What's the advantage of using getters and setters - that only get and set - instead of simply using public fields for those variables?
If getters and setters are ever doing more than just the simple get/set, I can figure this one out very quickly, but I'm not 100% clear on how:
public String foo;
is any worse than:
private String foo;
public void setFoo(String foo) { this.foo = foo; }
public String getFoo() { return foo; }
Whereas the former takes a lot less boilerplate code.
There are actually many good reasons to consider using accessors rather than directly exposing fields of a class - beyond just the argument of encapsulation and making future changes easier.
Here are the some of the reasons I am aware of:
Encapsulation of behavior associated with getting or setting the property - this allows additional functionality (like validation) to be added more easily later.
Hiding the internal representation of the property while exposing a property using an alternative representation.
Insulating your public interface from change - allowing the public interface to remain constant while the implementation changes without affecting existing consumers.
Controlling the lifetime and memory management (disposal) semantics of the property - particularly important in non-managed memory environments (like C++ or Objective-C).
Providing a debugging interception point for when a property changes at runtime - debugging when and where a property changed to a particular value can be quite difficult without this in some languages.
Improved interoperability with libraries that are designed to operate against property getter/setters - Mocking, Serialization, and WPF come to mind.
Allowing inheritors to change the semantics of how the property behaves and is exposed by overriding the getter/setter methods.
Allowing the getter/setter to be passed around as lambda expressions rather than values.
Getters and setters can allow different access levels - for example the get may be public, but the set could be protected.
Because 2 weeks (months, years) from now when you realize that your setter needs to do more than just set the value, you'll also realize that the property has been used directly in 238 other classes :-)
A public field is not worse than a getter/setter pair that does nothing except returning the field and assigning to it. First, it's clear that (in most languages) there is no functional difference. Any difference must be in other factors, like maintainability or readability.
An oft-mentioned advantage of getter/setter pairs, isn't. There's this claim that you can change the implementation and your clients don't have to be recompiled. Supposedly, setters let you add functionality like validation later on and your clients don't even need to know about it. However, adding validation to a setter is a change to its preconditions, a violation of the previous contract, which was, quite simply, "you can put anything in here, and you can get that same thing later from the getter".
So, now that you broke the contract, changing every file in the codebase is something you should want to do, not avoid. If you avoid it you're making the assumption that all the code assumed the contract for those methods was different.
If that should not have been the contract, then the interface was allowing clients to put the object in invalid states. That's the exact opposite of encapsulation If that field could not really be set to anything from the start, why wasn't the validation there from the start?
This same argument applies to other supposed advantages of these pass-through getter/setter pairs: if you later decide to change the value being set, you're breaking the contract. If you override the default functionality in a derived class, in a way beyond a few harmless modifications (like logging or other non-observable behaviour), you're breaking the contract of the base class. That is a violation of the Liskov Substitutability Principle, which is seen as one of the tenets of OO.
If a class has these dumb getters and setters for every field, then it is a class that has no invariants whatsoever, no contract. Is that really object-oriented design? If all the class has is those getters and setters, it's just a dumb data holder, and dumb data holders should look like dumb data holders:
class Foo {
public:
int DaysLeft;
int ContestantNumber;
};
Adding pass-through getter/setter pairs to such a class adds no value. Other classes should provide meaningful operations, not just operations that fields already provide. That's how you can define and maintain useful invariants.
Client: "What can I do with an object of this class?"
Designer: "You can read and write several variables."
Client: "Oh... cool, I guess?"
There are reasons to use getters and setters, but if those reasons don't exist, making getter/setter pairs in the name of false encapsulation gods is not a good thing. Valid reasons to make getters or setters include the things often mentioned as the potential changes you can make later, like validation or different internal representations. Or maybe the value should be readable by clients but not writable (for example, reading the size of a dictionary), so a simple getter is a nice choice. But those reasons should be there when you make the choice, and not just as a potential thing you may want later. This is an instance of YAGNI (You Ain't Gonna Need It).
Lots of people talk about the advantages of getters and setters but I want to play devil's advocate. Right now I'm debugging a very large program where the programmers decided to make everything getters and setters. That might seem nice, but its a reverse-engineering nightmare.
Say you're looking through hundreds of lines of code and you come across this:
person.name = "Joe";
It's a beautifully simply piece of code until you realize its a setter. Now, you follow that setter and find that it also sets person.firstName, person.lastName, person.isHuman, person.hasReallyCommonFirstName, and calls person.update(), which sends a query out to the database, etc. Oh, that's where your memory leak was occurring.
Understanding a local piece of code at first glance is an important property of good readability that getters and setters tend to break. That is why I try to avoid them when I can, and minimize what they do when I use them.
In a pure object-oriented world getters and setters is a terrible anti-pattern. Read this article: Getters/Setters. Evil. Period. In a nutshell, they encourage programmers to think about objects as of data structures, and this type of thinking is pure procedural (like in COBOL or C). In an object-oriented language there are no data structures, but only objects that expose behavior (not attributes/properties!)
You may find more about them in Section 3.5 of Elegant Objects (my book about object-oriented programming).
There are many reasons. My favorite one is when you need to change the behavior or regulate what you can set on a variable. For instance, lets say you had a setSpeed(int speed) method. But you want that you can only set a maximum speed of 100. You would do something like:
public void setSpeed(int speed) {
if ( speed > 100 ) {
this.speed = 100;
} else {
this.speed = speed;
}
}
Now what if EVERYWHERE in your code you were using the public field and then you realized you need the above requirement? Have fun hunting down every usage of the public field instead of just modifying your setter.
My 2 cents :)
One advantage of accessors and mutators is that you can perform validation.
For example, if foo was public, I could easily set it to null and then someone else could try to call a method on the object. But it's not there anymore! With a setFoo method, I could ensure that foo was never set to null.
Accessors and mutators also allow for encapsulation - if you aren't supposed to see the value once its set (perhaps it's set in the constructor and then used by methods, but never supposed to be changed), it will never been seen by anyone. But if you can allow other classes to see or change it, you can provide the proper accessor and/or mutator.
Thanks, that really clarified my thinking. Now here is (almost) 10 (almost) good reasons NOT to use getters and setters:
When you realize you need to do more than just set and get the value, you can just make the field private, which will instantly tell you where you've directly accessed it.
Any validation you perform in there can only be context free, which validation rarely is in practice.
You can change the value being set - this is an absolute nightmare when the caller passes you a value that they [shock horror] want you to store AS IS.
You can hide the internal representation - fantastic, so you're making sure that all these operations are symmetrical right?
You've insulated your public interface from changes under the sheets - if you were designing an interface and weren't sure whether direct access to something was OK, then you should have kept designing.
Some libraries expect this, but not many - reflection, serialization, mock objects all work just fine with public fields.
Inheriting this class, you can override default functionality - in other words you can REALLY confuse callers by not only hiding the implementation but making it inconsistent.
The last three I'm just leaving (N/A or D/C)...
Depends on your language. You've tagged this "object-oriented" rather than "Java", so I'd like to point out that ChssPly76's answer is language-dependent. In Python, for instance, there is no reason to use getters and setters. If you need to change the behavior, you can use a property, which wraps a getter and setter around basic attribute access. Something like this:
class Simple(object):
def _get_value(self):
return self._value -1
def _set_value(self, new_value):
self._value = new_value + 1
def _del_value(self):
self.old_values.append(self._value)
del self._value
value = property(_get_value, _set_value, _del_value)
Well i just want to add that even if sometimes they are necessary for the encapsulation and security of your variables/objects, if we want to code a real Object Oriented Program, then we need to STOP OVERUSING THE ACCESSORS, cause sometimes we depend a lot on them when is not really necessary and that makes almost the same as if we put the variables public.
EDIT: I answered this question because there are a bunch of people learning programming asking this, and most of the answers are very technically competent, but they're not as easy to understand if you're a newbie. We were all newbies, so I thought I'd try my hand at a more newbie friendly answer.
The two main ones are polymorphism, and validation. Even if it's just a stupid data structure.
Let's say we have this simple class:
public class Bottle {
public int amountOfWaterMl;
public int capacityMl;
}
A very simple class that holds how much liquid is in it, and what its capacity is (in milliliters).
What happens when I do:
Bottle bot = new Bottle();
bot.amountOfWaterMl = 1500;
bot.capacityMl = 1000;
Well, you wouldn't expect that to work, right?
You want there to be some kind of sanity check. And worse, what if I never specified the maximum capacity? Oh dear, we have a problem.
But there's another problem too. What if bottles were just one type of container? What if we had several containers, all with capacities and amounts of liquid filled? If we could just make an interface, we could let the rest of our program accept that interface, and bottles, jerrycans and all sorts of stuff would just work interchangably. Wouldn't that be better? Since interfaces demand methods, this is also a good thing.
We'd end up with something like:
public interface LiquidContainer {
public int getAmountMl();
public void setAmountMl(int amountMl);
public int getCapacityMl();
}
Great! And now we just change Bottle to this:
public class Bottle implements LiquidContainer {
private int capacityMl;
private int amountFilledMl;
public Bottle(int capacityMl, int amountFilledMl) {
this.capacityMl = capacityMl;
this.amountFilledMl = amountFilledMl;
checkNotOverFlow();
}
public int getAmountMl() {
return amountFilledMl;
}
public void setAmountMl(int amountMl) {
this.amountFilled = amountMl;
checkNotOverFlow();
}
public int getCapacityMl() {
return capacityMl;
}
private void checkNotOverFlow() {
if(amountOfWaterMl > capacityMl) {
throw new BottleOverflowException();
}
}
I'll leave the definition of the BottleOverflowException as an exercise to the reader.
Now notice how much more robust this is. We can deal with any type of container in our code now by accepting LiquidContainer instead of Bottle. And how these bottles deal with this sort of stuff can all differ. You can have bottles that write their state to disk when it changes, or bottles that save on SQL databases or GNU knows what else.
And all these can have different ways to handle various whoopsies. The Bottle just checks and if it's overflowing it throws a RuntimeException. But that might be the wrong thing to do.
(There is a useful discussion to be had about error handling, but I'm keeping it very simple here on purpose. People in comments will likely point out the flaws of this simplistic approach. ;) )
And yes, it seems like we go from a very simple idea to getting much better answers quickly.
Please note also that you can't change the capacity of a bottle. It's now set in stone. You could do this with an int by declaring it final. But if this was a list, you could empty it, add new things to it, and so on. You can't limit the access to touching the innards.
There's also the third thing that not everyone has addressed: getters and setters use method calls. That means that they look like normal methods everywhere else does. Instead of having weird specific syntax for DTOs and stuff, you have the same thing everywhere.
I know it's a bit late, but I think there are some people who are interested in performance.
I've done a little performance test. I wrote a class "NumberHolder" which, well, holds an Integer. You can either read that Integer by using the getter method
anInstance.getNumber() or by directly accessing the number by using anInstance.number. My programm reads the number 1,000,000,000 times, via both ways. That process is repeated five times and the time is printed. I've got the following result:
Time 1: 953ms, Time 2: 741ms
Time 1: 655ms, Time 2: 743ms
Time 1: 656ms, Time 2: 634ms
Time 1: 637ms, Time 2: 629ms
Time 1: 633ms, Time 2: 625ms
(Time 1 is the direct way, Time 2 is the getter)
You see, the getter is (almost) always a bit faster. Then I tried with different numbers of cycles. Instead of 1 million, I used 10 million and 0.1 million.
The results:
10 million cycles:
Time 1: 6382ms, Time 2: 6351ms
Time 1: 6363ms, Time 2: 6351ms
Time 1: 6350ms, Time 2: 6363ms
Time 1: 6353ms, Time 2: 6357ms
Time 1: 6348ms, Time 2: 6354ms
With 10 million cycles, the times are almost the same.
Here are 100 thousand (0.1 million) cycles:
Time 1: 77ms, Time 2: 73ms
Time 1: 94ms, Time 2: 65ms
Time 1: 67ms, Time 2: 63ms
Time 1: 65ms, Time 2: 65ms
Time 1: 66ms, Time 2: 63ms
Also with different amounts of cycles, the getter is a little bit faster than the regular way. I hope this helped you.
Don't use getters setters unless needed for your current delivery I.e. Don't think too much about what would happen in the future, if any thing to be changed its a change request in most of the production applications, systems.
Think simple, easy, add complexity when needed.
I would not take advantage of ignorance of business owners of deep technical know how just because I think it's correct or I like the approach.
I have massive system written without getters setters only with access modifiers and some methods to validate n perform biz logic. If you absolutely needed the. Use anything.
We use getters and setters:
for reusability
to perform validation in later stages of programming
Getter and setter methods are public interfaces to access private class members.
Encapsulation mantra
The encapsulation mantra is to make fields private and methods public.
Getter Methods: We can get access to private variables.
Setter Methods: We can modify private fields.
Even though the getter and setter methods do not add new functionality, we can change our mind come back later to make that method
better;
safer; and
faster.
Anywhere a value can be used, a method that returns that value can be added. Instead of:
int x = 1000 - 500
use
int x = 1000 - class_name.getValue();
In layman's terms
Suppose we need to store the details of this Person. This Person has the fields name, age and sex. Doing this involves creating methods for name, age and sex. Now if we need create another person, it becomes necessary to create the methods for name, age, sex all over again.
Instead of doing this, we can create a bean class(Person) with getter and setter methods. So tomorrow we can just create objects of this Bean class(Person class) whenever we need to add a new person (see the figure). Thus we are reusing the fields and methods of bean class, which is much better.
I spent quite a while thinking this over for the Java case, and I believe the real reasons are:
Code to the interface, not the implementation
Interfaces only specify methods, not fields
In other words, the only way you can specify a field in an interface is by providing a method for writing a new value and a method for reading the current value.
Those methods are the infamous getter and setter....
It can be useful for lazy-loading. Say the object in question is stored in a database, and you don't want to go get it unless you need it. If the object is retrieved by a getter, then the internal object can be null until somebody asks for it, then you can go get it on the first call to the getter.
I had a base page class in a project that was handed to me that was loading some data from a couple different web service calls, but the data in those web service calls wasn't always used in all child pages. Web services, for all of the benefits, pioneer new definitions of "slow", so you don't want to make a web service call if you don't have to.
I moved from public fields to getters, and now the getters check the cache, and if it's not there call the web service. So with a little wrapping, a lot of web service calls were prevented.
So the getter saves me from trying to figure out, on each child page, what I will need. If I need it, I call the getter, and it goes to find it for me if I don't already have it.
protected YourType _yourName = null;
public YourType YourName{
get
{
if (_yourName == null)
{
_yourName = new YourType();
return _yourName;
}
}
}
One aspect I missed in the answers so far, the access specification:
for members you have only one access specification for both setting and getting
for setters and getters you can fine tune it and define it separately
In languages which don't support "properties" (C++, Java) or require recompilation of clients when changing fields to properties (C#), using get/set methods is easier to modify. For example, adding validation logic to a setFoo method will not require changing the public interface of a class.
In languages which support "real" properties (Python, Ruby, maybe Smalltalk?) there is no point to get/set methods.
One of the basic principals of OO design: Encapsulation!
It gives you many benefits, one of which being that you can change the implementation of the getter/setter behind the scenes but any consumer of that value will continue to work as long as the data type remains the same.
You should use getters and setters when:
You're dealing with something that is conceptually an attribute, but:
Your language doesn't have properties (or some similar mechanism, like Tcl's variable traces), or
Your language's property support isn't sufficient for this use case, or
Your language's (or sometimes your framework's) idiomatic conventions encourage getters or setters for this use case.
So this is very rarely a general OO question; it's a language-specific question, with different answers for different languages (and different use cases).
From an OO theory point of view, getters and setters are useless. The interface of your class is what it does, not what its state is. (If not, you've written the wrong class.) In very simple cases, where what a class does is just, e.g., represent a point in rectangular coordinates,* the attributes are part of the interface; getters and setters just cloud that. But in anything but very simple cases, neither the attributes nor getters and setters are part of the interface.
Put another way: If you believe that consumers of your class shouldn't even know that you have a spam attribute, much less be able to change it willy-nilly, then giving them a set_spam method is the last thing you want to do.
* Even for that simple class, you may not necessarily want to allow setting the x and y values. If this is really a class, shouldn't it have methods like translate, rotate, etc.? If it's only a class because your language doesn't have records/structs/named tuples, then this isn't really a question of OO…
But nobody is ever doing general OO design. They're doing design, and implementation, in a specific language. And in some languages, getters and setters are far from useless.
If your language doesn't have properties, then the only way to represent something that's conceptually an attribute, but is actually computed, or validated, etc., is through getters and setters.
Even if your language does have properties, there may be cases where they're insufficient or inappropriate. For example, if you want to allow subclasses to control the semantics of an attribute, in languages without dynamic access, a subclass can't substitute a computed property for an attribute.
As for the "what if I want to change my implementation later?" question (which is repeated multiple times in different wording in both the OP's question and the accepted answer): If it really is a pure implementation change, and you started with an attribute, you can change it to a property without affecting the interface. Unless, of course, your language doesn't support that. So this is really just the same case again.
Also, it's important to follow the idioms of the language (or framework) you're using. If you write beautiful Ruby-style code in C#, any experienced C# developer other than you is going to have trouble reading it, and that's bad. Some languages have stronger cultures around their conventions than others.—and it may not be a coincidence that Java and Python, which are on opposite ends of the spectrum for how idiomatic getters are, happen to have two of the strongest cultures.
Beyond human readers, there will be libraries and tools that expect you to follow the conventions, and make your life harder if you don't. Hooking Interface Builder widgets to anything but ObjC properties, or using certain Java mocking libraries without getters, is just making your life more difficult. If the tools are important to you, don't fight them.
From a object orientation design standpoint both alternatives can be damaging to the maintenance of the code by weakening the encapsulation of the classes. For a discussion you can look into this excellent article: http://typicalprogrammer.com/?p=23
Code evolves. private is great for when you need data member protection. Eventually all classes should be sort of "miniprograms" that have a well-defined interface that you can't just screw with the internals of.
That said, software development isn't about setting down that final version of the class as if you're pressing some cast iron statue on the first try. While you're working with it, code is more like clay. It evolves as you develop it and learn more about the problem domain you are solving. During development classes may interact with each other than they should (dependency you plan to factor out), merge together, or split apart. So I think the debate boils down to people not wanting to religiously write
int getVar() const { return var ; }
So you have:
doSomething( obj->getVar() ) ;
Instead of
doSomething( obj->var ) ;
Not only is getVar() visually noisy, it gives this illusion that gettingVar() is somehow a more complex process than it really is. How you (as the class writer) regard the sanctity of var is particularly confusing to a user of your class if it has a passthru setter -- then it looks like you're putting up these gates to "protect" something you insist is valuable, (the sanctity of var) but yet even you concede var's protection isn't worth much by the ability for anyone to just come in and set var to whatever value they want, without you even peeking at what they are doing.
So I program as follows (assuming an "agile" type approach -- ie when I write code not knowing exactly what it will be doing/don't have time or experience to plan an elaborate waterfall style interface set):
1) Start with all public members for basic objects with data and behavior. This is why in all my C++ "example" code you'll notice me using struct instead of class everywhere.
2) When an object's internal behavior for a data member becomes complex enough, (for example, it likes to keep an internal std::list in some kind of order), accessor type functions are written. Because I'm programming by myself, I don't always set the member private right away, but somewhere down the evolution of the class the member will be "promoted" to either protected or private.
3) Classes that are fully fleshed out and have strict rules about their internals (ie they know exactly what they are doing, and you are not to "fuck" (technical term) with its internals) are given the class designation, default private members, and only a select few members are allowed to be public.
I find this approach allows me to avoid sitting there and religiously writing getter/setters when a lot of data members get migrated out, shifted around, etc. during the early stages of a class's evolution.
There is a good reason to consider using accessors is there is no property inheritance. See next example:
public class TestPropertyOverride {
public static class A {
public int i = 0;
public void add() {
i++;
}
public int getI() {
return i;
}
}
public static class B extends A {
public int i = 2;
#Override
public void add() {
i = i + 2;
}
#Override
public int getI() {
return i;
}
}
public static void main(String[] args) {
A a = new B();
System.out.println(a.i);
a.add();
System.out.println(a.i);
System.out.println(a.getI());
}
}
Output:
0
0
4
Getters and setters are used to implement two of the fundamental aspects of Object Oriented Programming which are:
Abstraction
Encapsulation
Suppose we have an Employee class:
package com.highmark.productConfig.types;
public class Employee {
private String firstName;
private String middleName;
private String lastName;
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getMiddleName() {
return middleName;
}
public void setMiddleName(String middleName) {
this.middleName = middleName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getFullName(){
return this.getFirstName() + this.getMiddleName() + this.getLastName();
}
}
Here the implementation details of Full Name is hidden from the user and is not accessible directly to the user, unlike a public attribute.
There is a difference between DataStructure and Object.
Datastructure should expose its innards and not behavior.
An Object should not expose its innards but it should expose its behavior, which is also known as the Law of Demeter
Mostly DTOs are considered more of a datastructure and not Object. They should only expose their data and not behavior. Having Setter/Getter in DataStructure will expose behavior instead of data inside it. This further increases the chance of violation of Law of Demeter.
Uncle Bob in his book Clean code explained the Law of Demeter.
There is a well-known heuristic called the Law of Demeter that says a
module should not know about the innards of the objects it
manipulates. As we saw in the last section, objects hide their data
and expose operations. This means that an object should not expose its
internal structure through accessors because to do so is to expose,
rather than to hide, its internal structure.
More precisely, the Law of Demeter says that a method f of a class C
should only call the methods of these:
C
An object created by f
An object passed as an argument to f
An object held in an instance variable of C
The method should not invoke methods on objects that are returned by any of the allowed functions.
In other words, talk to friends, not to strangers.
So according this, example of LoD violation is:
final String outputDir = ctxt.getOptions().getScratchDir().getAbsolutePath();
Here, the function should call the method of its immediate friend which is ctxt here, It should not call the method of its immediate friend's friend. but this rule doesn't apply to data structure. so here if ctxt, option, scratchDir are datastructure then why to wrap their internal data with some behavior and doing a violation of LoD.
Instead, we can do something like this.
final String outputDir = ctxt.options.scratchDir.absolutePath;
This fulfills our needs and doesn't even violate LoD.
Inspired by Clean Code by Robert C. Martin(Uncle Bob)
If you don't require any validations and not even need to maintain state i.e. one property depends on another so we need to maintain the state when one is change. You can keep it simple by making field public and not using getter and setters.
I think OOPs complicates things as the program grows it becomes nightmare for developer to scale.
A simple example; we generate c++ headers from xml. The header contains simple field which does not require any validations. But still as in OOPS accessor are fashion we generates them as following.
const Filed& getfield() const
Field& getField()
void setfield(const Field& field){...}
which is very verbose and is not required. a simple
struct
{
Field field;
};
is enough and readable.
Functional programming don't have the concept of data hiding they even don't require it as they do not mutate the data.
Additionally, this is to "future-proof" your class. In particular, changing from a field to a property is an ABI break, so if you do later decide that you need more logic than just "set/get the field", then you need to break ABI, which of course creates problems for anything else already compiled against your class.
One other use (in languages that support properties) is that setters and getters can imply that an operation is non-trivial. Typically, you want to avoid doing anything that's computationally expensive in a property.
One relatively modern advantage of getters/setters is that is makes it easier to browse code in tagged (indexed) code editors. E.g. If you want to see who sets a member, you can open the call hierarchy of the setter.
On the other hand, if the member is public, the tools don't make it possible to filter read/write access to the member. So you have to trudge though all uses of the member.
Getters and setters coming from data hiding. Data Hiding means We
are hiding data from outsiders or outside person/thing cannot access
our data.This is a useful feature in OOP.
As a example:
If you create a public variable, you can access that variable and change value in anywhere(any class). But if you create as private that variable cannot see/access in any class except declared class.
public and private are access modifiers.
So how can we access that variable outside:
This is the place getters and setters coming from. You can declare variable as private then you can implement getter and setter for that variable.
Example(Java):
private String name;
public String getName(){
return this.name;
}
public void setName(String name){
this.name= name;
}
Advantage:
When anyone want to access or change/set value to balance variable, he/she must have permision.
//assume we have person1 object
//to give permission to check balance
person1.getName()
//to give permission to set balance
person1.setName()
You can set value in constructor also but when later on when you want
to update/change value, you have to implement setter method.
This question already has answers here:
Are getters and setters poor design? Contradictory advice seen [duplicate]
(16 answers)
Closed 9 years ago.
I have been going through clean code book which states that the class should not expose the internal state of its data and only should be exposing the behavior. In case of a very simpl and dumb java bean exposing the internal state which getter's and setters, is it not worth just removing them and make the private members public? Or just treat the class as a data structure?
I don't think so. It depends of the lifetime of your Object and its "exposure" (external modification).
If you're only using it as a data structure, exposing fields in secure way (final) sounds enough:
public class Person {
public final String firstName;
public final String lastName;
public Person(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}
}
The term POJO was intended to distinguish classes from JavaBeans or any other convention. As such a POJO is by definition NOT required to do anything.
I have been going through clean code book which states that the class should not expose the internal state of its data and only should be exposing the behavior.
This is called encapsulation and a good principle.
In case of a very simpl and dumb java bean exposing the internal state which getter's and setters, is it not worth just removing them and make the private members public?
That is an alternative approach. Some projects may forbid this approach while others may encourage it. Personally, I would favour this approach for classes which are encapsulated in some way already e.g. they are package local.
There is a view that some day in some way your class might have additional requirements and changing the "API" will be impossible. This goes against the YAGNI principle and very rarely proves to be the case and when it does has a much lower cost than adding lots of methods which don't do anything.
However, this is not always the case and if you don't use accessor methods you should consider what the impact will be on the project if you have to change it later. Using accessor methods every where means you never need to worry about this.
In summary, if you are pretty sure accessor methods are pointless and it won't be a problem to add them later, I would say you should use your judgement. However if you are not sure if it could be a problem in the future or you don't want to have to worry about it, use accessor methods.
The definition of POJO doesn't mandate getter/setter.
Experimentally, I am not using getter and setter in my current project.
The approach I am taking is this one:
unless necessary, don't provide getter/setter.
So far, I didn't find a case where I really needed get/set.
Some friend told me: "having get/set is helpful if in the future you need xyz"; my reply has been: when -in the future- I need to do so, I will provide the getter and setter; I don't want to anticipate anything.
The objection about incapsulation, that some may raise, is not really a valid one: providing getter and setter breaks incapsulation in the same manner, plus you have additional lines of (useless) code. Bugs may also lay in getter and setters.
This is an example of one of a non-trivial domain class:
public class SSHKey implements IsSerializable {
public Long id;
public Long userId;
public String type;
public String bits;
public String fingerprint;
public String comment;
#SuppressWarnings("unused")
private SSHKey() { // required by gwt-rpc
}
public SSHKey(String text) throws InvalidSSHKeyException {
Ensure.that(text != null, new InvalidSSHKeyException("Invalid Key"));
text = text.trim();
String[] parts = text.split(" ", 3);
Ensure.that(parts.length >= 2,
new InvalidSSHKeyException("Invalid Key"));
type = getType(parts);
Ensure.that(type.equals("ssh-rsa") || type.equals("ssh-dss"),
new InvalidSSHKeyException(
"Key must start with 'ssh-rsa' or 'ssh-dss'"));
bits = getBits(parts);
comment = getComment(parts);
}
private String getBits(String[] parts) {
return parts[1];
}
private String getComment(String[] parts) {
if (parts.length == 3)
return parts[2];
return type + " " + bits.substring(0, min(15, bits.length())) + "...";
}
private String getType(String[] parts) {
return parts[0];
}
}
The constructor takes the responsibility to validate and prepare the data to be manageable. Thus this logic doesn't need to be in a setter/getter.
If I was shown object with public members some years ago, I would probably not like them; maybe I am doing something wrong now, but I am experimenting and so far it is ok.
Also, you need to consider if your class is designed to be extended or not (so, foresee the future is part of the requirements), and if you want your object to be immutable. Those things you can only do with get/set.
If your object must be immutable, and you can avoid the empty constructor, you can just add 'final' to the member instances, btw.
Unfortunately I had to add IsSerializable (similar to java.io.Serializable) and an empty constructor since this is required by gwt. So, you could tell me then "you see? you need the getter an setter"; well not so sure.
There are some jdbc frameworks which promote the use of public fields btw, like http://iciql.com
This doesn't imply that this project is correct, but that some people are thinking about it.
I suppose that the need of getter/setter is mostly cultural.
The issue with making the members accessible is that you no longer control them from inside the class.
Let's say that you make Car.speed accessible. Now, everywhere in you program there can be some reference to it. Now, if you want to make sure that speed is never set a negative value (or to make the change synchronized because you need to make it thread safe), you have to either:
in all the points where speed is accessible, rewrite the program to add the control. And hope that everybody that changes the program in the future remembers to do so.
make the member private again, create the getter and setter methods, and rewrite the program to use them.
Better get used to write getter and setter from the beginning. Nowadays, most IDEs do it automatically for you, anyway.
The canonical answer to this is: You don't know whether your simple data structure will stay so simple in the future. It might evolve more than you expect now. It might be also possible, that anytime soon you want some "value changed" observer in that bean. With getter and setter methods you can do this very simply later without changing you existing codebase.
Another pro point for getter/setter is: If in Rome, do like the Romans... Which means in this case: Many generic frameworks expect getter/setter. If you don't want to rule all these usefulls frameworks out right from the start then do you and your colleagues a favour and simply implement standard getter/and setters.
Only if you expose a class in a library that's used beyond your control.
If you do release such a library, the Uniform Access Principle dictates that you should use getters and setters in order to be able to change the underlying implementation later without requiring clients to change their code. Java doesn't give you other mechanisms to do this.
If you use this class in your own system, there's no need: your IDE can easily encapsulate a public field and update all its usages in one safe step. In this case, brevity wins, and you lose nothing for the time where you need encapsulation.
I think it's a good idea to use getters and setters, unless you have very specific speed/memory/efficiency requirements or very simple objects.
A good example is a Point, where it is probably both nicer and more efficient to expose it's .x and .y variables.
That said, it will actually not be a big effort to change the visibility of a few member variables and introduce getters and setters even for a large codebase, if you suddenly require some logic in a setter.
JavaBeans require getters and setters. POJOs do not, anyway this has its benefits
The objetive of the getters and setters is to achieve encapsulation, which manages the internal state of object. This allows you to add or change business rules in your application after the application has been implemented only change the getter or setter code, example, if you have a text field that only allows for more than 3 characters can check before assigning it to an attribute and throw an exception, other reason for not doing this is if it's possible you'll want to change the implementation or change variable names or something like. This cannot be enforced if the field is publicly accessible and modifyable
anyway you can use your IDE to generate setters and getters.
If you are developing a simple application can be recommended, if your application is complex and must give maintenance is not recommend.
for the data-type objects, like POJO / PODS / JavaBean, at python you have only public members
you can set those and get those easily, without generating boilerplate setter and getter code(in java these boilerplate code usually(98%) exposes the inner private tag as noted in the question)
and at python in the case you would need to interact with a getter, then you just define extra code only for that purpose
clean and effective at the language level
at java they chose the IDE development instead of changing base java, see JavaBean e.g. how old that is and java 1.0.2 is how old...
JDK 1.0 (January 23, 1996)
The EJB specification was originally developed in 1997 by IBM and later adopted by Sun Microsystems (EJB 1.0 and 1.1) in 1999
so just live with it, use the setter getter because those are enforced by java surroundings
That's the true what #Peter Lawrey explains about encapsulation.
Only one note: it's more important, when you are working with complex objects (for example in the domain model in a ORM project), when you have attributes that aren't simple Java types. For example:
public class Father {
private List childs = new ArrayList();
public Father() {
// ...
}
private List getChilds() {
return this.childs;
}
public void setChilds(List newChilds) {
this.childs = newChilds;
}
}
public class Child {
private String name;
// ...
private String getName() {
return this.name;
}
public void setName(String newName) {
this.name = newName;
}
}
If you expose one attribute (like the childs attribute in the Father class) as a public, you won't be able to identify what part of your code are setting or changing one property of your exposed attribute (in the case, for example, adding new Child to a Father or even changing the name of a existing Child). In the example, only a Father object can retrieve the childs content and all the rest of the classes can change it, using its setter.
I'm just trying to find some examples of Martin Fowler's Domain Model pattern and I can't.
From what I found on the Internet Domain Model is just adding some "logic" methods to classes. For example
public class Income {
private String title;
private String comment;
private String date;
private Double amount;
private Integer category;
public ExIn(String title, String comment, Double amount, Integer category, String date) {
this.title = title;
this.comment = comment;
this.date = date;
this.amount = amount;
this.category = category;
}
public Integer getCategory() {
return category;
}
public void setCategory(Integer category) {
this.category = category;
}
// more getters and setters
// Domain Model part starts
public boolean isPositive()
{
return amount > 0 ? true : false;
}
// more methods like that
}
Did I understand it correctly? If not, I'd be grateful for a little example of Domain Model Pattern usage.
Did I understand it correctly? If not, I'd be grateful for a little
example.
Broadly, yes.
From Martin Fowler, domain model is an object model of the domain that incorporates both behavior and data.
The domain model is frequently opposed to a model where you have specific classes to bear data and some other specific classes to bear behavior/processings.
If I take your Income class, it looks like more as a class that holds properties/data than an domain model with a real behavior.
public boolean isPositive(){
return amount > 0 ? true : false;
}
is a kind of utility function that has no relation with the model.
You could put that in a Math class.
I will try to give you a domain model example and then the version where the model separates data and processing.
Suppose in the requirements of the domain of the application you are modeling, we need to add a bonus for incomes. This bonus may take place in winter for Christmas for example (but why not for other events)
Rather than having a service class to do this processing, we let domain model objects perform the task.
Incomes, a high level object could iterate on Income instances and apply the bonus and we could have a bonus rule class that defines the bonus according to some input values.
I introduce multiple classes since the idea is to allow each objects to collaborate according to their responsibilities.
Incomes :
public class Incomes {
List<Income> incomes = ...
....
public void applyBonus(BonusRule bonusRule){
for (Income income : incomes){
income.applyBonus(bonusRule);
}
}
Income :
public class Income {
private float amount;
...
public void applyBonus(BonusRule bonusRule){
float bonus = bonusRule.compute(this);
amount += bonus;
}
...
}
ChristmasRule :
public class ChristmasBonusRule implements BonusRule {
...
#Override
public float compute(Income income){
float bonus = ...
return bonus;
}
...
}
And finally, we could apply the processing in this way :
void foo(){
// create a domain object that has both behavior and data
Incomes incomes = ...;
// invoke a functional method on the object by passing another domain object
incomes.applyBonus(new ChristmasBonusRule());
}
In a design where you separate data and logic in distinct classes, it could look like more like that :
public class IncomeBonusService {
// stateless : no incomes data inside it
....
public void applyChristmasBonus(List<Income> incomes){
for (Income income : incomes){
// Christmas bonus computation here
float bonus = ...
income.setAmount(bonus + income.getAmount());
}
}
}
And we could apply the processing in this way :
// inject the service
#Autowired
IncomeBonusService incomeBonusService;
void foo(){
// create a domain object that has only data
List<Income> incomes = ...;
// invoke a service method by passing data as parameter
incomeBonusService.applyChristmasBonus(incomes);
}
A model design where the objects have no behavior (only getter/setter) is called Anemic Domain Model.
Big differences between the two ways illustrated by this example :
Domain model :
The objects are meaningful.
Behavioral responsibility finely defined between classes.
So good isolation, testability and maintainability.
For example, adding/removing/unit-testing a BonusRule is easy.
Objects responsible of their state.
Indeed, no need to provide setters as the object can itself update its state after collaborating with other objects.
We could see that in Amount.applyBonus() :
float bonus = bonusRule.compute(this);
amount += bonus;
Anemic Domain Model :
All the logic is in the service class.
So a single place to get the code.
With few lines, it is fine.
But note that this advantage has a certain limit because as the logic becomes big or complex, the best thing is often splitting the logic in multiple service classes.
But whatever the number of Service classes you need, the whole logic is located in the service classes and not somewhere else. Which may ease development norms if we compare it to the domain model where the logic may be exploded in some different "types" of classes.
Necessity to provide getter/setter for domain classes.
The domain is not responsible of its state and its invariant rules either.
So any class that depends on the domain class can "break" its state.
As a side note, some frameworks (for persistence, mapping, serialization, ...) rely by default on getter/setter.
That's why this model, despite its drawbacks, leads in some projects.
Fowler's book that you cite refers to Larman's book for introductory understanding and examples.
Interestingly, Larman's approach to domain modeling doesn't ever add behavior to domain classes.
There is a notion in a domain model that a class is conceptual and is not a software class, but that software classes are based on domain (conceptual) classes. Larman's approach to implementing behavior follows responsibility driven design, and GoF design patterns.
The domain model remains a separate element in the software development process. The good aspect of this way of modeling is you separate the problem from the solution. A domain model is supposed to be true to the problem (capture requirements from the problem domain without addressing implementation details).
Larman presents "operation contracts" as a way to assure behaviors are consistent inside a domain model. Again, contracts are supposed to be independent of a solution (an implementation). Contracts have postconditions that describe a constraint in the domain model after an operation has taken place. An example of a postcondition would be that when a customer completes a purchase in a store, the sale object is associated with each of the items the customer purchased.
The implementation of the business logic should respect the contracts (postconditions) defined for the domain model. Larman's approach with the Controller GRASP pattern as well as other GRASP patterns ends up putting this logic in various classes (usually the domain layer, which is software classes inspired by conceptual classes in the domain model) or Façade (Controller) classes that handle the system operations.
Larman's approach is more complicated than this explanation, but the point is that behavior is never only defined alone in the domain model as methods. Larman says many times that domain (conceptual) classes do not have methods, as they are not software classes.
Fowler's book also refers to another book that he wrote on Analysis Patterns for examples.
The patterns come from various domains, including health care, financial trading, and accounting. Each of the patterns is described both textually and in a simple pre-UML notation (this book was written before the UML had stabilized into a usable form).
None of the examples in that book show software classes, that is with methods defined in a programming language (that I could find).
I know of at least one book where domain models in fields such as molecular biology have been published in UML. Here's an example (note the UML is modified -- sub-type boxes are shown in super-type boxes -- to save space):
The book above does not model behaviors, probably because they really depend on the requirements of the software application. These models capture some business rules, such as:
Each Chemical Formulation must be composed of 2 or more of either Chemical Elements or Chemical Compounds, or both.
But the models in that book are mostly data models.
Googling will find you this huge model for the Biomedical Research Integrated Domain Group (BRIDG). Drilling down to the Molecular Biology sub-domain, and to the class Gene, for example, you'll see it has no behavior, nor do any of the other (conceptual) classes in this domain model.
Are domain models in a programming language or not?
Larman's philosophy clearly shows them as programming language-independent (conceptual as opposed to software classes), as a separate artifact from code, to tie them explicitly to the problem domain (requirements).
On the other hand, you'll find Fowler saying, "I prefer POJO domain models.", which is pretty much saying domain models are defined in code.
Eric Evans' DDD makes the assumption that an important degree of complexity in much of software development comes from the domain, and so a model of such complex domains is essential to managing the complexity. Therefore, Domain Modeling is necessary when domains are complex. DDD suggests using a domain modeling language that is ubiquitous; that is, common to the domain experts and the developers. This would imply that in at least some cases, a domain model would not be defined in a programming language.
There is a related question that might shed some light (although it has generated a lot of heat). Some have criticized the question's example as being too trivial (not complex enough) for a justified domain model.
A "domain model" is simply an object which represents some discernible concept in your business domain. A "Customer", an "Order", etc. Whatever the business logic is, the tangible entities which make up that logic are the models in your domain.
Some will have lots of business logic (perhaps worth breaking up into other classes), some will have very little (or even none).
The difference between a "domain model" and any other class isn't a construct of the Java language itself, it's mainly a semantic construct of the business logic that you define.
Following are the two approaches:
constructor with all the class properties
Pros: I have to put an exact number of types of parameters so if I make an error the compiler warns me (by the way, is there a way to prevent the problem of having erroneously switched two Integer on the parameter list?)
Cons: if I have lots of properties the instantiation line can become really long and it could span over two or more lines
setters and the default empty constructor
Pros: I can clearly see what I'm setting, so if I'm doing something wrong I can pinpoint it as soon as I'm typing it (I can't make the previuos error of switching two variables of the same type)
Cons: the instantiation of an object with lots of properties could take several lines (don't know if this is really a con) and if I forget to set a property the compiler doesn't say anything.
What will you do and why?
Do you know of any light pattern (consider that it should be used everytime an object wth 7+ properties is instantiated) to suggest?
I'm asking this because I tend to dislike large constructors where I can't figure out fast where is the variable I'm looking for, on the other hand I find the "set all properties" vulnerable to missing some of the properties.
Feel free to argument my assumptions in pros and cons as they are only mine thoughts :)
Update - a question I've found which is related to this: Building big, immutable objects without using constructors having long parameter lists
You've missed the biggest pro of having a constructor with loads of parameters: it lets you create immutable types.
The normal way of creating immutable types without huge constructor nastiness is to have a helper type - a builder which maintains the values you'll want in your final object, then builds the immutable object when you're ready.
You might look at the Builder pattern advocated by Joshua Bloch, and described in Effective Java. There's a presentation with the main points at http://developers.sun.com/learning/javaoneonline/2007/pdf/TS-2689.pdf; no doubt you could dig up a better reference.
Basically, you have another class, probably an inner class, which provides methods named after the properties being set, and which return the original builder so you can chain calls. It makes for quite a readable chunk of code.
For example, let's suppose I have a simple Message with a few properties. The client code constructing this could use a builder to prepare a Message as follows:
Message message = new Message.Builder()
.sender( new User( ... ) )
.recipient( new User( ... ) )
.subject( "Hello, world!" )
.text( messageText )
.build();
A fragment of Message.Builder might look similar to the following:
public class Builder {
private User sender = null;
// Other properties
public Builder sender( User sender ) {
this.sender = sender;
return this;
}
// Methods for other properties
public Message build() {
Message message = new Message();
message.setSender( sender );
// Set the other properties
return message;
}
}
Recent academic research (CMU and Microsoft) on API usability suggests that default constructors with setters would be the way to go in terms of usability.
This is from "Usability Implications of Requiring Parameters in Objects' Constructors" by Jeff Stylos and Steven Clarke and was presented at the International Conference on Software Engineering:
Abstract:
The usability of APIs is increasingly important to programmer productivity. Based on experience with usability studies of specific APIs, techniques were explored for studying the usability of design choices common to many APIs. A comparative study was performed to assess how professional programmers use APIs with required parameters in objects' constructors as opposed to parameterless "default" constructors. It was hypothesized that required parameters would create more usable and self-documenting APIs by guiding programmers toward the correct use of objects and preventing errors. However, in the study, it was found that, contrary to expectations, programmers strongly preferred and were more effective with APIs that did not require constructor parameters. Participants' behavior was analyzed using the cognitive dimensions framework, and revealing that required constructor parameters interfere with common learning strategies, causing undesirable premature commitment.
You mention it in your post, but I think this is an important point that deserves more attention: unless every input parameter is a different type, the big problem with huge constructors is that it's very easy to transpose a couple of variables. The compiler is an unreliable safety net -- it will catch some mistakes, but the ones that slip through are going to be much more difficult to identify and debug. Especially because the input list for a huge constructor is quite opaque unless you've got the API open in another window.
Getters and setters are vastly easier to debug, especially if you institute safeguards that throw a runtime exception if the object isn't properly populated. And I'm a huge fan of "easy to debug."
Prior to this thread I'd never heard of the Builder pattern Rob mentions. Never used it myself (obviously), but it's damned intriguing.
I prefer taking constructor arguments, for the aforementioned immutability reasons. If that gives you a constructor that takes lots of arguments (say more than four or so), that's a code smell to me: some of those arguments should be bundled together into their own types.
For example, if you have something like this:
class Contact
{
public Contact(string firstName, string lastName, string phoneNumber,
string street, string city, string state, int zipCode) { ... }
}
I'd refactor it to:
class Contact
{
public Contact(Person person, PhoneNumber number, Address address) { ... }
}
class Person
{
public Person(string firstName, string lastName) { ... }
}
class PhoneNumber
{
public PhoneNumber(string digits) { ... }
}
class Address
{
public Address(string street, string city, string state, int zipCode) { ... }
}
Too-large classes are a really common design problem in OOP codebases.
There are other aspects as well. If you want to be able to certain things with your class at design time rather than just at runtime, for example adding your class as an object in the Object Palette (this is Java using Netbeans) you need to provide a no-argument constructor in order to be able to do so.
There are other strategies here, too. Before trying to figure out how to deal with lots of parameters, I think it is important to re-visit your design and look at whether your class is doing too much. See if you can group some of the parameters together into a new class, and move some behavior into that class.
setters and the default empty constructor
JRL obliquely touched on it, but one reason to consider using setters is to have the object conform to the JavaBean specification. This makes instances amenable to editing via introspection tools and persistence using certain serialization techniques.
Who says you can't do both? I'd say mandatory properties go into the constructor, optional ones are handled with setters. BTW, who says you always need one setter per property? If two properties belong together conceptually, why not set them together?
I like the Builder pattern too, but the most important rule is: always use your brain and find the design that best fits the specific problem. There's no one-size-fits-all solution.