-extensions library. It won't work as final works in Java though as Python is still a dynamically typed language. But if you use it together with a static type checker like mypy it will give you very similar benefits.
There is also a final decorator that can be applied to mark class methods as final and preventing from being overridden. Again this is only checked at "compile-time", so you'd need to include a static type checker in your workflow.
Python has no equivalent of "final". It doesn't have "public" and "protected" either, except by naming convention. It's not that "bondage and discipline".
http://code.activestate.com/recipes/576527/ defines a freeze function, although it doesn't work perfectly.
I would consider just leaving it mutable though.
Python indeed does not have a final type, it does have immutable types such as tuples but that is something else.
Some of the other Answers here make classes full of pseudo final variables and I prefer my class to only have a few Final types, so I suggest using an descriptor to create the final type:
from typing import TypeVar, Generic, Type
T = TypeVar('T')
class FinalProperty(Generic[T]):
def __init__(self, value: T):
self.__value = value
def __get__(self, instance: Type, owner) -> T:
return self.__value
def __set__(self, instance: Type, value: T) -> None:
raise ValueError("Final types can't be set")
If you use this class like so:
class SomeJob:
FAILED = FinalProperty[str]("Failed")
Then you will not be able to set that variable in any instance of that class.
Unfortunately as with the WriteOnceReadWhenever answer you can still set the class variable.
job = SomeJob()
job.FAILED = "Error, this will trigger the ValueError"
SomeJob.FAILED = "However this still works and breaks the protection afterwards"
Although this is an old question, I figured I would add yet another potential option: You can also use assert to verify a variable is set to what you originally intended it to be set to – a double checking if you will. Although this is not the same as final in Java, it can be used to create a similar effect:
PI = 3.14
radius = 3
try:
assert PI == 3.14
print PI * radius**2
except AssertionError:
print "Yikes."
As seen above, if PI were for some reason not set to 3.14, an AssertionError would be thrown, so a try/except block would probably be a wise addition. Regardless, it may come in handy depending on your situation.
As of Python 3.8, Python does have a keyword "final". However, it is a hint and is not enforced at runtime.
Related
I have a method that will process a Collection<Nodes> that is passed in as a parameter. This Collection will be modified, therefore I thought it would be good to first make a copy of it. How do I name the parameter and local variable, e.g. nodes in the example below?
List<Nodes> process(Collection<Nodes> nodes) {
List<Nodes> nodes2 = new ArrayList<>(nodes);
...
}
As another example consider the following where the variable is an int parsed from a String parameter:
public void processUser(final String userId) {
final int userId2 = Integer.parseInt(userId);
...
A good approach to the name variables problem is to use names that suggest the actual meaning of the variable. In your example, you are using names that do not say anything about the method functionality or variables meaning, that's why it is hard to pick a name.
There are many cases like yours in the JDK, e.g. Arrays#copyOf:
public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
#SuppressWarnings("unchecked")
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
}
In this case they call the parameter original and the local variable copy which perfectly expresses that the returned value is a copy of the parameter. Precisely, copying is what this method does and it is named accordingly.
Using the same reasoning for your case (consider refactoring to give more meaningful names to your method and variables) I would name your local copy of nodes something like processedNodes, to express what that variable is and to be consistent with your method's name.
Edit:
The name of the new method you added in your edit does not provide hints about what it does either. I'll assume that it modifies some properties (maybe in a database) of the user whose id is passed via parameter.
If that is the case (or similar), I think that an appropriate approach you
could apply would be that every method should have a single responsibility. According to your method's name it should process the user, for that you need an int userId. The responsibility of parsing an String userId should be out of the scope of this method.
Using the proposed approach has, among others, the following advantages:
Your class won't change if you have to add additional validation to your input.
Your class won't be responsible for handling NumberFormatException which must be the application responsibility.
Your processUser method won't change if you have to handle different types of inputs (e.g. float userId).
It ultimately comes down to what you want to communicate to future programmers. The computer obviously doesn't care; it's other people you're talking to. So the biggest factor is going to be what those people need to know:
What is the logical (abstract, conceptual) meaning of this variable?
What aspects of how this variable is used could be confusing to programmers?
What are the most important things about this variable?
Looking at your first example, it's kind of hard to understand enough about your program to really choose a good name. The method is called process; but methods generally speaking implement computational processes, so this name really doesn't tell me anything at all. What are you processing? What is the process? Who are you processing it for, and why? Knowing what the method does, and the class it's in, will help to inform your variable name.
Let's add some assumptions. Let's say you're building an application that locates Wi-fi access points in a building. The Node in question is a wireless node, with subclasses Repeater, AccessPoint, and Client. Let's also say it's an online-processed dataset, so the collection of nodes given may change at any time in response to a background thread receiving updates in what nodes are currently visible. Your reason for copying the collection at the head of the method is to isolate yourself from those changes for the duration of local processing. Finally, let's assume that your method is sorting the nodes by ping time (explaining why the method takes a generic Collection but returns the more specific List type).
Now that we better understand your system, let's use that understanding to choose some names that communicate the logical intention of your system to future developers:
class NetworkScanner {
List<Node> sortByPingTime(Collection<Node> networkNodes) {
final ArrayList<Node> unsortedSnapshot;
synchronized(networkNodes) {
unsortedSnapshot = new ArrayList<>(networkNodes);
}
return Utils.sort(unsortedSnapshot, (x,y) -> x.ping < y.ping);
}
}
So the method is sortByPingTime to define what it does; the argument is networkNodes to describe what kind of node we're looking at. And the variable is called unsortedSnapshot to express two things about it that aren't visible just by reading the code:
It's a snapshot of something (implying that the original is somehow volatile); and
It has no order that matters to us (suggesting that it might have, by the time we're done with it).
We could put nodes in there, but that's immediately visible from the input argument. We could also call this snapshotToSort but that's visible in the fact that we hand it off to a sort routine immediately below.
This example remains kind of contrived. The method is really too short for the variable name to matter much. In real life I'd probably just call it out, because picking a good name would take longer than anyone will ever waste figuring out how this method works.
Other related notes:
Naming is inherently a bit subjective. My name will never work for everyone, especially when multiple human languages are taken into account.
I find that the best name is often no name at all. If I can get away with making something anonymous, I will--this minimizes the risk of the variable being reused, and reduces symbols in IDE 'find' boxes. Generally this also pushes me to write tighter, more functional code, which I view as a good thing.
Some people like to include the variable's type in its name; I've always found that a bit odd because the type is generally immediately obvious, and the compiler will usually catch me if I get it wrong anyway.
"Keep it Simple" is in full force here, as everywhere. Most of the time your variable name will not help someone avoid future work. My rule of thumb is, name it something dumb, and if I ever end up scratching my head about what something means, choose that occasion to name it something good.
I used to give names, which reflect and emphasize the major things. So a potential reader (including myself after a couple of months) can get immediately, what is done inside the method just by its signature.
The API in discussion receives an input , does some processing and returns the output. These are the three main things here.
If it is not important, what processing is done and what is the type of input, the most generic is this form:
List<Nodes> process(Collection<Nodes> input) {
List<Nodes> output = new ArrayList<>(input);
...
}
and
public void process(final String input) {
final int output = Integer.parseInt(input);
...
If it is important to provide more information about processing and type of an input, names like: processCollection, inputCollection and processUser, inputUserId are more appropriate, but the local variable is still the output - it is clear and self-explained name:
List<Nodes> processCollection(Collection<Nodes> inputCollection) {
List<Nodes> output = new ArrayList<>(inputCollection);
...
}
and
public void processUser(final String inputUserId) {
final int output = Integer.parseInt(inputUserId);
...
It depends on the use case and sometimes it is even more appropriate to elaborate the processing, which is done: asArray or asFilteredArray etc instead of processCollection.
Someone may prefer the source-destination terminology to the input-output - I do not see the major difference between them. If this serves telling the method story with its title, it is good enough.
It depends on what you are going to do with the local variable.
For example in the first example it seems that is likely that variable nodes2 will actually be the value returned in the end. My advice is then to simply call it result or output.
In the second example... is less clear what you may want to achieve... I guess that userIdAsInt should be fine for the local. However if an int is always expected here and you still want to keep the parameter as a String (Perhaps you want to push that validation out of the method) I think it is more appropriate to make the local variable userId and the parameter userIdAsString or userIdString which hints that String, although accepted here, is not the canonic representation of an userId which is an int.
For sure it depends on the actual context. I would not use approaches from other programming languages such as _ which is good for instance for naming bash scripts, IMO my is also not a good choice - it looks like a piece of code copied from tutorial (at least in Java).
The most simple solution is to name method parameter nodesParam or nodesBackup and then you can simply go with nodes as a copy or to be more specific you can call it nodesCopy.
Anyway, your method process has some tasks to do and maybe it is not the best place for making copies of the nodes list. You can make a copy in the place where you invoke the method, then you can simply use nodes as a name of your object:
List<Nodes> process(Collection<Nodes> nodes) {
// do amazing things here
// ...
}
// ...
process(new ArrayList<>(nodes))
// ...
Just my guess, you have got a collection and you want to keep the original version and modify the copy, maybe a real solution for you is to use java.util.stream.Stream.
Simply put, when naming the variable, I consider a few things.
How is the copy created? (Is it converted from one type to another?...)
What am I going to do with the variable?
Is the name short, but/and meaningful?
Considering the same examples you have provided in the question, I will name variables like this:
List<Nodes> process(Collection<Nodes> nodes) {
List<Nodes> nodesCopy = new ArrayList<>(nodes);
...
}
This is probably just a copy of the collection, hence the name nodesCopy. Meaningful and short. If you use nodesList, that can mean it is not just a Collection; but also a List (more specific).
public void processUser(final String userId) {
final int userIdInt = Integer.parseInt(userId);
...
The String userId is parsed and the result is an integer (int)! It is not just a copy. To emphasize this, I would name this as userIdInt.
It is better not to use an underscore _, because it often indicates instance variables. And the my prefix: not much of a meaning there, and it is nooby (local will do better).
When it comes to method parameter naming conventions, if the thing a method parameter represents will not be represented by any other variable, use a method parameter name that makes it very clear what that method parameter is in the context of the method body. For example, primaryTelephoneNumber may be an acceptable method parameter name in a JavaBean setter method.
If there are multiple representations of a thing in a method context (including method parameters and local variables), use names that make it clear to humans what that thing is and how it should be used. For example, providedPrimaryTelephoneNumber, requestedPrimaryTelephoneNumber, dirtyPrimaryTelephoneNumber might be used for the method parameter name and parsedPrimaryTelephoneNumber, cleanPrimaryTelephoneNumber, massagedPrimaryTelephoneNumber might be used for the local variable name in a method that persists a user-provided primary telephone number.
The main objective is to use names that make it clear to humans reading the source code today and tomorrow as to what things are. Avoid names like var1, var2, a, b, etc.; these names add extra effort and complexity in reading and understanding the source code.
Don't get too caught up in using long method parameter names or local variable names; the source code is for human readability and when the class is compiled method parameter names and local variable names are irrelevant to the machine.
Suppose I have a dumb Frege function that constructs a pair of Nums.
newPair :: (Num α, Num β) => α -> β -> (α, β)
newPair = (,)
-- alternatively -- newPair x y = (x, y)
Attempting to call this function from Java, however, a PreludeBase.CNum<α> and a PreludeBase.CNum<β> are demanded in addition to the expected Lazy<α> and Lazy<β>. Likewise with Show types, where
showSomething :: (Show α) => α -> String
showSomething = show
-- alternatively -- showSomething x = show x
would require a PreludeBase.CShow<α> in addition to the expected parameter.
What is the proper way to pass constrained Frege objects to and from Java?
Good question, since this is not explained in the wiki yet.
As in all cases like this, I recommend to use the
:java
command in the REPL. For example:
frege> newPair 1 2.3
frege> :java
You will then get a window that contains among all active definitions one that corresponds to this call. A simple text search can help find the place where newPair is called. This should help to resolve such issues most of the time.
In your case, the relevant part would look like:
Console.<Integer, Double>numPair(
PreludeBase.INum_Int.it,
PreludeBase.IReal_Double.it,
Thunk.<Integer>lazy(1),
Thunk.<Double>lazy(2.3))
Here is a short overwiew about how type classes and instances are named and how you can get at them.
module x.y.Z where
class Xable where ...
This results in a Java-interface with the fully qualified name
x.y.Z.CXable
And this:
module a.b.C where
import x.y.Z
data MyType ... = ....
instance Xable MyType where ...
results in some class
a.b.C.IXable_MyType /* implements CXable<TMyType> */
If your instance definition does not have constraints themselves, there will be a singleton instance that you can use.
a.b.C.IXable_MyType.it
Otherwise, you need to construct a new instance by passing all constraints as argument to the constructor. For example, the Show instance for
Maybe Int
would look something like this:
new IShow_Maybe(IShow_Int.it)
since the instance head lists a constraint for the Maybe element type:
instance Show a => Show (Maybe a)
Note that you need to know the actual type fully, you can't make a generic type class instance. This is never a problem in Frege itself, as all needed instances are passed to a polymorphic function from the caller. However, as it stands, we don't have constraints in native functions.
Should you need something like this, you can achieve the functionality in most cases by just passing the function you wanted to call as argument.
For example, this doesn't work:
pure native myMethod :: Show a => a -> ...
but this should:
pure native myMethod :: (a -> String) -> a -> ....
myMethod show (Just 47)
The example java code above also reveals that it is not always as easy as described. For example, it so happens that the Double type doesn't have a separate Num instance, but just one for Real which is a subclass of Num. Unfortunately, only the compiler has the knowledge of what instances are actually present for some type, and which ones are implicit, that is, provided by an instance for a sub-class. Again, the REPL is the best way to find this out.
I vaguely remember from my days of Minecraft modding that in java, you can set multiple class variables in one line, like this:
SomeBlock.hardness(0.5).isOpaque(True).blastResistance(2).etc.etc
Well I was wondering if there was a way to do something similar in python. I have tried doing the same thing in python, but the second one ends up thinking it is supposed to modify the return of the first one instead of the original class.
Is there a way to do this?
Yes, you can, in fact it is done the same way it is done in Java: by returning the self reference.
class SomeBlock(object):
def hardness(self, blah):
self.hardness = blah
return self
def isOpaque(self, value):
self.isOpaque = value
return self
a = SomeBlock()
a.hardness(0.5).isOpaque(true)
This is just a fluent interface, which is desgined to be like that. It is mostly created by returning the same object:
class Test:
def setX(self, x):
self.x = x
return self
def setY(self, x):
self.y = y
return self
t = Test()
t.setX(12).setY(11)
the second one ends up thinking it is supposed to modify the return of the first one instead of the original class. Yes that is the normal Python syntax. Returning self does not create a special syntax. It just ensures the returned object is the correct one for chaining.
Your initialization task is more commonly coded as
class SomeBlock():
def __init__(self, hardness=None, opaque=False, blastResistance=None):
self.hardness = hardness
self.opaque = opaque
self.blastResistance = blastResistance
ablock = SomeBlock(hardness=0.5, opaque=True, blastResistance=2)
And because Python does not require methods for accessing attributes, you can also write
ablock = SomeBlock()
ablock.hardness = 0.5
...
So if a method does return self, it's usually to implement a more complex that setting attributes.
It's more common to have method return a new object of the same class. This is typical, for example, of many of the numpy array methods. A.dot(B) returns new array, the result of taking the dot product of A and B. A.dot(B).dot(C) may look like chaining, but is just a sequence of actions on successive arrays.
Is there any such equivalent of Java
String myMethod (MyClass argument) {...}
in Python?
Thank you, Tomas
No. (And more stuff to round this up to 15 characters...)
No, there is not.
In fact, checking types is considered "un-Pythonic", because an object of any type that looks enough like the expected type should be treated equally.
Python 3.x has function annotations where you can declare argument and return types:
def myMethod(argument: MyClass) -> str:
...
But currently Python does nothing with them, they serve as documentation only.
I just want to say that I'm in full agreement that type checking is evil. But python is also incredibly flexible and I'm in the mood to be evil. This code will take effect at runtime and not compile time. You could do something similar for return type. Something like this could be useful for debugging and, because it's a decorator, it's easy enough to remove.
For it to be useful for debugging you would have to have a situation where two types had all the same attributes that were getting accessed but with different semantics. So that's a pretty limited case. Other than that, you're about to get a typerror anyways when this code runs. The good news is that this is almost never a problem. I really don't know why people from statically typed languages make such a big deal over it.
def types(*args, **kwargs):
arg_types = args
kwarg_types = kwargs
def decorator(f):
def func(*args, **kwargs):
for arg, arg_type in zip(args, arg_types):
if not isinstance(arg, arg_type):
raise TypeError("Wrong type suckah")
for kw, arg in kwargs.items():
if not isinstance(arg, kwarg_types[kw]):
raise TypeError("this is a bad error message")
return f(*args, **kwargs)
return func
return decorator
#types(int, str, bool, flag=bool)
def demo(i, strng, flag=False):
print i, strng, flag
demo(1, "foo", True)
try:
demo("foo", "bar", flag="foobar")
except TypeError:
print "busted on posargs"
try:
demo(1, "foo", flag=2)
except TypeError:
print "busted on keyargs"
try:
demo(1, "foo", 3)
except TypeError:
print "no use sneaking it through"
No.
In Python, it's the program's
responsibility to use built-in
functions like isinstance() and
issubclass() to test variable types
and correct usage. Python tries to
stay out of your way while giving you
all you need to implement strong type
checking.
from Why is Python a dynamic language and also a strongly typed language. Also
In a dynamically typed language, a
variable is simply a value bound to a
name; the value has a type -- like
"integer" or "string" or "list" -- but
the variable itself doesn't. You could
have a variable which, right now,
holds a number, and later assign a
string to it if you need it to change.
Further, isinstance() and issubclass() can be used to do type-checking. If you want to make sure that argument is of MyClass type, you can have a check inside the function. You can even type-cast the value of the argument (if you have a constructor accepting such value) and assign it to my_object.
One use of the var keyword in C# is implicit type declaration. What is the Java equivalent syntax for var?
There is none. Alas, you have to type out the full type name.
Edit: 7 years after being posted, type inference for local variables (with var) was added in Java 10.
Edit: 6 years after being posted, to collect some of the comments from below:
The reason C# has the var keyword is because it's possible to have Types that have no name in .NET. Eg:
var myData = new { a = 1, b = "2" };
In this case, it would be impossible to give a proper type to myData. 6 years ago, this was impossible in Java (all Types had names, even if they were extremely verbose and unweildy). I do not know if this has changed in the mean time.
var is not the same as dynamic. variables are still 100% statically typed. This will not compile:
var myString = "foo";
myString = 3;
var is also useful when the type is obvious from context. For example:
var currentUser = User.GetCurrent();
I can say that in any code that I am responsible for, currentUser has a User or derived class in it. Obviously, if your implementation of User.GetCurrent return an int, then maybe this is a detriment to you.
This has nothing to do with var, but if you have weird inheritance hierarchies where you shadow methods with other methods (eg new public void DoAThing()), don't forget that non-virtual methods are affected by the Type they are cast as.
I can't imagine a real world scenario where this is indicative of good design, but this may not work as you expect:
class Foo {
public void Non() {}
public virtual void Virt() {}
}
class Bar : Foo {
public new void Non() {}
public override void Virt() {}
}
class Baz {
public static Foo GetFoo() {
return new Bar();
}
}
var foo = Baz.GetFoo();
foo.Non(); // <- Foo.Non, not Bar.Non
foo.Virt(); // <- Bar.Virt
var bar = (Bar)foo;
bar.Non(); // <- Bar.Non, not Foo.Non
bar.Virt(); // <- Still Bar.Virt
As indicated, virtual methods are not affected by this.
No, there is no non-clumsy way to initialize a var without an actual variable.
var foo1 = "bar"; //good
var foo2; //bad, what type?
var foo3 = null; //bad, null doesn't have a type
var foo4 = default(var); //what?
var foo5 = (object)null; //legal, but go home, you're drunk
In this case, just do it the old fashioned way:
object foo6;
If you add Lombok to your project you can use its val keyword.
http://projectlombok.org/features/val.html
JEP - JDK Enhancement-Proposal
http://openjdk.java.net/jeps/286
JEP 286: Local-Variable Type Inference
Author Brian Goetz
// Goals:
var list = new ArrayList<String>(); // infers ArrayList<String>
var stream = list.stream(); // infers Stream<String>
With the release of JDK 10 on March 20, Java now includes a var reserved type name (not a keyword—see below) as specified in JEP 286. For local variables, the following is now valid in Java 10 or higher:
var map = new HashMap<String, Integer>();
The var reserved type name in Java is nearly identical to the var keyword in C# in that both allow for implicit typing (see below for important differences). var in Java can only be used for implicit type inference in the following contexts (as enumerated in JEP 286: Goals):
local variables with initializers
indexes in the enhanced for-loop
locals declared in a traditional for-loop
Therefore var cannot be used for fields, return types, class names, or interface names. Its rationale is to remove the need for including long type names when declaring and defining local variables, as stated in JEP 286 (authored by Brian Goetz):
We seek to improve the developer experience by reducing the ceremony
associated with writing Java code, while maintaining Java's commitment
to static type safety, by allowing developers to elide the
often-unnecessary manifest declaration of local variable types.
var Scoping in Java
It should be noted that var is not a keyword in Java, but rather a reserved type name. As quoted from JEP 286:
The identifier var is not a keyword; instead it is a reserved type
name. This means that code that uses var as a variable, method, or
package name will not be affected; code that uses var as a class or
interface name will be affected (but these names are rare in practice,
since they violate usual naming conventions).
Note that since var is a reserved type name and not a keyword, it can still be used for package names, method names, and variable names (along with its new type-interference role). For example, the following are all examples of valid uses of var in Java:
var i = 0;
var var = 1;
for (var i = 0; i < 10; i++) { /* ... */ }
public int var() { return 0; }
package var;
As quoted from JEP 286:
This treatment would be restricted to local variables with
initializers, indexes in the enhanced for-loop, and locals declared in
a traditional for-loop; it would not be available for method formals,
constructor formals, method return types, fields, catch formals, or
any other kind of variable declaration.
Differences Between var in Java & C
This is one notable difference between var in C# and Java include the following: var can be used as a type name in C# but cannot be used as a class name or interface name in Java. According to the C# documentation (Implicitly Typed Local Variables):
If a type named var is in scope, then the var keyword will resolve to
that type name and will not be treated as part of an implicitly typed
local variable declaration.
The ability to use var as a type name in C# creates some complexity and introduces some intricate resolution rules, which are avoided by var in Java by disallowing var as a class or interface name. For information on the complexities of var type names in C#, see Restrictions apply to implicitly-typed variable declarations. For more information on the rationale behind the scoping decision for `var in Java, see JEP 286: Scoping Choices.
I have cooked up a plugin for IntelliJ that – in a way – gives you var in Java. It's a hack, so the usual disclaimers apply, but if you use IntelliJ for your Java development and want to try it out, it's at https://bitbucket.org/balpha/varsity.
It will be supported in JDK 10. It's even possible to see it in action in the early access build.
The JEP 286:
Enhance the Java Language to extend type inference to declarations of local variables with initializers.
So now instead of writing:
List<> list = new ArrayList<String>();
Stream<> stream = myStream();
You write:
var list = new ArrayList<String>();
var stream = myStream();
Notes:
var is now a reserved type name
Java is still commitment to static typing!
It can be only used in local variable declarations
If you want to give it a try without installing Java on your local system, I created a Docker image with JDK 10 installed on it:
$ docker run -it marounbassam/ubuntu-java10 bash
root#299d86f1c39a:/# jdk-10/bin/jshell
Mar 30, 2018 9:07:07 PM java.util.prefs.FileSystemPreferences$1 run
INFO: Created user preferences directory.
| Welcome to JShell -- Version 10
| For an introduction type: /help intro
jshell> var list = new ArrayList<String>();
list ==> []
A simple solution (assuming you're using a decent IDE) is to just type 'int' everywhere and then get it to set the type for you.
I actually just added a class called 'var' so I don't have to type something different.
The code is still too verbose, but at least you don't have to type it!
As of Java 10, the equivalent is ... var.
You can take a look to Kotlin by JetBrains, but it's val. not var.
Java 10 did get local variable type inference, so now it has var which is pretty much equivalent to the C# one (so far as I am aware).
It can also infer non-denotable types (types which couldn't be named in that place by the programmer; though which types are non-denotable is different). See e.g. Tricks with var and anonymous classes (that you should never use at work).
The one difference I could find is that in C#,
If a type named var is in scope, then the var keyword will resolve to that type name and will not be treated as part of an implicitly typed local variable declaration.
In Java 10 var is not a legal type name.
I know this is older but why not create a var class and create constructors with different types and depending on what constructors gets invoked you get var with different type. You could even build in methods to convert one type to another.
Lombok supports var but it's still classified as experimental:
import lombok.experimental.var;
var number = 1; // Inferred type: int
number = 2; // Legal reassign since var is not final
number = "Hi"; // Compilation error since a string cannot be assigned to an int variable
System.out.println(number);
Here is a pitfall to avoid when trying to use it in IntelliJ IDEA. It appears to work as expected though including auto completion and everything. Until there is a "non-hacky" solution (e.g. due to JEP 286: Local-Variable Type Inference), this might be your best bet right now.
Note that val is support by Lombok as well without modifying or creating a lombok.config.
You can, in Java 10, but only for Local variables, meaning,
You can,
var anum = 10; var aString = "Var";
But can't,
var anull = null; // Since the type can't be inferred in this case
Check out the spec for more info.
In general you can use Object class for any type, but you have do type casting later!
eg:-
Object object = 12;
Object object1 = "Aditya";
Object object2 = 12.12;
System.out.println(Integer.parseInt(object.toString()) + 2);
System.out.println(object1.toString() + " Kumar");
System.out.println(Double.parseDouble(object2.toString()) + 2.12);
This feature is now available in Java SE 10. The static, type-safe var has finally made it into the java world :)
source: https://www.oracle.com/corporate/pressrelease/Java-10-032018.html