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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)
This question already has answers here:
Why use getters and setters/accessors?
(37 answers)
Closed 9 years ago.
Consider the following Java code:
public class SomeClass{
private int data;
public void setData(int data){
this.data = data;
}
public int getData(){
return this.data;
}
}
In the above code the value of data can be accessed from anywhere. So why not just make the field data public?
Why Getters and Setters?
Many people wonder why we need accessor and mutator methods in Java (a.k.a. getters and setters), why can’t we just access the data directly? But the purpose of encapsulation here is is not to hide the data itself, but the implementation details on how this data is manipulated. So, once more what we want is a way to provide a public interface through which we can gain access to this data. We can later change the internal representation of the data without compromising the public interface of the class. On the contrary, by exposing the data itself, we compromise encapsulation, and therefore, the capacity of changing the ways to manipulate this data in the future without affecting its users. We would create a dependency with the data itself, and not with the public interface of the class. We would be creating a perfect cocktail for trouble when “change” finally finds us.
There are several compelling reasons why we might want to encapsulate access to our fields. The best compendium of these reasons I have ever found is described in Joshua Bloch’s book Effective Java. There in Item 14: Minimize the accessibility of classes and members, he mentions several reasons, which I mention here:
You can limit the values that can be stored in a field (i.e. gender must be F or M).
You can take actions when the field is modified (trigger event, validate, etc).
You can provide thread safety by synchronizing the method.
You can switch to a new data representation (i.e. calculated fields, different data type)
You could make a field read-only
Etc.
However, it is very important to understand that encapsulation is more than hiding fields. In Java we can hide entire classes, by this, hiding the implementation details of an entire API.
My understanding of this important concept was broaden and enriched by my reading of a great article by Alan Snyder called Encapsulation and Inheritance in Object-Oriented Programming Languages which I recommend to all readers.
Because you can control how other classes set the data.
You can preform checks on the inputs, or even modify the input according to certain rules you could have.
For example:
public class SomeClass{
private int data;
public void setData(int data){
if (data < 1000){
this.data = data;
}
else{
data = -1;
}
}
public int getData(){
return this.data;
}
}
Even if it looks like there's no point in using the getter/setter methods now, since they don't do anything interesting, you need to think about what might happen in the future when the requirements change. You might find that you need to add some validation when setting the value; you might find that you need to change the whole implementation so that the "get" method is actually going to compute a value from several pieces of data, or read it from a file, or something. And when this happens, if you have other classes that are accessing data directly, you will be screwed. You won't be able to change the implementation the way you need to, without a whole lot of pain. I speak from long, bitter experience going through the pain of maintaining other people's sloppy code.
Sometimes the situation demands that some variables can access for read or write only purpose. You can achieve it through getter and setter method.
If you make the variable public then it can be access and can be changed its value using class object.
This type of pattern is call POJO. POJO is an acronym for Plain Old Java Object. The name is used to emphasize that a given object is an ordinary Java Object, not a special object.
This Link can be answered your question and google up the advantages of POJO class for more info.
class Person
{
private BankAccount account;
Person(BankAccount account)
{
this.account = account;
}
public Person someMethod(Person person)
{
//Why accessing private field is possible?
BankAccount a = person.account;
}
}
Please forget about the design. I know that OOP specifies that private objects are private to the class. My question is, why was OOP designed such that private fields have class-level access and not object-level access?
I am also a bit curious with the answer.
The most satisfying answer that I find is from Artemix in another post here (I'm renaming the AClass with Person class):
Why have class-level access modifiers instead of object-level?
The private modifier enforces Encapsulation principle.
The idea is that 'outer world' should not make changes to Person internal processes because Person implementation may change over time (and you would have to change the whole outer world to fix the differences in implementation - which is nearly to impossible).
When instance of Person accesses internals of other Person instance - you can be sure that both instances always know the details of implementation of Person. If the logic of internal to Person processes is changed - all you have to do is change the code of Person.
EDIT:
Please vote Artemix' answer. I'm just copy-pasting it.
Good question. It seems that object level access modifier would enforce the Encapsulation principle even further.
But actually it's the other way around. Let's take an example. Suppose you want to deep copy an object in a constructor, if you cannot access the private members of that object. Then the only possible way is to add some public accessors to all of the private members. This will make your objects naked to all other parts of the system.
So encapsulation doesn't mean being closed to all of the rest of the world. It means being selective about whom you want to be open to.
See the Java Language Specification, Section 6.6.1. Determining Accessibility
It states
Otherwise, if the member or constructor is declared private, then
access is permitted if and only if it occurs within the body of the
top level class (§7.6) that encloses the declaration of the member or
constructor.
Click the link above for more details. So the answer is: Because James Gosling and the other authors of Java decided it to be that way.
This works because you are in the class Person - a class is allowed to poke inside it's own type of class. This really helps when you want to write a copy constructor, for example:
class A
{
private:
int x;
int y;
public:
A(int a, int b) x(a), y(b) {}
A(A a) { x = a.x; y = y.x; }
};
Or if we want to write operator+ and operator- for our big number class.
Just my 2 cents on the question of why the semantics of the private visibility in Java is class level rather than object level.
I would say that convenience seems to be the key here. In fact, a private visibility at object level would have forced to expose methods to other classes (e.g. in the same package) in the scenario illustrated by the OP.
In truth I was not able neither to concoct nor to find an example showing that the visibility at class-private level (like offered by Java) creates any issues if compared to visibility at object-private level.
That said, programming languages with a more fine-grained system of visibility policies can afford both object visibility at object level and class level.
For example Eiffel, offers selective export: you can export any class feature to any class of your choice, from {NONE} (object-private) to {ANY} (the equivalent of public, and also the default), to {PERSON} (class-private, see the OP's example), to specific groups of classes {PERSON, BANK}.
It's also interesting to remark that in Eiffel you don't need to make an attribute private and write a getter to prevent other classes from assigning to it. Public attributes in Eiffel are by default accessible in read-only mode, so you don't need a getter just to return their value.
Of course you still need a setter to set an attribute, but you can hide it by defining it as "assigner" for that attribute. This allows you, if you wish, to use the more convenient assignment operator instead of the setter invocation.
Because the private access modifier renders it visible only within the class. This method is still IN the class.
the private field is accessible in the class/object in which the field is declared. It is private to other classes/objects outside of the one it is located in.
First thing here we have to understand is all we have to do is must follow oops principles so encapsulation is say that wrap data within package(i.e. class) and than represent all data as Object and easy to access. so if we make field as non-private than
it's accessed indivisually. and it result into bad paratice.
With reflection concept in Java is possible modify fields and methods privates
Modificando metodos y campos privados con Refleccion en Java
This question already has answers here:
Is it in an anti-pattern to always use get and set methods to access a class's own member fields? [duplicate]
(11 answers)
Closed 9 years ago.
Usually, in my own projects I use getters and setters for any field access, and I followed to do the same on my job. Some time ago, the tech lead of our project asked me why I was doing that and why is this better than just using fields themselves (with an option of declaring them protected if they needed to be accessed by subclasses). I couldn't come up with a clear answer.
So, are there any reasons to using getters and setters inside a class for class' own fields, or is it better to use fields directly?
The most obvious answer is side effects:
int getCost()
{
if (cost == null) {
calculateCost();
}
return cost;
}
If you need the cost, use getCost(). If you want to see if cost has been calculated, use cost.
If there is any business logic around those values (or there is the potential for such logic), then there is a benefit to using getters and setters even for internal calls.
For example, your setter might do validation on its inputs, and throw an exception rather than store an invalid value. Having all your code use that setter rather than simply setting values directly means that the error is caught at the time it is made rather than a long time later when that value is used. A similar case for a getter is when there is a logical default value, which should be used in case of a null. By using a getter, you can safely write local methods without needing continuous null checks or default options.
That said, if there's no business logic in those methods, and no side effects caused by them, then it's mostly a stylistic thing. It is essentially the responsibility of the class to be internally consistent, and as long as it remains so then it's mostly personal/professional preference whether you access the variables directly or through wrapping methods.
You want to declare them as public getters and setters, and private fields. This means external classes (not subclasses) who want to modify the variables all do so through the setters, and get them through the getters. The benefit of this is that if you want to control how or what condition they get or set them, or want to add information or even print debug, it means you only have to put it in the getters and setters.
There's a really good explanation of the benefits on stackoverflow actually:
In Java, difference between default, public, protected, and private
Of course, only make methods when they're actually needed, and similarly, only public when needed by external classes.
Hope that helps the defense!
This is part of the general question as to why you use getters and setters. Many developers use them without though, as a matter of practice. Personally, I only put in getters/setters if I need to.
I would suggest you do what is clearest/simplest to you.
In general, if I can easily add a getter/setter later should I need it, I won't add it. If it would be difficult to add later (or you have an immediate use for them), I would include them.
Some of us are web developers so, we resort to creating JavaBeans and JavaBeans has its own specification. In the specification, it clearly states:
The class must have a public default constructor (no-argument).
The class properties must be accessible using get, set, is (used for boolean properties instead of get) and other methods.
The class should be serializable.
The reason being, JavaBeans were designed for Reusability where JavaBeans could travel through any Java technologies (e.g. Servlets, JSPs, RMI, Web Services, etc.).
That's my 2cent worth on why we have getters/setters. I mostly create JavaBeans.
Some people think that they should always encapsulate all fields by using setters/getters.
Others think that this practice should not be used at all.
If your class does not have any logic for the fields and just is used as a holder, you can skip using methods and just declare your fields as public. This concept is also called a Data Transfer Object (or Messenger.) But as a rule you should use final attribute for such fields to make your class immutable:
public class TwoTuple<A,B> {
public final A first;
public final B second;
public TwoTuple(A a, B b) { first = a; second = b; }
}
However you must/or it's strongly recommended to use setters/getters:
in web applications sometimes there are requirements to use setters/getters. See POJO/JavaBean objects.
if your class is going to be used in concurrent environment. See Java Concurrency in Practice, Section 3.2:
"Whether another thread actually does something with a published reference doesn't really matter, because the risk of misuse is still present.[7] Once an object escapes, you have to assume that another class or thread may, maliciously or carelessly, misuse it. This is a compelling reason to use encapsulation: it makes it practical to analyze programs for correctness and harder to violate design constraints accidentally"
if you want to add some extra logic when you set/get values you must use setters/getters. Just read about encapsulation and its advantages.
My own opinion always declare fields as "private final" and only then, if needed change these properties.
What is better practise and why: accessing base class variables through a protected field or a public getter on the private field.
(The getter will be public regardless)
If there's going to be a public getter anyway, why would you want to expose the field itself more widely than absolutely necessary? That means it's immediately writable by subclasses (unless it's final to start with).
Personally I like all my fields to be private: it provides a cleaner separation between API and implementation. I regard the relationship between a superclass and a subclass as similar to that of a caller and callee - changes to the underlying implementation shouldn't break subclasses any more than they should break callers. The name of a field is an implementation detail which shouldn't impact other classes.
Admittedly my view is occasionally seen as somewhat extreme...
You should always program against the public API of a class, that is, use the public methods.
The reason is simple. Someday in the future, you or someone else might want to change the implementation. This should always be possible. If you rely on instance variable, you limit yourself.
Also, when accessing the variable, you can not control if that variable is read-only nor can you add checks when this variable is changed.
If you use setters/getters, you can allways add validation, checking etc later on. You can also only provide a getter to make a variable read only.
Direct field access is not preferred. Use public or protected setters and getters.
The getter need not be public - if you wan to hide the data from "outsiders", but give the data to subclasses, use protected
Some of Sun's recommendations on controlling access to fields are here. Note that making a field protected exposes it to the package as well, not only to subclasses. Generally, as stated at the link above, fields should be private unless there is a very good reason not to do so.
Effective Java 2nd Edition says
Item 13: Minimize the accessibility of classes and members
The rule of thumb is simple: make each class or member as inaccessible as
possible. In other words, use the lowest possible access level consistent with the
proper functioning of the software that you are writing.
So if you are not sure why you need a protected class member (ie you don't need the field to be accessible to subclasses or classes in the same package), then declare it private. If you wish to set it from outside the class, then make a public setter.
However, if your member is final, then making it protected might be ok in some cases (ie it doesn't reveal sensitive information).
One potential security issue I would like to mention is that if you have an array declared protected final (even public final), the array reference is final (cannot be modified), but the objects held in the array are not final (an intruder could change the array contents).
If you know c++, you probably know that
const int * someMember
is different from
int * const someMember
The latter is like the final array in java.
The fix for the aforementioned security hole is to return a deep copy of the array or return it as a read only list.
Generally, you should use Sun's recommendations. There is one big exception: if you're programming for Android.
The reason is performance. With every virtual method invocation, there is overhead involved in using the lookup table to route the method to its object. This overhead is not involved when accessing a local variable.
Here are some links that explain this in a little more depth:
http://developer.android.com/training/articles/perf-tips.html#GettersSetters
http://blog.leocad.io/why-you-shouldnt-use-getters-and-setters-on-android/
It's important to know what you're trying to accomplish:
The field's value should be accessible to client code, using a public interface.
The field is meant to be used by subclasses.
In plain ol' Java, getters and setters accomplish both tasks. But Android is different. If you're doing #1, then you should use public getters and setters. If you're doing #2, then you should use protected fields. If you're doing both, use both.
I would like to present you with some arguments protecting "protected" fields in Java:
"You may favor accessing base class members using protected fields over public accessors in situation where you need to avoid value validation".
However if this is not the case, then private fields with public accessors should be used, to compliment hermetization.
The principle of getters and setters is to provide validation to the values inputted and outputted to the class member. However, in OOP languages, we operate on objects not classes. Base class and specialized class represent a single object, that is why it is perfectly fine to access specific class members over protected field.
Consider the following abstract example with a car:
- you have a base class Car and the derived class Porshe.
- Car class may have field like engine, which value is not set in Cars constructor (maybe the type of engine is known only after object initialization)
- You create a Porshe class object that contains some logic used to calculate engine type using some external data.
In this example, it is expected that engine field has a public getter, so car users know what engine the car has. However, there is no public setter as we expect car drivers not to temper with the engine! That is why, it is perfectly fine to make engine a protected field, so the Porshe class can set its value at some time in future.
Yes, some people will probably say "then use protected setter!".
And I will repeat: in OOP languages we work with objects not classes.
Single responsibility principle - yes, but as object not as class.
If you say: "at some point if we use protected fields over 3 or 5 inheritance levels, it may be troublesome to understand what happens to the field if each class performs some operation with it". And then I answer: That is another antipattern - your object is probably too big at this point and voids Single Responsibility principle.
Accessing protected fields from a subclass is one of the ways that inheritance violates encapsulation. Using the public API is better for this reason.