abstract class shape{
public double volume;
public double surface;
}
class cube extends shape{
public static double side;
public static double newSide;
cube(double newSide){
side = newSide;
}
public double volume(){
return side;
}
public double surface(){
return newSide;
}
}
public class shapes {
public static void main(String[] args) {
cube a = new cube(5);
System.out.println (a.volume);
System.out.println (a.surface);
}
}
This produces:
0.0
0.0
The parameter I am passing through the constructor does not seem to be processed properly by the cube class.
What am I doing wrong here?
So my guess is you haven’t figured out what objects are. If you look at the JDK code you will see examples of classes that define objects java.lang.String, java.lang.Integer, java.lang.StringBuilder are classes you can look at to get some idea of what objects are. Once you see how some of these classes work you can get the idea for the motivation behind the rest of this.
There are objects and there are classes. We use classes as templates to create objects. Each object has its own state. For example you can have many different strings in your program. Each string is an object with its own state, using its own instance variables. But static variables don’t belong to objects, they belong to the class.
“Instance” means one occurrence of an object. We instantiate objects using a constructor.
Your methods like
public double volume(){
are called accessors, or getters, because they’re used to access the state of the object.
Having an instance method accessor access a static variable is confusing. Static members have a different scope than instance members and combining them here is confusing. It’s hard to tell what is intended but you shouldn’t be using static variables to describe state that should be specific to an instance.
Your static variables are public and have the same name as the accessor. If you call the member without parens you are calling the variable directly. If you use parens you will call the accessor. .
In the event you do need static variables, you don’t initialize them in a constructor. Constructors are for initializing an instance of the class, where static variables don’t belong to one instance but to the class. Use constructors to set instance variables, not static ones.
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");
}
}
Im curious if there exists an abbreviation form for getter/setter methods of objects
SimpleObject oSimple = new SimpleObject();
oSimple.setCounterValue(oSimple.getCounterValue() + 1);
like one for simple datatypes
int counter = 0;
counter += 2;
Info
The getter/setter methods are required.
Addition
If there isn't a language feature thats support the idea, what is the most convenient way to deal with that in context of good and clean code?
You have C# background I can imagine ;-)
It's not possible in Java (apart from not-quite-the-same solutions such as having public properties etc). Same as operator overloading, which could also have solved your issue.
However have a look at http://www.eclipse.org/xtend/ though, it's a JVM language similar to Java that supports operator overloading and some other nifty things.
As geert3 said there is no shortcut to setters/getters in Java without accessing the property directly.
In your case your SimpleObject-class should just have a method increaseCounter() and maybe increaseCounterBy(int add) (or simply add(int a)).
For this case, I create annotation
/**
* Annotation for PropertiesContainer class (that has field without getter and setter)
* This class look like as class with Properties in C#
*
*/
public #interface PropertiesContainer {
}
Add add to any class (annotation shows that isn't error)
#PropertiesContainer // class without getter and setter
public Class SomeObject{
public int counter;
}
And just use:
oSimple.counter++;
If the value doesn't have to be checked for any wrong values / execute code when setting it, you could solve it by just referencing the variable directly:
public Class SomeObject{
public int value;
}
public Class Main{
public static void main(String[] args){
SomeObject o = new SomeObject();
o.value += 1;
}
}
If the example you gave is all you want to do (increment and assign an index for a new object every time you make one) then a common practice is use a static variable as a counter and a local variable for the specific index.
class SomeObject {
static int objectCounter;
private int index;
public SomeObject() {
index = objectCounter++;
// additional constructor code
}
}
If you wish to add a number to a variable, I'd consider making a separate method that does just that rather than trying to rewrite set(), or if it makes sense, make it part of the code for the set().
I want to create a simple game in Java.
I'm struggling to understand how to use inheritance to accomplish how to implement subclasses that use encapsulation without needing to write out the same methods in the subclasses.
Ideally I'd like to make one base class "character" with a bunch of methods that use encapsulation, so that I can just declare different values for the private members of the subclasses. So something like,
public class Character {
private int hitPoints;
public int getHitPoints(){return hitPoints;}
}
And then just declare different values for the variables.
public class subCharacter extends Character {
private int hitPoints=100;
//getHitPoints() already inherited and should return 100
}
But to properly get the hit points of the subclass. I have to declare the same method in the subclass to actually get the method to work.
So isn't encapsulation incompatible with inheritance? Is there something basic here I'm misunderstanding or completely overlooking?
You should make the variable hitPoints protected in you Character class, and set it to 100 in the constructor of the subCharacter class. There is no need for the declaration of the getHitPoints method in the subclass. The code would look like this:
public class Character {
protected int hitPoints;
public int getHitPoints(){return hitPoints;}
}
public class subCharacter extends Character {
public subCharacter () {
hitPoints = 100;
}
}
Example of a subCharacter object:
subCharacter sub = new subCharacter();
System.out.println(sub.getHitPoints()); // prints 100
The reason this doesn't work like you think it should is because the subclass's hitpoints field is different from the superclass's hitpoints field. So while the superclass method is defined, it's trying to refer to a variable that you never actually initialized because it's not the same variable named hitpoints.
As others have already said, you should use the protected access modifier instead of the private access modifier on fields you want to have inherited to a subclass.
Then again, you probably don't actually need the SubCharacter class to begin with, if this is what you're actually writing for. You just need to have a constructor that takes a variable argument for hitpoints, or any other field in Character that needs to take different values.
//I'm not going to reproduce everything.
Character(int hp, String nm, boolean someBooleanThatIJustMadeUpToGetTheConceptAcross){
hitpoints = hp;
name = nm;
randomBoolean = someBooleanThatIJustMadeUpToGetTheConceptAcross;
}
This is not to say, however, that you don't need a superclass/subclass if, say, you're using this Character class for both enemies and player characters, for instance.
For an example of when you'd use inheritance...
public class Circle{
protected int radius;
Circle(){//It's always a good idea to have default constructors, by the way.
radius = 1;
}
Circle(int rad){
radius = rad;
}
}
public class Wheel extends Circle{
protected int numspokes;
Wheel(){
super(); //Calls the constructor for Circle, instead of reimplementing the wheel. badpuns++;.
numspokes = 0;
}
Wheel(int rad, int spokes){
super(rad); //This passes the radius up to the Circle this Wheel also is, so that any calls to this Wheel AS IF IT WAS a Circle, like an array or ArrayList of Circles, will function, which is the point of inheritance.
numspokes = spokes;
}
}
Hello I am curious to know that Is there any purpose to make private class variable to public in Java.
public class XYZ {
public String ID;
public ABC abc;
private class ABC {
public boolean isExist;
}
}
Thanks in advance.
Yes, there's a purpose. If you do that then those program elements which can access the class can manipulate that variable directly. Otherwise (say if the variable is private), those elements would still be able to access the class but won't be able to manipulate the variable (unless you provide a getter/setter for it).
Think about it this way: the class modifier defines the level of access to the class, the variable modifier then defines the level of access to the variable itself (for those elements which can access the class).
This is sometimes done for data-only classes. For example, this is sometimes done to represent the models stored in databases (see Objectify for a real example of how this is used, in conjunction with annotations, to represent the database models that are stored in an App Engine database).
That being said, this sort of thing makes for a very poor API. If you do this, I'd suggest doing it with classes that are either package-level access or in private nested classes, only. When exposing functionality or data to code outside your package, it is generally better to do it with a carefully designed interface that would allow you to change the implementation if your underlying structure were to change.
That is to make isExist visible to XYZ class.
Note, ABC is only visible to XYZ and not to any outside classes and its variable is public so you can have access to it. private has not meaning to XYZ, only outside classes
From inside XYZ,
ABC abc = new ABC(); //can only be accessed by XYZ.
abc.isExists = true; //can only be accessed by XYZ
Making isExist public means you do not care about encapsulating (prevent it from unwanted manipulation from outside) it. If you make it private, you will need a get accessor to expose it
private class ABC {
private boolean _isExist; //only through accessors
public boolean isExist()
{
return _isExist;
}
}
You can do either of the following two things to your class instance variables:
THING # 1: Keep your instance variables private. Then have public getter and setter methods to get and set the value of that variable. The good thing about it is that you get to put checks inside the setter method. For example, lengths can never be negative. So, you can't just make lengths public and let anyone assign it whatever value they want. You need to make sure the value being assigned to it is not negative. So:
class myClass {
private int length;
public void setLength(int i) {
if ( i > 0 ) {
length = i;
}
}
}
Also, you can make your instance variables read-only, write-only, or read-and-write, depending on the availability of getter and setter methods for that private variable.
THING # 2 : If you don't need any restrictions on the value of your instance variable, and you want it to neither be read-only nor write-only, then it's fine to keep that variable public. For example: babies can have any name - no restrictions:
class Baby {
public name;
}
class Mother {
public void nameTheBaby() {
Baby baby = new Baby();
baby.name = "Sarah";
}
}