I could not find any references online about this. But just wanted to know if final fields in a class should always be static or is it just a convention. Based on my understanding of their uses, I feel that it is more of a logical thing to do than something that is imposed by the language.
Of course not. They must be static if they belong to the class, and not be static if they belong to the instance of the class:
public class ImmutablePerson {
private static final int MAX_LAST_NAME_LENGTH = 255; // belongs to the type
private final String firstName; // belongs to the instance
private final String lastName; // belongs to the instance
public ImmutablePerson(String firstName, String lastName) {
if (lastName.length() > MAX_LAST_NAME_LENGTH) {
throw new IllegalArgumentException("last name too large");
}
this.firstName = firstName;
this.lastName = lastName;
}
// getters omitted for brevity
}
No, absolutely not - and it's not a convention.
static and final are entirely different things. static means that the field relates to the type rather than any particular instance of the type. final means that the field can't change value after initial assignment (which must occur during type/instance initialization).
static final fields are usually for constants - whereas instance fields which are final are usually used when creating immutable types.
They don't always come together and it's not a convention. final fields are often used to create immutable types:
class Person {
private final String name;
private final int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
}
On the other hand static but not final fields are not that common and are quite tricky. static final is seen often because it means application1-wide constant.
1 - well, class loader-wide, to be precise
Final fields do not need to be static, and sometimes it can be useful to have a non-static final instance variable. Fields that are marked both static and final are usually used for constants, like this:
public static final int BORDER_WIDTH = 5;
However, sometimes you'll see a non-static final field when an object has a immutable property. Usually, non-static final fields are still marked private for the usual reasons, though, so it's more of an extra check so the compiler can make sure you're never setting the property again.
If you want to access them like ClassName.FIELD, then yes, you have to do that. If you don't make it static, you have to do something like new ClassName().FIELD, which is unnecessary and a pointless creation of an object.
However, if you are only using it in the class or making it private, then don't make it static. If you are within the actual class, you can just do FIELD.
To fully grasp this concept, you have to know what static means. Static means that it belongs to the actual class, not an instance of it.
Absolutely not. Consider:
class Point {
public final int x;
public final int y;
public Point(int _x, int _y) {
x = _x;
y = _y;
}
}
Drop the final, and the class becomes mutable. Add a static, and all your points are the same, and there is no legal way to write the constructor.
Absolutely not. Immutable objects, for example, have final properties, that can be set only once, by the constructor.
For more information, please see: http://docs.oracle.com/javase/tutorial/essential/concurrency/imstrat.html
Immutable objects are not the only case in which final properties are used, but they provide a evident example of their usefulness.
The answer is no.
static
"Indicates that only one such data field is available for all
instances of this class. Without this modifier, each instance has its
own copy of a data field"
...meaning there can only be one of this
final
"The value provided for the data field cannot be modified"
...meaning that this is a constant
Related
The below class doesn't have final keyword but its member variables are private and final and the class exposes no mutate/set methods. Is this class immutable or not?
public class Abc {
private final int id;
private final String name;
public Abc(int id, String name) {
this.id = id;
this.name = name;
}
public String getName() {
return name;
}
public int getId() {
return id;
}
}
The class itself is immutable, yes - if you create an instance of just Abc, no aspect of that can be changed after the instance has been created.
However, that doesn't mean that any code receiving a parameter of type Abc can assume it's immutable with all the benefits that carries... because the class isn't final. It's entirely possible for an object of a type compatible with Abc to be mutable:
public class Mutable extends Abc {
private String value;
public Mutable(int id, String name) {
super(id, name);
}
public void setValue(String value) {
this.value = value;
}
#Override public String toString() {
return value;
}
}
Now imagine you've got code which deals with an Abc:
public class AbcConsumer {
private final Abc abc;
public AbcConsumer(Abc abc) {
this.abc = abc;
}
// No need to create a defensive copy or anything like that...
// abc is immutable, right?
public Abc getAbc() {
return abc;
}
}
Here the consumer assumes it's fine to treat Abc as if it's an immutable class - but if someone creates an AbcConsumer by passing in a Mutable instance instead of "vanilla" Abc instance, it could cause problems.
That's why it's generally a good idea when you're creating an immutable type to make it final as well - that way any consumers know that if they receive a reference with that type, it's definitely immutable.
To put it another way: yes, the Abc class is immutable... but you can't assume that a reference with a compile-time type of Abc refers to an immutable object.
As presented, yes, the class is immutable.
The "final" keyword on a class declaration prevents it from being extended - it's not related to immutability (unless your variables are declared public or protected).
Edit; "not related" is a poor choice of words, please see Jon Skeet's answer below
No, it is most likely not.
A problem is terminology. What do you mean by class? If you mean this code, sure, it's immutable. But 'this code' is not something that is particularly relevant to the concept of immutability. That usually makes a lot more sense if we consider it: this type.
As in, is the type Abc immutable?
As in, given:
public void foo(Abc abc) { ... }
is it safe to assume that the received abc couldn't possibly change?
And then the answer is no. That is not safe to assume: The type Abc is mutable.
The reason is that someone could make this:
class SneakyAbc extends Abc {
private int id;
public void setId(int id) {
this.id = id;
}
public String getId() {
return id;
}
}
This is why immutable classes are virtually always made final, to fully guarantee it.
Depending on how fancy you want to paint with the brush of 'what does this term mean', if all methods of Abc are final, you can consider it immutable as well if you really want to: Whilst the class does not need to be immutable (a subclass can add a new non-final field and create getters and setters for this), all the stuff you can 'witness' from the Abc type, assuming you don't use reflection, do appear immutable.
Exactly what definition of immutable you go with would be required knowledge to delve any further.
Note that something like java.io.File has only final fields and is final, and yet, it has easily observable state that can be modified: just.. delete the file, and voila you can see it. You can pull similar stunts with an IdentityHashMap to create a faux but nevertheless very observable 'field'.
Thus, 'immutable' as a concept: Useful. As a boolean flag to bestow upon a certain type or some java source file: Not useful.
Records
Other Answers directly addressed your questions about immutability, class being marked final, and subclasses being mutable. I’ll add an alternative option to more briefly accomplish your goal of immutability: Records.
Java 16 brings the new records feature. If the main purpose of your class is to immutably and transparently carry data, define your class as a record. The compiler implicitly creates default constructor, getters, equals & hashCode, and toString.
A record is implicitly final, so no risk of a subclass becoming mutable.
Declare the properties in parentheses. By default, you need not put anything in the curly braces body of a record.
record Abc ( int id , String name ) {}
Instantiate like any other class.
Abc x = new Abc ( 42 , "Snuffleupagus" ) ;
The implicit getter methods are simply the property names. The JavaBeans-style get…/is… method naming is not used. (You could add such methods if required.)
System.out.println( x.name() ) ;
Snuffleupagus
It is mutable if its internal states can be changed after the class is created.
In your example, although there is no class final, the inside situations cannot be changed again because of final keyword. In this way, the class becomes immutable again
This question already has answers here:
What is meant by immutable?
(17 answers)
Closed 3 years ago.
I was reading about immutable classes, and the ways to make a class immutable were said to be:
1 - Make the class final to prevent inheritance
2 - Make the mutable variables final
3 - Don't provide setter methods.
I think the third condition is unnecessary. When we make a variable final and provide any value to it, after that new value can't be assigned to it even through a setter method (because final variables can't be changed once a value is assigned to it). So why do we need the third condition of not having setter methods?
Am I understanding something in a wrong way?
public class Person{
private String name;
public Person(String name){
this.name = name;
}
public String getName(){
return this.name;
}
public void setName(String name) {
this.name = name;
}
}
Now, it is clear that Person is not an Immutable class. This doesn't mean, an instance of Person can't be a member of another class that is (supposedly) immutable.
public final class MyImmutableClass {
// p is final, so it can't be re-referenced
private final Person p;
public MyImmutableClass(Person p) {
this.p = p;
}
// it can be altered, though
public void setPersonName(String name) {
this.p.setName(name);
}
public String toString() {
return "Person: " + p.getName();
}
}
Now, we have an immutable class, but, it does contain a setter. This setter actively changes a member of the (final) field p.
public static void main(String[] args) {
MyImmutableClass c = new MyImmutableClass(new Person("OriginalName"));
System.out.println(c);
c.setPersonName("AlteredName");
System.out.println(c);
}
And .. there you have it. The member is changed, through the setter (even though the variable was final). Do understand, a 'final variable' is NOT necessarily a constant, in most cases it's state can be changed. The point of a final variable, is that it can not be re-referenced. Note, we could also have a method like this:
public void setPerson(Person p) {
this.p.setName(p.getName());
}
A final variable itself is only a constant in case the type itself is an immutable type, or if it's a primitive, but you should understand that most types out there are mutable.
Is the type immutable or a primitive and it's declared final? Sure, add a setter. But, to what end? Misleading people who use your class?
You are sort of right. A setter, by definition, replaces a field with the given value. If all of the fields are final then you couldn't possibly provide a setter anyway.
My description of how to write an immutable class would be:
Make all fields final
Make sure the type of every field is itself immutable
It's possible to write an immutable class with fields that are mutable if you take great care to ensure that they never change but you need to be very careful in this case.
Yes, it could be reduced to
Make the class final to prevent inheritance;
Make the mutable variables final, and so don't bother providing any setters;
However for education purposes the shorter bullet points probably work better - even if slightly redundant.
The variable mentioned in item 2 can be a reference, which can be mutable (like a list or set) even when the variable itself is final.
This is why we have utilities like Collections.unmodifiableList to make mutable classes virtually immutable.
So the prohibition on setters is to prevent accidentally changing a final variable's state.
Making fields final is not sufficient (or even required) to guarantee immutability. You need to make defensive copies of mutable objects within the immutable class.
class Foo {
private String str;
public Foo(String str) {
this.str = str;
}
public String getString() {
return str;
}
}
The above class is immutable because:
String is immutable.
The field 'str' is private and can't be altered.
Now considering the following MyDate class.
public class MyDate {
private Date; // date is not immutable
public MyDate(Date date) {
this.date = date;
}
public Date getDate() {
return date;
}
}
The MyDate class above is not immutable because the user can do the following:
Date d = new Date(<someDate>);
MyDate md = new MyDate(d);
d.set(<someDate>); // oops, just changed value in MyDate via external reference.
The same could also be done via getDate().
To make MyClass immutable, make defensive copies of Date in the constructor and getter. These prevent the user of the class from changing the date field either:
Using a reference to the constructor argument.
Upon retrieving the date field via a getter
public class MyDate {
private Date; // date is not immutable
public MyDate(Date date) {
this.date = new Date(date);
}
public Date getDate() {
return new Date(date);
}
}
This question already has answers here:
Should I instantiate instance variables on declaration or in the constructor?
(15 answers)
Closed 7 years ago.
so let's say i have a class called city. what's the difference from where i initialize its fields? e.g.
public class City {
private String cityName;
private int population;
private boolean goodPeopleLiveThere;
City() {
cityName = "las vegas";
population = 603488;
goodPeopleLiveThere = true;
}
}
why would i initialize in the constructor rather than the fields or vice versa?
see the ambiguity i face is typically i would set them as parameters in the constructor and then initialize them in the main() when i instantiate my class, but then some tutorials i've seen used initialized them like aforementioned, and i'm yet to fully understand the implications of initialize in fields/constructor rather than in the object.
Usually, people use that format for functionality. Take the following for example
private int houseNumber;
private String houseStreet;
public House(int houseNumber, String houseStreet) {
this.houseNumber = houseNumber;
this.houseStreet = houseStreet;
}
Now this way, you can do things like the following much more easily.
public static void main(String[] args) {
House randomHouse = new House(12, "Main Street");
House otherHouse = new House(69, "Random Ave.");
}
instead of having to create a new class for each house.
There's no difference with these set variables. There may be a difference, however, if these variables depend on user input. For instance:
public class City{
private String cityName;
private int population;
private boolean goodPeopleLiveThere;
City(String city, int pop, boolean good)){
cityName = city;
population = pop;
goodPeopleLiveThere = good;
}
}
Now, in this example, a these variables rely on a value submitted upon instanciation, which is not at all rare when it comes to a constructor.
There is only a slight difference, besides personal preference. However, fields are initialized before constructor bodies are ran. This can cause errors if one it to override a field in the constructor that was initialized previously.
Most important is that one should be consistent throughout their code.
Initializing object-fields outside of the constructor is acceptable in some cases, for example, the builder-pattern is one of the static factory patterns that uses a nested class to initialize the instance members.
There's no "one better than the other" - simply different methods with pros and cons for each one of them.
When you initialize variables outside the constructor you should be careful not to publish the object before it's fully constructed otherwise you'll get yourself in a mess that will be difficult to debug.
How can I use the set and get methods, and why should I use them? Are they really helpful? And also can you give me examples of set and get methods?
Set and Get methods are a pattern of data encapsulation. Instead of accessing class member variables directly, you define get methods to access these variables, and set methods to modify them. By encapsulating them in this manner, you have control over the public interface, should you need to change the inner workings of the class in the future.
For example, for a member variable:
Integer x;
You might have methods:
Integer getX(){ return x; }
void setX(Integer x){ this.x = x; }
chiccodoro also mentioned an important point. If you only want to allow read access to the field for any foreign classes, you can do that by only providing a public get method and keeping the set private or not providing a set at all.
I want to add to other answers that setters can be used to prevent putting the object in an invalid state.
For instance let's suppose that I've to set a TaxId, modelled as a String. The first version of the setter can be as follows:
private String taxId;
public void setTaxId(String taxId) {
this.taxId = taxId;
}
However we'd better prevent the use to set the object with an invalid taxId, so we can introduce a check:
private String taxId;
public void setTaxId(String taxId) throws IllegalArgumentException {
if (isTaxIdValid(taxId)) {
throw new IllegalArgumentException("Tax Id '" + taxId + "' is invalid");
}
this.taxId = taxId;
}
The next step, to improve the modularity of the program, is to make the TaxId itself as an Object, able to check itself.
private final TaxId taxId = new TaxId()
public void setTaxId(String taxIdString) throws IllegalArgumentException {
taxId.set(taxIdString); //will throw exception if not valid
}
Similarly for the getter, what if we don't have a value yet? Maybe we want to have a different path, we could say:
public String getTaxId() throws IllegalStateException {
return taxId.get(); //will throw exception if not set
}
I think you want something like this:
public class Person {
private int age;
//public method to get the age variable
public int getAge(){
return this.age
}
//public method to set the age variable
public void setAge(int age){
this.age = age;
}
}
You're simply calling such a method on an object instance. Such methods are useful especially if setting something is supposed to have side effects. E.g. if you want to react to certain events like:
public void setAge(int age){
this.age = age;
double averageCigarettesPerYear = this.smokedCigarettes * 1.0 / age;
if(averageCigarettesPerYear >= 7300.0) {
this.eventBus.fire(new PersonSmokesTooMuchEvent(this));
}
}
Of course this can be dangerous if somebody forgets to call setAge(int) where he should and sets age directly using this.age.
Setters and getters are used to replace directly accessing member variables from external classes. if you use a setter and getter in accessing a property, you can include initialization, error checking, complex transformations, etc. Some examples:
private String x;
public void setX(String newX) {
if (newX == null) {
x = "";
} else {
x = newX;
}
}
public String getX() {
if (x == null) {
return "";
} else {
return x;
}
}
Having accessor methods is preferred to accessing fields directly, because it controls how fields are accessed (may impose data checking etc) and fits with interfaces (interfaces can not requires fields to be present, only methods).
Some benefits of using getters and setters (known as encapsulation or data-hiding):
(originally answered here)
1. The fields of a class can be made read-only (by only providing the getter) or write-only (by only providing the setter). This gives the class a total control of who gets to access/modify its fields.
Example:
class EncapsulationExample {
private int readOnly = -1; // this value can only be read, not altered
private int writeOnly = 0; // this value can only be changed, not viewed
public int getReadOnly() {
return readOnly;
}
public int setWriteOnly(int w) {
writeOnly = w;
}
}
2. The users of a class do not need to know how the class actually stores the data. This means data is separated and exists independently from the users thus allowing the code to be more easily modified and maintained. This allows the maintainers to make frequent changes like bug fixes, design and performance enhancements, all while not impacting users.
Furthermore, encapsulated resources are uniformly accessible to each user and have identical behavior independent of the user since this behavior is internally defined in the class.
Example (getting a value):
class EncapsulationExample {
private int value;
public int getValue() {
return value; // return the value
}
}
Now what if I wanted to return twice the value instead? I can just alter my getter and all the code that is using my example doesn't need to change and will get twice the value:
class EncapsulationExample {
private int value;
public int getValue() {
return value*2; // return twice the value
}
}
3. Makes the code cleaner, more readable and easier to comprehend.
Here is an example:
No encapsulation:
class Box {
int widthS; // width of the side
int widthT; // width of the top
// other stuff
}
// ...
Box b = new Box();
int w1 = b.widthS; // Hm... what is widthS again?
int w2 = b.widthT; // Don't mistake the names. I should make sure I use the proper variable here!
With encapsulation:
class Box {
private int widthS; // width of the side
private int widthT; // width of the top
public int getSideWidth() {
return widthS;
}
public int getTopWIdth() {
return widthT;
}
// other stuff
}
// ...
Box b = new Box();
int w1 = b.getSideWidth(); // Ok, this one gives me the width of the side
int w2 = b.getTopWidth(); // and this one gives me the width of the top. No confusion, whew!
Look how much more control you have on which information you are getting and how much clearer this is in the second example. Mind you, this example is trivial and in real-life the classes you would be dealing with a lot of resources being accessed by many different components. Thus, encapsulating the resources makes it clearer which ones we are accessing and in what way (getting or setting).
Here is good SO thread on this topic.
Here is good read on data encapsulation.
The above answers summarize the role of getters and setters better than I could, however I did want to add that your code should ideally be structured to reduce the use of pure getters and setters, i.e. those without complex constructions, validation, and so forth, as they break encapsulation. This doesn't mean you can't ever use them (stivlo's answer shows an example of a good use of getters and setters), just try to minimize how often you use them.
The problem is that getters and setters can act as a workaround for direct access of private data. Private data is called private because it's not meant to be shared with other objects; it's meant as a representation of the object's state. Allowing other objects to access an object's private fields defeats the entire purpose of setting it private in the first place. Moreover, you introduce coupling for every getter or setter you write. Consider this, for example:
private String foo;
public void setFoo(String bar) {
this.foo = bar;
}
What happens if, somewhere down the road, you decide you don't need foo anymore, or you want to make it an integer? Every object that uses the setFoo method now needs to be changed along with foo.
just because the OOP rule: Data Hiding and Encapsulation. It is a very bad practice to declare a object's as public and change it on the fly in most situations. Also there are many other reasons , but the root is Encapsulation in OOP. and "buy a book or go read on Object Oriented Programming ", you will understand everything on this after you read any book on OOP.
The benefits of get() set() methods are as follows ..
You can serialize you object easily.
You can create a persistent object from the containing class.
You can convert the properties to JSON easily.
In the DAO layer (Frameworks like Hibernate) you can directly save the object to DB.
Easy understanding of object oriented concept.
Needs in all design pattern except possibly in single tone pattern.
Security for properties protecting direct access.
Polymorphism, Encapsulation can be easily understood and implemented by this type of class.
Example:
private String personName;
private int personId;
public void setPersonName(String name) throws Exception{
if(!(name.equals("")||name=="")){
this.personName = name;
}
}
public String getPersonName(){
return this.personName;
}
public void setPersonId(int id) throws Exception{
this.personId = id;
}
public int getPersonId(){
return this.personId;
}
Above answers all assume that the object in question is an object with behaviour.
An advanced strategy in OOP is to separate data objects (that do zip, only have fields) and behaviour objects.
With data objects, it is perfectly fine to omit getters and instead have public fields. They usually don't have setters, since they most commonly are immutable - their fields are set via the constructors, and never again.
Have a look at Bob Martin's Clean Code or Pryce and Freeman's Growing OO Software... for details.
public class Person{
private int age;
public int getAge(){
return age;
}
public void setAge(int age){
this.age = age;
}
}
i think this is you want..
and this also called pojo
this is the code for set method
public void setAge(int age){
this.age = age;
}
It looks like you trying to do something similar to C# if you want setAge create method setAge(int age){
this.age = age;}
I don't see a simple answer to the second question (why) here. So here goes.
Let's say you have a public field that gets used very often in your code. Whenever you decide you need to do something extra before you give or set this field you have a problem. You have to create a special getter and setter for this field and change your complete code from using the field directly to using the getter and setters.
Now imagine you are developing a library widely used by many people. When you need to make a change like the above and set direct access of the field to private the code of all the people using this field will break.
Using getters and setters is about future planning of the code, it makes it more flexible. Of course you can use public fields, especially for simple classes that just hold some data. But it's always a good idea to just make the field privately and code a get and set method for it.
This answer is merged from another question.
Your getAge() method is called instance method in Java.
To invoke an instance method, you should have a object of the Class in which this method is defined.
For Example, If this method in a Class called Person, then
Create a Person object using new operator
Person p = new Person();
To get the age of a Person object, use this method
p.getAge()
Although still a second year undergraduate student I will say my opinion. I believe that Java and private variables within your class are "RULES". Therefore because the variables in your class are private I think you use getters and setters to be able to define these variables outside the class.
How can I use the set and get methods, and why should I use them? Are they really helpful? And also can you give me examples of set and get methods?
Set and Get methods are a pattern of data encapsulation. Instead of accessing class member variables directly, you define get methods to access these variables, and set methods to modify them. By encapsulating them in this manner, you have control over the public interface, should you need to change the inner workings of the class in the future.
For example, for a member variable:
Integer x;
You might have methods:
Integer getX(){ return x; }
void setX(Integer x){ this.x = x; }
chiccodoro also mentioned an important point. If you only want to allow read access to the field for any foreign classes, you can do that by only providing a public get method and keeping the set private or not providing a set at all.
I want to add to other answers that setters can be used to prevent putting the object in an invalid state.
For instance let's suppose that I've to set a TaxId, modelled as a String. The first version of the setter can be as follows:
private String taxId;
public void setTaxId(String taxId) {
this.taxId = taxId;
}
However we'd better prevent the use to set the object with an invalid taxId, so we can introduce a check:
private String taxId;
public void setTaxId(String taxId) throws IllegalArgumentException {
if (isTaxIdValid(taxId)) {
throw new IllegalArgumentException("Tax Id '" + taxId + "' is invalid");
}
this.taxId = taxId;
}
The next step, to improve the modularity of the program, is to make the TaxId itself as an Object, able to check itself.
private final TaxId taxId = new TaxId()
public void setTaxId(String taxIdString) throws IllegalArgumentException {
taxId.set(taxIdString); //will throw exception if not valid
}
Similarly for the getter, what if we don't have a value yet? Maybe we want to have a different path, we could say:
public String getTaxId() throws IllegalStateException {
return taxId.get(); //will throw exception if not set
}
I think you want something like this:
public class Person {
private int age;
//public method to get the age variable
public int getAge(){
return this.age
}
//public method to set the age variable
public void setAge(int age){
this.age = age;
}
}
You're simply calling such a method on an object instance. Such methods are useful especially if setting something is supposed to have side effects. E.g. if you want to react to certain events like:
public void setAge(int age){
this.age = age;
double averageCigarettesPerYear = this.smokedCigarettes * 1.0 / age;
if(averageCigarettesPerYear >= 7300.0) {
this.eventBus.fire(new PersonSmokesTooMuchEvent(this));
}
}
Of course this can be dangerous if somebody forgets to call setAge(int) where he should and sets age directly using this.age.
Setters and getters are used to replace directly accessing member variables from external classes. if you use a setter and getter in accessing a property, you can include initialization, error checking, complex transformations, etc. Some examples:
private String x;
public void setX(String newX) {
if (newX == null) {
x = "";
} else {
x = newX;
}
}
public String getX() {
if (x == null) {
return "";
} else {
return x;
}
}
Having accessor methods is preferred to accessing fields directly, because it controls how fields are accessed (may impose data checking etc) and fits with interfaces (interfaces can not requires fields to be present, only methods).
Some benefits of using getters and setters (known as encapsulation or data-hiding):
(originally answered here)
1. The fields of a class can be made read-only (by only providing the getter) or write-only (by only providing the setter). This gives the class a total control of who gets to access/modify its fields.
Example:
class EncapsulationExample {
private int readOnly = -1; // this value can only be read, not altered
private int writeOnly = 0; // this value can only be changed, not viewed
public int getReadOnly() {
return readOnly;
}
public int setWriteOnly(int w) {
writeOnly = w;
}
}
2. The users of a class do not need to know how the class actually stores the data. This means data is separated and exists independently from the users thus allowing the code to be more easily modified and maintained. This allows the maintainers to make frequent changes like bug fixes, design and performance enhancements, all while not impacting users.
Furthermore, encapsulated resources are uniformly accessible to each user and have identical behavior independent of the user since this behavior is internally defined in the class.
Example (getting a value):
class EncapsulationExample {
private int value;
public int getValue() {
return value; // return the value
}
}
Now what if I wanted to return twice the value instead? I can just alter my getter and all the code that is using my example doesn't need to change and will get twice the value:
class EncapsulationExample {
private int value;
public int getValue() {
return value*2; // return twice the value
}
}
3. Makes the code cleaner, more readable and easier to comprehend.
Here is an example:
No encapsulation:
class Box {
int widthS; // width of the side
int widthT; // width of the top
// other stuff
}
// ...
Box b = new Box();
int w1 = b.widthS; // Hm... what is widthS again?
int w2 = b.widthT; // Don't mistake the names. I should make sure I use the proper variable here!
With encapsulation:
class Box {
private int widthS; // width of the side
private int widthT; // width of the top
public int getSideWidth() {
return widthS;
}
public int getTopWIdth() {
return widthT;
}
// other stuff
}
// ...
Box b = new Box();
int w1 = b.getSideWidth(); // Ok, this one gives me the width of the side
int w2 = b.getTopWidth(); // and this one gives me the width of the top. No confusion, whew!
Look how much more control you have on which information you are getting and how much clearer this is in the second example. Mind you, this example is trivial and in real-life the classes you would be dealing with a lot of resources being accessed by many different components. Thus, encapsulating the resources makes it clearer which ones we are accessing and in what way (getting or setting).
Here is good SO thread on this topic.
Here is good read on data encapsulation.
The above answers summarize the role of getters and setters better than I could, however I did want to add that your code should ideally be structured to reduce the use of pure getters and setters, i.e. those without complex constructions, validation, and so forth, as they break encapsulation. This doesn't mean you can't ever use them (stivlo's answer shows an example of a good use of getters and setters), just try to minimize how often you use them.
The problem is that getters and setters can act as a workaround for direct access of private data. Private data is called private because it's not meant to be shared with other objects; it's meant as a representation of the object's state. Allowing other objects to access an object's private fields defeats the entire purpose of setting it private in the first place. Moreover, you introduce coupling for every getter or setter you write. Consider this, for example:
private String foo;
public void setFoo(String bar) {
this.foo = bar;
}
What happens if, somewhere down the road, you decide you don't need foo anymore, or you want to make it an integer? Every object that uses the setFoo method now needs to be changed along with foo.
just because the OOP rule: Data Hiding and Encapsulation. It is a very bad practice to declare a object's as public and change it on the fly in most situations. Also there are many other reasons , but the root is Encapsulation in OOP. and "buy a book or go read on Object Oriented Programming ", you will understand everything on this after you read any book on OOP.
The benefits of get() set() methods are as follows ..
You can serialize you object easily.
You can create a persistent object from the containing class.
You can convert the properties to JSON easily.
In the DAO layer (Frameworks like Hibernate) you can directly save the object to DB.
Easy understanding of object oriented concept.
Needs in all design pattern except possibly in single tone pattern.
Security for properties protecting direct access.
Polymorphism, Encapsulation can be easily understood and implemented by this type of class.
Example:
private String personName;
private int personId;
public void setPersonName(String name) throws Exception{
if(!(name.equals("")||name=="")){
this.personName = name;
}
}
public String getPersonName(){
return this.personName;
}
public void setPersonId(int id) throws Exception{
this.personId = id;
}
public int getPersonId(){
return this.personId;
}
Above answers all assume that the object in question is an object with behaviour.
An advanced strategy in OOP is to separate data objects (that do zip, only have fields) and behaviour objects.
With data objects, it is perfectly fine to omit getters and instead have public fields. They usually don't have setters, since they most commonly are immutable - their fields are set via the constructors, and never again.
Have a look at Bob Martin's Clean Code or Pryce and Freeman's Growing OO Software... for details.
public class Person{
private int age;
public int getAge(){
return age;
}
public void setAge(int age){
this.age = age;
}
}
i think this is you want..
and this also called pojo
this is the code for set method
public void setAge(int age){
this.age = age;
}
It looks like you trying to do something similar to C# if you want setAge create method setAge(int age){
this.age = age;}
I don't see a simple answer to the second question (why) here. So here goes.
Let's say you have a public field that gets used very often in your code. Whenever you decide you need to do something extra before you give or set this field you have a problem. You have to create a special getter and setter for this field and change your complete code from using the field directly to using the getter and setters.
Now imagine you are developing a library widely used by many people. When you need to make a change like the above and set direct access of the field to private the code of all the people using this field will break.
Using getters and setters is about future planning of the code, it makes it more flexible. Of course you can use public fields, especially for simple classes that just hold some data. But it's always a good idea to just make the field privately and code a get and set method for it.
This answer is merged from another question.
Your getAge() method is called instance method in Java.
To invoke an instance method, you should have a object of the Class in which this method is defined.
For Example, If this method in a Class called Person, then
Create a Person object using new operator
Person p = new Person();
To get the age of a Person object, use this method
p.getAge()
Although still a second year undergraduate student I will say my opinion. I believe that Java and private variables within your class are "RULES". Therefore because the variables in your class are private I think you use getters and setters to be able to define these variables outside the class.