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Different form of encapsulation

According to the OOP concepts, encapsulation is considered as defined private variables and public getter and setter methods.
Example:
public class Student {
private String name;
private int id;
public void setName(String name){
name = this.name;
}
public void setID(int Id){
id= this.id;
}
public String getName(){
return name;
}
public int getID(){
return id;
}
}
But if I wrote this code in the following way, could I say this class follows encapsulation concept ?
Because here, we return department name by using public method.
public class Student {
private department;
public String getDepartmentOfStudent(String name){
// write java code to get department name based on name from DB
return department;
}
}
Case II: If private variable department was not declared and just return value retrieved from DB, would we say that this class follows encapsulation?

But if I wrote this code in the following way, could I say this class follows encapsulation concept ?. Because here, we return department name by using public method.
=> Usually getter and setter are public. variables are private.
Case II : If private variable "department" was not declared and just return value retrieved from DB, would we say that this class follows encapsulation?.
=> Yes, its still encapsulation . It is not mandatory to have both getter and setter for every variable. So how you set data in variable, does not matter.
Encapsulation is defined as the wrapping up of data under a single
unit. It is the mechanism that binds together code and the data it
manipulates.Other way to think about encapsulation is, it is a
protective shield that prevents the data from being accessed by the
code outside this shield.
1 - Technically in encapsulation, the variables or data of a class is
hidden from any other class and can be accessed only through any
member function of own class in which they are declared.
2 - As in encapsulation, the data in a class is hidden from other classes, so it is also known as data-hiding.
3 - Encapsulation can be achieved by: Declaring all the variables in the class as private and writing public methods in the class to set
and get the values of variables.
Source:- https://www.geeksforgeeks.org/encapsulation-in-java/

For both cases, the answer is also "Yes". It is because your private field variable is not being accessed directly by other classes (e.g. declaring it as public String department). Accessible department is still being controlled by the code path of public String getDepartmentOfStudent(String name) which is playing the role of a public getter.

Getter gets it's names due to it's functionality that it returns some private attributes not by it's name. For example: getName() or getAge().
If a method named abc() returns some private data member/attribute then it is a getter method.

Yes your code follows encapsulation. In encapsulation the variables of the object cannot be directly accessed and modified by any other object.
In Case II:
If private variable "department" was not declared and there is no other public variables in your class, then yes you have encapsulation as the class's variables or data cannot directly accessed by another class.

To answer both questions we need take into account the concept of encapsulation
The localization of knowledge within a module. Because objects
encapsulate data and implementation, the user of an object can view
the object as a black box that provides services. Instance variables
and methods can be added, deleted, or changed, but as long as the
services provided by the object remain the same, code that uses the
object can continue to use it without being rewritten. (...)
So, we can say YES, it does; because to the client consuming the information provided by Student there is no knowledge of how the value of department is being retrieved/handled and returned, it just knows that from the call to getDepartmentOfStudent will be a String result. It actually doesn't care whether it is comming from database or memory, only Student knows that.

Why use getters instead of public fields in immutable objects? [duplicate]

This question already has answers here:
Immutable Type: public final fields vs. getter
(9 answers)
Closed 6 years ago.
I am aware that using getters and making fields private has many advantages in general cases (data-hiding, decoupling, blah, blah, blah). What I'm asking is related specifically to immutable classes.
Let's say I've created a model class to store the data from a REST response, for example:
public final class Profile {
private final int id;
private final String name;
private final String info;
private final String location;
private final URI avatar;
private final Gender gender;
// about 10 more fields
}
The class and its fields are all final and cannot be changed or overridden. Each field is an instance of an immutable class and is validated in the constructor. Also, every field needs to be publicly accessible (no data-hiding).
In such a case, what possible advantage could there be to tripling the size of the class to add getters for every field instead of just making the fields public?

Implementing the getters provides flexibility for future changes to the Profile class. If your class provides a getter you can change the underlying private member in the Profile class and it won't require changes to the consumers of your class. You do want to hide the data types of the class-level variables just as you want to hide the values.

why would one initialize fields in the constructor? [duplicate]

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.

What names should getter and setter methods have

I am still very confused about getter and setter methods. I had this code;
public class MethodsInstances {
public MethodsInstances(String name){
girlName = name;
}
private String girlName;
public String getName(){
return girlName;
}
public void sayName(){
System.out.printf("Your first gf was %s", getName());
}
}
But for "sayName", why couldnt you instead of using getName(), just type girlName? It would be the same, because getName() returns girlName, as seen in the code. Also, do the methods have to start with get and set, or can be named whatever you want?
Huge thanks from the newbie coder, Dan B

The point of getters and setters is that only they are meant to be used to access the private varialble, which they are getting or setting. This way you provide encapsulation and it will be much easier to refactor or modify your code later.
Imagine you use girlName instead of its getter. Then if you want to add something like a default (say the default name is 'Guest' if it wasn't set before), then you'll have to modify both the getter and the sayName function.
There is no requirement for getters and setter to start with get and set - they are just normal member functions. However it's a convention to do that. (especially if you use Java Beans)

You absolutely can use the variable directly in your example, mostly because sayName() is in the same class.
Other than that, I see 3 reasons for having getters and setters:
1.) It's a principle of object oriented programming to keep values (state) private and provide public methods for interaction with other classes.
2.) Classes with getters and setters often follow the Java beans design pattern. This pattern allows those objects to be used in template engines or expression languages like JSP or Spring.
3.) In some cases it prevents actual errors. An example:
public class DateHolder() {
public Date date;
public static void main(String... args) {
DateHolder holder = new DateHolder();
holder.date = new Date();
System.out.println("date in holder: "+holder.date);
Date outsideDateRef = holder.date;
outsideDateRef.setTime(1l);
//will be different, although we did not change anything in the holder object.
System.out.println("date in holder: "+holder.date);
}
}
wrapping the date variable with getter and setter that operate only with the value, not the reference, would prevent this:
public class DateHolder() {
private Date date;
public Date getDate() {
return (Date)this.date.clone();
}
public void setDate(Date date) {
this.date = (Date) date.clone();
}
public static void main(String... args) {
DateHolder holder = new DateHolder();
holder.setDate( new Date() );
System.out.println("date in holder: "+holder.getDate());
Date outsideDateRef = holder.getDate();
outsideDateRef.setTime(1l);
//Date in holder will not have changed
System.out.println("date in holder: "+holder.getDate());
}
}

You can use girlName here you really don't have to call getName(). The reason you need getName() is if you you want to get the name outside of this class. For example if you create a new class and then create the above class object in the new class and assign that object a name (value for girlName) you won't be able to access girlName from new class since it is private .. so you need a public method which will get the value for you.
Also it doesn't have to be getName or setName but this just makes it easy to understand what function is doing.

It's a common design patter to encapsulate the process of "getting" and "setting" variables in methods. This gives you more freedom if you ever want to change the underlying implementation.
For an example, lets say you one day want to change the parameter girlName to be an object Girl instead;
If you directly access girlName from your outer classes, you will have to change all your external code.
With a setter method, you could simply change one method and do
public void setGirlname(String name)
{
girlname = new Girl(name, some_other_data);
}
Or perhaps you want to make sure girlname always is returned with uppercase.
public String getGirlname()
{
return girlName.toUpperCase();
}
Thus giving you a loot more flexibility in your code design.

You must first read about abstraction, encapsulation and OOP to understand about accessors, mutators, immutability and data access.

We want to prevent direct access to the variable, we make the variable a private variable.
When the variable is private, other classes are not able to access that variable.
If we create variable as public it is accessible for all.
to change the actual private variable we will now need public getter() or setter().
The basic naming conventions say that we will take the name of the variable and prefix it with get and/or set.
in your specific case the getGirlname would be correct.
We call this encapsulation

This way you can inspect classes and invoke them at runtime using Reflection. See more here
HTH
Ivo Stoykov

How to create immutable objects in Java?

How to create immutable objects in Java?
Which objects should be called immutable?
If I have class with all static members is it immutable?

Below are the hard requirements of an immutable object.
Make the class final
make all members final, set them
explicitly, in a static block, or in the constructor
Make all members private
No Methods that modify state
Be extremely careful to limit access to mutable members(remember the field may be final but the object can still be mutable. ie private final Date imStillMutable). You should make defensive copies in these cases.
The reasoning behind making the class final is very subtle and often overlooked. If its not final people can freely extend your class, override public or protected behavior, add mutable properties, then supply their subclass as a substitute. By declaring the class final you can ensure this won't happen.
To see the problem in action consider the example below:
public class MyApp{
/**
* #param args
*/
public static void main(String[] args){
System.out.println("Hello World!");
OhNoMutable mutable = new OhNoMutable(1, 2);
ImSoImmutable immutable = mutable;
/*
* Ahhhh Prints out 3 just like I always wanted
* and I can rely on this super immutable class
* never changing. So its thread safe and perfect
*/
System.out.println(immutable.add());
/* Some sneak programmer changes a mutable field on the subclass */
mutable.field3=4;
/*
* Ahhh let me just print my immutable
* reference again because I can trust it
* so much.
*
*/
System.out.println(immutable.add());
/* Why is this buggy piece of crap printing 7 and not 3
It couldn't have changed its IMMUTABLE!!!!
*/
}
}
/* This class adheres to all the principles of
* good immutable classes. All the members are private final
* the add() method doesn't modify any state. This class is
* just a thing of beauty. Its only missing one thing
* I didn't declare the class final. Let the chaos ensue
*/
public class ImSoImmutable{
private final int field1;
private final int field2;
public ImSoImmutable(int field1, int field2){
this.field1 = field1;
this.field2 = field2;
}
public int add(){
return field1+field2;
}
}
/*
This class is the problem. The problem is the
overridden method add(). Because it uses a mutable
member it means that I can't guarantee that all instances
of ImSoImmutable are actually immutable.
*/
public class OhNoMutable extends ImSoImmutable{
public int field3 = 0;
public OhNoMutable(int field1, int field2){
super(field1, field2);
}
public int add(){
return super.add()+field3;
}
}
In practice it is very common to encounter the above problem in Dependency Injection environments. You are not explicitly instantiating things and the super class reference you are given may actually be a subclass.
The take away is that to make hard guarantees about immutability you have to mark the class as final. This is covered in depth in Joshua Bloch's Effective Java and referenced explicitly in the specification for the Java memory model.

Just don't add public mutator (setter) methods to the class.

Classes are not immutable, objects are.
Immutable means: my public visible state cannot change after initialization.
Fields do not have to be declared final, though it can help tremendously to ensure thread safety
If you class has only static members, then objects of this class are immutable, because you cannot change the state of that object ( you probably cannot create it either :) )

To make a class immutable in Java , you can keep note of the following points :
1. Do not provide setter methods to modify values of any of the instance variables of the class.
2. Declare the class as 'final' . This would prevent any other class from extending it and hence from overriding any method from it which could modify instance variable values.
3. Declare the instance variables as private and final.
4. You can also declare the constructor of the class as private and add a factory method to create an instance of the class when required.
These points should help!!

From oracle site, how to create immutable objects in Java.
Don't provide "setter" methods — methods that modify fields or objects referred to by fields.
Make all fields final and private.
Don't allow subclasses to override methods. The simplest way to do this is to declare the class as final. A more sophisticated approach is to make the constructor private and construct instances in factory methods.
If the instance fields include references to mutable objects, don't allow those objects to be changed:
I. Don't provide methods that modify the mutable objects.
II. Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.

An immutable object is an object that will not change its internal state after creation. They are very useful in multithreaded applications because they can be shared between threads without synchronization.
To create an immutable object you need to follow some simple rules:
1. Don't add any setter method
If you are building an immutable object its internal state will never change. Task of a setter method is to change the internal value of a field, so you can't add it.
2. Declare all fields final and private
A private field is not visible from outside the class so no manual changes can't be applied to it.
Declaring a field final will guarantee that if it references a primitive value the value will never change if it references an object the reference can't be changed. This is not enough to ensure that an object with only private final fields is not mutable.
3. If a field is a mutable object create defensive copies of it for
getter methods
We have seen before that defining a field final and private is not enough because it is possible to change its internal state. To solve this problem we need to create a defensive copy of that field and return that field every time it is requested.
4. If a mutable object passed to the constructor must be assigned to a
field create a defensive copy of it
The same problem happens if you hold a reference passed to the constructor because it is possible to change it. So holding a reference to an object passed to the constructor can create mutable objects. To solve this problem it is necessary to create a defensive copy of the parameter if they are mutable objects.
Note that if a field is a reference to an immutable object is not necessary to create defensive copies of it in the constructor and in the getter methods it is enough to define the field as final and private.
5. Don't allow subclasses to override methods
If a subclass override a method it can return the original value of a mutable field instead of a defensive copy of it.
To solve this problem it is possible to do one of the following:
Declare the immutable class as final so it can't be extended
Declare all methods of the immutable class final so they can't be overriden
Create a private constructor and a factory to create instances of the immutable class because a class with private constructors can't be extended
If you follow those simple rules you can freely share your immutable objects between threads because they are thread safe!
Below are few notable points:
Immutable objects do indeed make life simpler in many cases. They are especially applicable for value types, where objects don't have an identity so they can be easily replaced and they can make concurrent programming way safer and cleaner (most of the notoriously hard to find concurrency bugs are ultimately caused by mutable state shared between threads).
However, for large and/or complex objects, creating a new copy of the object for every single change can be very costly and/or tedious. And for objects with a distinct identity, changing an existing objects is much more simple and intuitive than creating a new, modified copy of it.
There are some things you simply can't do with immutable objects, like have bidirectional relationships. Once you set an association value on one object, it's identity changes. So, you set the new value on the other object and it changes as well. The problem is the first object's reference is no longer valid, because a new instance has been created to represent the object with the reference. Continuing this would just result in infinite regressions.
To implement a binary search tree, you have to return a new tree every time: Your new tree will have had to make a copy of each node that has been modified (the un-modified branches are shared). For your insert function this isn't too bad, but for me, things got fairly inefficient quickly when I started to work on delete and re-balance.
Hibernate and JPA essentially dictate that your system uses mutable objects, because the whole premise of them is that they detect and save changes to your data objects.
Depending on the language a compiler can make a bunch of optimizations when dealing with immutable data because it knows the data will never change. All sorts of stuff is skipped over, which gives you tremendous performance benefits.
If you look at other known JVM languages (Scala, Clojure), mutable objects are seen rarely in the code and that's why people start using them in scenarios where single threading is not enough.
There's no right or wrong, it just depends what you prefer. It just depends on your preference, and on what you want to achieve (and being able to easily use both approaches without alienating die-hard fans of one side or another is a holy grail some languages are seeking after).

Don't provide "setter" methods — methods that modify fields or
objects referred to by fields.
Make all fields final and private.
Don't allow subclasses to override methods. The simplest way to do this is to declare the class as final. A more sophisticated approach is to make the constructor private and construct instances in factory methods.
If the instance fields include references to mutable objects, don't allow those objects to be changed:
Don't provide methods that modify the mutable objects.
Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.

First of all, you know why you need to create immutable object, and what are the advantages of immutable object.
Advantages of an Immutable object
Concurrency and multithreading
It automatically Thread-safe so synchronization issue....etc
Don't need to copy constructor
Don't need to implementation of clone.
Class cannot be override
Make the field as a private and final
Force callers to construct an object completely in a single step, instead of using a no-Argument constructor
Immutable objects are simply objects whose state means object's data can't change after the
immutable object are constructed.
please see the below code.
public final class ImmutableReminder{
private final Date remindingDate;
public ImmutableReminder (Date remindingDate) {
if(remindingDate.getTime() < System.currentTimeMillis()){
throw new IllegalArgumentException("Can not set reminder" +
" for past time: " + remindingDate);
}
this.remindingDate = new Date(remindingDate.getTime());
}
public Date getRemindingDate() {
return (Date) remindingDate.clone();
}
}

Minimize mutability
An immutable class is simply a class whose instances cannot be modified. All of the information contained in each instance is provided when it is created and is fixed for the lifetime of the object.
JDK immutable classes: String, the boxed primitive classes(wrapper classes), BigInteger and BigDecimal etc.
How to make a class immutable?
Don’t provide any methods that modify the object’s state (known as mutators).
Ensure that the class can’t be extended.
Make all fields final.
Make all fields private.
This prevents clients from obtaining access to mutable objects referred to by fields and modifying these objects directly.
Make defensive copies.
Ensure exclusive access to any mutable components.
public List getList() {
return Collections.unmodifiableList(list); <=== defensive copy of the mutable
field before returning it to caller
}
If your class has any fields that refer to mutable objects, ensure that clients of the class cannot obtain references to these objects. Never initialize such a field to a client-provided object reference or return the object reference from an accessor.
import java.util.Date;
public final class ImmutableClass {
public ImmutableClass(int id, String name, Date doj) {
this.id = id;
this.name = name;
this.doj = doj;
}
private final int id;
private final String name;
private final Date doj;
public int getId() {
return id;
}
public String getName() {
return name;
}
/**
* Date class is mutable so we need a little care here.
* We should not return the reference of original instance variable.
* Instead a new Date object, with content copied to it, should be returned.
* */
public Date getDoj() {
return new Date(doj.getTime()); // For mutable fields
}
}
import java.util.Date;
public class TestImmutable {
public static void main(String[] args) {
String name = "raj";
int id = 1;
Date doj = new Date();
ImmutableClass class1 = new ImmutableClass(id, name, doj);
ImmutableClass class2 = new ImmutableClass(id, name, doj);
// every time will get a new reference for same object. Modification in reference will not affect the immutability because it is temporary reference.
Date date = class1.getDoj();
date.setTime(date.getTime()+122435);
System.out.println(class1.getDoj()==class2.getDoj());
}
}
For more information, see my blog:
http://javaexplorer03.blogspot.in/2015/07/minimize-mutability.html

an object is called immutable if its state can not be changed once created. One of the most simple way of creating immutable class in Java is by setting all of it’s fields are final.If you need to write immutable class which includes mutable classes like "java.util.Date". In order to preserve immutability in such cases, its advised to return copy of original object,

Immutable Objects are those objects whose state can not be changed once they are created, for example the String class is an immutable class. Immutable objects can not be modified so they are also thread safe in concurrent execution.
Features of immutable classes:
simple to construct
automatically thread safe
good candidate for Map keys and Set as their internal state would not change while processing
don't need implementation of clone as they always represent same state
Keys to write immutable class:
make sure class can not be overridden
make all member variable private & final
do not give their setter methods
object reference should not be leaked during construction phase

The following few steps must be considered, when you want any class as an immutable class.
Class should be marked as final
All fields must be private and final
Replace setters with constructor(for assigning a value to a
variable).
Lets have a glance what we have typed above:
//ImmutableClass
package younus.attari;
public final class ImmutableExample {
private final String name;
private final String address;
public ImmutableExample(String name,String address){
this.name=name;
this.address=address;
}
public String getName() {
return name;
}
public String getAddress() {
return address;
}
}
//MainClass from where an ImmutableClass will be called
package younus.attari;
public class MainClass {
public static void main(String[] args) {
ImmutableExample example=new ImmutableExample("Muhammed", "Hyderabad");
System.out.println(example.getName());
}
}

Commonly ignored but important properties on immutable objects
Adding over to the answer provided by #nsfyn55, the following aspects also need to be considered for object immutability, which are of prime importance
Consider the following classes:
public final class ImmutableClass {
private final MutableClass mc;
public ImmutableClass(MutableClass mc) {
this.mc = mc;
}
public MutableClass getMutClass() {
return this.mc;
}
}
public class MutableClass {
private String name;
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
}
public class MutabilityCheck {
public static void main(String[] args) {
MutableClass mc = new MutableClass();
mc.setName("Foo");
ImmutableClass iMC = new ImmutableClass(mc);
System.out.println(iMC.getMutClass().getName());
mc.setName("Bar");
System.out.println(iMC.getMutClass().getName());
}
}
Following will be the output from MutabilityCheck :
Foo
Bar
It is important to note that,
Constructing mutable objects on an immutable object ( through the constructor ), either by 'copying' or 'cloing' to instance variables of the immutable described by the following changes:
public final class ImmutableClass {
private final MutableClass mc;
public ImmutableClass(MutableClass mc) {
this.mc = new MutableClass(mc);
}
public MutableClass getMutClass() {
return this.mc;
}
}
public class MutableClass {
private String name;
public MutableClass() {
}
//copy constructor
public MutableClass(MutableClass mc) {
this.name = mc.getName();
}
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
}
still does not ensure complete immutability since the following is still valid from the class MutabilityCheck:
iMC.getMutClass().setName("Blaa");
However, running MutabilityCheck with the changes made in 1. will result in the output being:
Foo
Foo
In order to achieve complete immutability on an object, all its dependent objects must also be immutable

From JDK 14+ which has JEP 359, we can use "records". It is the simplest and hustle free way of creating Immutable class.
A record class is a shallowly immutable, transparent carrier for a fixed set of fields known as the record components that provides a state description for the record. Each component gives rise to a final field that holds the provided value and an accessor method to retrieve the value. The field name and the accessor name match the name of the component.
Let consider the example of creating an immutable rectangle
record Rectangle(double length, double width) {}
No need to declare any constructor, no need to implement equals & hashCode methods. Just any Records need a name and a state description.
var rectangle = new Rectangle(7.1, 8.9);
System.out.print(rectangle.length()); // prints 7.1
If you want to validate the value during object creation, we have to explicitly declare the constructor.
public Rectangle {
if (length <= 0.0) {
throw new IllegalArgumentException();
}
}
The record's body may declare static methods, static fields, static initializers, constructors, instance methods, and nested types.
Instance Methods
record Rectangle(double length, double width) {
public double area() {
return this.length * this.width;
}
}
static fields, methods
Since state should be part of the components we cannot add instance fields to records. But, we can add static fields and methods:
record Rectangle(double length, double width) {
static double aStaticField;
static void aStaticMethod() {
System.out.println("Hello Static");
}
}