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Does Java have "properties" that work the same way properties work in C#?
(5 answers)
Closed 9 years ago.
I understand that this question is similar to others asking about Java Properties, and the answers in those questions seem to be "no, you have to use getters/setters".
I searched for C#-like Java properties, but have only found one "hit" so far. And I'm not sure if it is a property or something else.
Reading the Java Tutorial, I came across some text and code that says (** emphasis and comment mine):
Finally, you can use the built-in **length property** to determine the
size of any array. The code:
//notice lack of brackets after anArray.length :
System.out.println(anArray.length);
will print the array's size to standard output.
Since Array has a length property, Java does have properties, right?
Can someone point to some documentation about them?
It really depends what definition you're using. Accessing some class's data without using parentheses is a very weak definition of "property." You can make any class/instance member field public and thus have it accessible without using a method.
That doesn't make it like a C# property where you can add custom logic to how getting and setting is done on that field though! You're straight up reading from/writing to a variable. The length field of an array is not writable and is in fact immutable, so it's not horrible style for that to have been done. But other classes that expose their fields have gotten into trouble, like Point and Dimension.
No. There's no documentation about them. Not real properties as in C#, Groovy, etc.
The JavaBean specification is as close as it gets, and that has more to do with satisfying the needs of tooling; it's still just getters, setters, and naming conventions.
"Length" is somewhat anomalous, along with other things about how arrays were implemented.
Java objects can have attributes, which you can think of as a type of property, and they also can have methods. However, "property" in the realm of C# means something specific, which Java does not have.
Of course, Java can mimic them with other features.
In Java, public member variables can be considered properties but they are just simple attributes; there is no way to provide getter/setter methods to intercept their reading/writing. For example:
class Foo {
public String name;
}
Foo f = new Foo();
f.name = "Alice";
f.name; // "Alice"
You can also make them "read-only", as long as you provide a constructor with assignment (or a default value):
class Bar {
public final String name;
public Bar(String name) { this.name = name; }
}
Bar b = new Bar("Bob");
b.name; // "Bob"
b.name = "Carol"; // Compile Error: cannot assign to final field.
No.
Java uses the JavaBeans specification (roughly: getters and setters) to accomplish this.
Java primitive objects have a handful of things you might regard as "properties", since they are specified as variableName.someName rather than variableName.someName(). length (for an array) is one (and the only one I can think of at present). However, in a bit of sleight-of-hand they are referred to as "fields" in the spec, even though they are not implemented at all like fields.
They are all read-only.
Related
Java 14 introduced records feature. Record creates getter with the same name as field, so one would write print(person.name()) for example. But old Java bean convention dictates that one should name this method as getName().
Using both styles in the same code base does not look very nice. Migrating everything to records is not possible, as they are too limited to replace all use-cases.
Is there any official or semi-official guidelines how to name getters and setters after Java 14 in new code?
Quote from JEP 359:
It is not a goal to declare "war on boilerplate"; in particular, it is not a goal to address the problems of mutable classes using the JavaBean naming conventions.
My understanding, based on the same document is that records are transparent holders for shallowly immutable data.
That being said:
Records are not the place to look for getters/setters syntactical sugar, as they are not meant to replace JavaBeans.
I strongly agree with you that JavaBeans are too verbose. Maybe an additional feature (called beans instead of records) could be implemented - very similar behavior with the records feature but that would permit mutability. In that case, records and beans would not be mutually exclusive.
As it has been mentioned, records are in preview mode. Let's see what the feedback from community would be.
All in all, IMHO they are a step forward... I wrote this example set where you can see a code reduction to ~15% LOC from standard JavaBeans.
Also, note that records behave like normal classes: they can be declared top level or nested, they can be generic, they can implement interfaces (from the same document). You can actually partly simulate JavaBeans (only getters would make sense, though) by extracting an interface containing the getters - however that would be a lot of work and not a really clean solution...
So, based on the logic above, to address your question, no - I didn't see any (semi)official guideline for getters and setters and I don't think that there is a motivation for it right now because, again, records are not a replacement for JavaBeans...
The record spec is now "final" as of Java 17 and this naming convention discrepancy has unfortunately not been addressed. I stumbled upon it when attempting to leverage Records as shallow holder classes to implement interfaces part of an existing domain model.
Whilst this isn't as neat a solution as I'd like, Records can have methods, so you could add "legacy" getters to your record, as in the following (contrived but simple) example.
public interface Nameable {
public String getName();
}
public record Person(String name) implements Nameable {
public String getName() {
return name; // or return name();
}
}
At least this allows client code to continue to use that tried and tested (over 20 years old) convention, which - let's face it - is used far more than in pure JavaBeans context.
You could say that the language designers have lived up to their remit of "not declaring war on boilerplate"
I stumbled up this when researching naming conventions for my project. Looking at the "recent" additions to the std lib (e.g. Path, FileSystem, HttpRequest, ...) the only more-or-less "pattern" I could detect was that .prop() implies direct, unmodified access to the field value, and thus existance of the field with that very type.
Whereas "getXXX" conveys that you cannot/should not assume the existence of a field. This property might be calculated, direct field access or read-only wrapped (e.g. List.copyOf) or converted.
So my conclusion is: if you want to communicate "structure" or enforce the precence of fields use .prop(). In all other cases stick to getXXX as it is more flexible (implementers can be entity classes, records or service classes.
Btw: I am aware that there are big offenders to this logic even in the jdk. e.g. BigDecimal that's why I focused on more recent additions.
In Java records, object fields must be private and final.
So there is just one kind of getter and one kind of setter possible.
In Java classes, object fields may be private or public.
In the latter type of field, one can get or set them simply by adding a period and the field name, e.g.
Employee emp = new Employee(); // Nullary constructor
emp.name = "John Schmidt"; // Setter
. . .
. . .
if (emp.name != "Buddy") // Getter
{
emp.bonus = 100.00;
}
Non-private fields are used a lot in Android apps to save memory and time extracting data. But there's no reason not to use them in Java where it's safe to do so.
Now, if you change away from the usual way in Java classes to something like that used in record types, e.g.
String name = emp.name(); // New getter convention for private field
you have a serious risk of confusion by code readers who might misinterpret this as a non-private object field.
And if you change the record getter to what is used in Java objects, i.e.
obj.getField()
then there is a risk of confusion by coder reviewers and possibly a compiler may treat it as a Java object, depending on execution decision criteria.
In short, it's a different type of object to the normal Java class or enum. Its accessors indicate this new type unambiguously.
That's how I see it anyhow.
Maybe someone on the Java development committee may be able to enlighten us further.
This question already has answers here:
Why are getter and setter method important in java? [duplicate]
(6 answers)
Closed 7 years ago.
Encapsulation is hiding the data. I would like to hear some really interesting answers here.
What is the point behind keeping variables as private when we already declare public setter methods for variables?
I understand the usage of encapsulation but when we are making the setters as public what is the point behind keeping the variables as private, we can directly use public access modifiers.
Is it because we do not want others to know the exact way we are storing data or managing data on the back-end?
Is it because we do not want others to know the exact way we are
storing data or managing data on the back-end?
Yes, that's the point. It is related to the concepts of abstraction and information hiding too.
You provide a public setter that when invoked by the class client will have the effect that you have documented. It is none of the client's business how this effect is actually achieved. Are you modifying one of the class attributes? Ok, let the client know that, but not the fact that you are actually modifying a variable. In the future, you could want to modify your class so that instead of a simple backup variable it uses something completely different (a dictionary of attributes? An external service? Whatever!) and the client will not break.
So your setter is an abstraction that you provide to the client for "modify this class attribute". At the same time you are hiding the fact that you are using an internal variable because the client doesn't need to know that fact.
(Note: here I'm using the word "attribute" as a generic concept, not related to any concrete programming language)
I fully agree with Konamiman's answer, but I'd like to add one thing:
There are cases where you really don't want that abstraction. And that's fine.
A simple example I like to use here is a class for a 3-dimensional float vector:
class Vector3f {
public:
float x;
float y;
float z;
};
Could you make those fields private and provide setters instead? Sure, you could. But here you might argue that the class is really just supposed to provide a tuple of floats and you don't want any additional functionality. Thus adding setters would only complicate the class and you'd rather leave the fields public.
Now, you can easily construct scenarios where that might bite you later on. For instance, you might one day get a requirement that Vector3fs are not allowed to store NaNs and should throw an exception if anyone tries to do so. But such a hypothetical future problem should not be enough to justify introducing additional abstractions.
It's your call as a programmer to decide which abstractions make sense for the problem at hand and which ones would only get in your way of getting the job done. Unnecessary abstractions are over-engineering and will hurt your productivity just as much as not abstracting enough.
Bottom line: Don't blindly use setters everywhere just because someone claimed that's good practice. Instead, think about the problem at hand and consider the tradeoffs.
Because by encapsulation we provide single point of access. Suppose you define a variable and its setter as follows
String username;
public void setUsername(String username){
this.username = username;
}
Later you like to add some validation before setting username property. If you are setting the username at 10 places by directly accessing the property then you don't have single point of access and you need to make this change at 10 places. But if you have one setter method then by making a change at one place you can easily achieve the result.
Think about this : I'm representing a real life object, a Lion through a class. I'd do something like this.
class Lion {
public int legs;
}
Now my class is needed by some other developer to create an object and set its legs field. He'd do something like this
Lion jungleKing = new Lion();
jungleKing.legs = 15;
Now the question is, Java won't restrict him to setting any number more than 4 as the number of legs for that object. It's not an error, and it'll run just fine. But it's a logical blunder, and the compiler won't help you there. This way a Lion may have any number of legs.
But if we write the code this way
class Lion {
private int legs;
public void setLegs(int legs){
if(legs > 4){
this.legs = 4;
}
else this.legs = legs;
}
}
Now you won't have any Lion with more than 4 legs because the policy of updating the fields of the class has been defined by the class itself and there's no way anyone not knowing the policy is going to update the legs field because the only way to update the legs field is through the setLegs() method and that method knows the policy of the class.
Although Konamiman's answer is spot on, I'd like to add that, in the particular case of public setters versus directly exposing public fields you are asking, there is another very important distinction to keep in mind apart from information hiding and decoupling implementation from the public surface, or API, of a class; validation.
In a public field scenario, there is no way to validate the field's value when it's modified. In case of a public setter (be it a Foo {get; set;} property or a SetFoo(Foo value)) method you have the possibility to add validation code and launch required side-effects and this way ensure that your class is always in a valid or predictable state.
What if you do want to a range check before assignment? That's one of the cases I use setters and getters
More or less simple and realistic example I encountered in practice is an Options class, which has a lot of setters and getters. At some point you might want to add new option which depends on others or has side effects. Or even replace group of options with Enum. In this case setA function will not just modify a field, but will hide some additional configuration logic. Similarly getA will not just return value of a, but something like config == cStuffSupportingA.
Wikipedia has a good overview of [mutator methods(https://en.wikipedia.org/wiki/Mutator_method), which is what setter methods are and how they work in different languages.
The short version: if you want to introduce validation or other logic that gets executed on object modification it is nice to have a setter to put that logic in. Also you may want to hide how you store things. So, those are reasons for having getters/setters. Similarly, for getters, you might have logic that provides default values or values that are dependent on e.g. configuration for things like Locale, character encoding, etc. There are lots of valid reasons to want to have logic other than getting or setting the instance variable.
Obviously, if you have getters and setteres, you don't want people bypassing them by manipulating the object state directly, which is why you should keep instance variables private.
Other things to consider include whether you actually want your objects to be mutable at all (if not, make fields final), whether you want to make modifying the object state threadsafe with e.g. locks, synchronized, etc.
Setting fields as private documents a powerful fact: these private fields are only directly used within the current class. This helps maintainers by not having to track down field usage. They can reason better on the code by looking at the class and determining that the effects on and from these fields with the class' environment go through public and protected method calls. It limits the exposure surface on the class.
In turn, defining a "setter" for a private field is not about giving it publicity again. It is about declaring another powerful fact: an object belonging to this class has a property that can be modified from the outside. (The terms object and property are used in the sense of a bounded part of the whole and an observable fact about this part, not in the OOP sense)
Why then declare a "setter" on a field when making the field public would suffice? Because declaring a field not only binds a name to a property of the objects of the class, but also commits to use memory storage for this property.
Therefore, if you declare a "private field with a setter", you declare three things:
You declare that the name you gave to the field/setter cluster represents a property of the object which is of interest when the object is seen as a black box.
You declare that the value of this property is modifiable by the environment of the object.
You declare that in this particular concrete class, the property of the object is realized by committing some memory storage to it.
I advocate that you never make your fields private with getters and setters indiscriminately. Fields are for describing storage. Methods are for interactions with the environment. (And the particular case of "getters" and "setters" are for describing properties of interest)
Through the Java API you see numerous occurrences of conflicting naming and practices which are really confusing to me.
For example:
The String class has a private variable (Integer) by the name of count which keeps track of the size of the string, however this is returned by a getter by the name of length().
If you move over to any type of arrays, instead of having a getter method for length, they just pass the variable through with a public accessor, and it can be obtained through arrayInstance.length.
Moving back to the String class we have the String#getBytes() method which is a getter, similar to the length() getter, however performs slightly more logic to get and return the value.
To me, personally, creating a getter with the prefix of get seems redundant, for example I rather type GamePacket#data() versus GamePacket#getData() however I feel like there may be a deeper meaning behind this naming instead of just inconsistency.
Also, why doesn't the Array[] use a getter for length?
Would anybody be kind enough to shed some light on this for me?
Getters (and setters) come from the Java Bean specification. The reasons to use them are multiple:
most Java developers expect accessors to be named like that
an API respecting these conventions is easier to discover. For example, in my IDE, I'll often press get CtrlSpace to discover all the information available in an object.
many APIs and frameworks rely on these conventions to work: the JSP EL, MVC frameworks populating beans from request parameters, JPA, dependency injection frameworks like Spring, etc.
You usually name the getter the same way as the private variable that holds the information, but what matters is encapsulation and the public API, so nothing prevents you from computing a value in a getter, or to name the private field a different way.
This is not going to be a full answer, as a real answer would probably require interviewing the original developers of Java or otherwise researching historical records. But here are a few notes:
As you were told in the comments, the length field of an array is final, therefore it cannot be set.
Both arrays and the String class were designed in the earliest versions of Java, before the conventions for getters and setters were settled upon. The get... is... and set... conventions have only really been settled upon at the introduction of JavaBeans. Changing the API would make old code stop working, and so the old names persist.
In general (e.g. classes which are not JavaBeans), there is actually no rule that says the getter and setter methods have to reflect the name of any particular field in the class. The whole idea of accessor methods is that they hide implementation, so whatever is behind them can be a field, a combination of fields, or something else altogether.
Naming conventions appear to vary throughout the Java codebase, but one early standard was the JavaBeans naming convention; this basically formed an nomenclature solution to Java's lack of true properties.
Object/primitive getters had the form getXXX(), except for booleans, which had the preferred form isXXX(). Setters were always in the form setXXX().
From this single point, millions of lines of reflective code were written.
This convention pre-dated annotations, which would have been a trade-off between increased intent and increased verbosity when writing something like this pseudo-ish code
#Setter
void data(Data data) {
this.data = data;
};
#Getter
Data data() {
return data;
};
I have a class that has many settable/gettable attributes. I'd like to use reflection to set these attributes, but I have 2 questions about my implementation
Here is some stripped down code from my class
class Q {
public String question_1;
public String question_2;
public String question_3;
public String answer_1;
public String answer_2;
public String answer_3;
//etc. etc. Many String attributes
// … constructor and other stuff are omitted
// here is my method for "dynamically" setting each attribute
public void set_attribute(String a_raw_string, String my_field) {
try {
Class cls = Class.forName("com.xyz.models.Q");
Field fld = cls.getField(my_field);
fld.set(this, a_raw_string);
}
catch (Throwable e) {
System.err.println(e);
}
}
I then set various fields like this:
Q q = new Q();
q.set_attribute("abcde", "question_1");
q.set_attribute("defgh", "question_2");
// etc.
This works (i.e., the instance variables are set when I call set_attribute.
However, they only work when the instance variables are declared public. When they are declared private I get a NoSuchFieldException
QUESTION 1: Why do I get that error when the fields are private? My naive assumption is that since the set_attribute function is part of the class, it should have unfettered access to the instance variables.
QUESTION 2: I think I may be overthinking this problem (i.e., I shouldn't be using reflection to set variables in this way). Is there a more recommended approach?
The reason that I want to use reflection is because it's a pain in the ass to declare a ton of setter methods…so I'm wondering if someone has solved this annoyance in a better way.
Thanks!
I think I may be overthinking this problem (i.e., I shouldn't be using reflection to set variables in this way)
Yep. Reflection is fairly slow and should only be used as a last resort. If this is simply to avoid having so much redundant code, consider using automatic code generation. For pure data objects, I would strongly recommend using protocol buffers; it will generate the getters / setters (you only need to declare the fields). Plus it allows for easy communication of the data between C++, Java, and Python.
If you have a class that has a lot of fields but isn't a pure data object... well
You should consider whether all the fields should be mutable. (Do you really need setters?)
Whether the fields should even be visible. (Do you need any accessors at all?)
It is often a good idea to make fields "final", initialize them in the constructor(s), and provide no access or provide limited access through an implemented interface.
Using setter methods is the accepted way to set values for class member variables, reflection should definitely not be used for that as the code will be harder to understand and run much more slowly.
Most IDEs (eg Eclipse or NetBeans) include tools for automatically creating getter and setter methods for a class's fields.
When they are private you need to call fld.setAccessible(true);
Yes, why don't you just set the fields directly and avoid reflection? It doesn't look like you're doing anything dynamic. It's just that they are private -- why? Perhaps you mean to expose getters/setters and make the fields private? If so, then you should just invoke the public setters.
I'm a bit confused as to why languages have these. I'm a Java programmer and at the start of my career so Java is the only language I've written in since I started to actually, you know, get it.
So in Java of course we don't have properties and we write getThis() and setThat(...) methods.
What would we gain by having properties?
Thanks.
EDIT: another query: what naming conventions arise in languages with properties?
Which one looks more natural to you?
// A
person.setAge(25)
// B
person.age = 25;
// or
person.Age = 25; //depending on conventions, but that's beside the point
Most people will answer B.
It's not only syntaxic sugar, it also helps when doing reflection; you can actually make the difference between data and operations without resorting to the name of the methods.
Here is an example in C# for those not familiar with properties:
class Person
{
public int Age
{
set
{
if(value<0)
throw new ArgumentOutOfRangeException();
OnChanged();
age = value;
}
get { return age; }
}
private int age;
protected virtual void OnChanged() { // ... }
}
Also, most people always use properties rather than promote a public member later for the same reason we always use get/set; there is no need to rewrite the old client code bound to data members.
The syntax is much nicer:
button.Location += delta;
than:
button.setLocation(button.getLocation() + delta);
Edit:
The code below assumes that you are doing everything by hand. In my example world the compiler would generate the simple get/set methods and convert all direct variable access to those methods. If that didn't then the client code would have to be recompiled which defeats a big part of the purpose.
Original:
The main argument for properties is that it removes the need to recompile your code if you go from a variable to a method.
For instance:
public class Foo
{
public int bar;
}
If we later decided to validation to "bar" we would need to do this:
public class Foo
{
private int bar;
public void setBar(final int val)
{
if(val <= 0)
{
throw new IllegalArgumentException("val must be > 0, was: " + val);
}
bar = val;
}
public int getBar()
{
return (bar);
}
}
But adding the set/get method would break all of the code. If it was done via properties then you would be able to add the validation after the fact without breaking client code.
I personally don't like the idea - I am much happier with the idea of using annotation and having the simple set/get geterated automatically with the ability to profive your own set/get implementations as needed (but I don't like hidden method calls).
Two reasons:
Cleaned/terser syntax; and
It more clearly indicates to the user of the class the difference between state (properties) and behaviour (methods).
In Java the getters and setters are in essence properties.
In other modern languages (c#) , etc it just makes the syntax easier to work with/comprehend.
They are unnecessary, and there are workarounds in most cases.
It's really a matter of preference, but if the language you're using supports them I would recommend using them :)
I struggled with this at first, too, however I've really come to appreciate them. The way I see it, properties allow me to interact with the exposed data in a natural way without losing the encapsulation provided by getter/setter methods. In other words, I can treat my properties as fields but without really exposing the actual fields if I choose not to. With automatic properties in C# 3.0 it gets even better as for most fields -- where I want to allow the consumer to read/write the data -- I have even less to write:
public string Prop { get; set; }
In the case where I want partial visibility, I can restrict just the accessor I want easily.
public string Prop { get; private set; }
All of this can be done with getter/setter methods, but the verbiage is much higher and the usage is much less natural.
A general rule of object oriented programming is that you never change an existing interface. This ensures that while in inner content may change for the objects calling the object don't need to know this.
Properties in other languages are methods masquerading as a specific language feature. In Java a property is distinguished only by convention. While in general this works, there are cases where it limits you. For example sometimes you would to use hasSomething instead of isSomething of getSomething.
So it allows flexibility of names, while tools and other code depending on can still tell the difference.
Also the code can be more compact and the get and set are grouped together by design.
In Object Oriented Software Construction 2 Bertrand Meyer calls this the "Uniform Access Principle" and the general idea is that when a property goes from a simple one (i.e. just an integer) to a derived one (a function call), the people using it shouldn't have to know.
You don't want everyone using your code to have to change from
int x = foo.y;
to
int x = foo.y();
That breaks encapsulation because you haven't changed your "interface" just your "implementation".
You can also create derived fields and read-only/write-only fields. Most Properties that I've seen in languages I've worked in allow you to not only assign simple fields, but also full functions to properties.
Properties provide a simple method to abstract the details behind a set of logic in an object down to a single value to the outside world.
While your property may start out only as a value, this abstraction decouples the interface such that it's details can be changed later with minimal impact.
A general rule of thumb is that abstraction and loose coupling are good things. Properties are a pattern that achieve both.
Properties at the language level are a bad idea. There's no good convention for them and they hide performance deficits in the code.
It's all about bindings
There was a time when I considered properties to just be syntactic sugar (i.e. help the developer by having them type a bit less). As I've done more and more GUI development, and started using binding frameworks (JGoodies, JSR295), I have discovered that language level properties are much, much more than syntactic sugar.
In a binding scenario, you essentially define rules that say 'property X of object A should always be equal to property Y of object B'. Shorthand is: A.x <-> B.y
Now, imagine how you would go about actually writing a binding library in Java. Right now, it is absolutely not possible to refer to 'x' or 'y' directly as language primitives. You can only refer to them as strings (and access them via reflection). In essence, A."x" <-> B."y"
This causes massive, massive problems when you go to refactor code.
There are additional considerations, including proper implementation of property change notifications. If you look at my code, every blessed setter requires a minimum of 3 lines to do something that is incredibly simple. Plus one of those 3 lines includes yet another string:
public void setFoo(Foo foo){
Foo old = getFoo();
this.foo = foo;
changeSupport.firePropertyChange("foo", old, foo);
}
all of these strings floating around is a complete nightmare.
Now, imagine if a property was a first class citizen in the language. This starts to provide almost endless possibilities (for example, imagine registering a listener with a Property directly instead of having to muck with PropertyChangeSupport and it's 3 mystery methods that have to get added to every class). Imagine being able to pass the property itself (not the value of the property, but the Property object) into a binding framework.
For web tier developers, imagine a web framework that can build it's own form id values from the names of the properties themselves (something like registerFormProperties(myObject.firstname, myObject.lastname, someOtherObject.amount) to allow for round-trip population of object property values when the form is submitted back to the server. Right now to do that, you'd have to pass strings in, and refactoring becomes a headache (refactoring actually becomes downright scary once you are relying on strings and reflection to wire things up).
So anyway, For those of us who are dealing with dynamic data updates via binding, properties are a much needed feature in the language - way more than just syntactic sugar.