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Java : final constructor [duplicate]

Why can't constructors be final, static, or abstract in Java?
For instance, can you explain to me why this is not valid?
public class K {
abstract public K() {
// ...
}
}

When you set a method as final it means: "I don't want any class override it." But according to the Java Language Specification:
JLS 8.8 - "Constructor declarations are not members. They are never inherited and therefore are not subject to hiding or overriding."
When you set a method as abstract it means: "This method doesn't have a body and it should be implemented in a child class." But the constructor is called implicitly when the new keyword is used so it can't lack a body.
When you set a method as static it means: "This method belongs to the class, not a particular object." But the constructor is implicitly called to initialize an object, so there is no purpose in having a static constructor.

The question really is why you want constructor to be static or abstract or final.
Constructors aren't inherited so can't be overridden so whats the use
to have final constructor
Constructor is called automatically when an instance of the class is
created, it has access to instance fields of the class. What will be
the use of a static constructor.
Constructor can't be overridden so what will you do with an abstract
constructor.

A Java constructor is implicitly final, the static / non-static aspects of its semantics are implicit1, and it is meaningless for a Java constructor to be abstract.
This means that the final and static modifiers would be redundant, and the abstract keyword would have no meaning at all.
Naturally, the Java designers didn't see in any point in allowing redundant and/or meaningless access modifiers on constructors ... so these are not allowed by the Java grammar.
Aside: It is a shame that they didn't make the same design call for interface methods where the public and abstract modifiers are also redundant, but allowed anyway. Perhaps there is some (ancient) historical reason for this. But either way, it cannot be fixed without rendering (probably) millions of existing Java programs uncompilable.
1 - Actually, constructors have a mixture of static and non-static semantics. You can't "call" a constructor on an instance, and it they are not inherited, or overridable. This is similar to the way static methods work. On the other hand, the body of a constructor can refer to this, and call instance methods ... like an instance method. And then there is constructor chaining, which is unique to constructors. But the real point is that these semantics are fixed, and there is no point allowing a redundant and probably confusing static modifier.

public constructor: Objects can be created anywhere.
default constructor: Objects can be created only in the same package.
protected constructor: Objects can be created by classes outside the package only if it's a subclass.
private constructor: Object can only be created inside the class (e.g., when implementing a singleton).
The static, final and abstract keywords are not meaningful for a constructor because:
static members belong to a class, but the constructor is needed to create an object.
An abstract class is a partially implemented class, which contains abstract methods to be implemented in child class.
final restricts modification: variables become constant, methods can't be overridden, and classes can't be inherited.

Final: Because you can't overwrite/extend a constructor anyway. You can extend a class (to prevent that you make it final) or overwrite a method (to prevent that you make it final), but there is nothing like this for constructors.
Static: If you look at the execution a constructor is not static (it can access instance fields), if you look at the caller side it is (kind of) static (you call it without having an instance. Its hard to imagine a constructor being completely static or not static and without having a semantic separation between those two things it doesn't make sense to distinguish them with a modifier.
Abstract: Abstract makes only sense in the presence of overwriting/extension, so the same argument as for 'final' applies

No Constructors can NEVER be declared as final. Your compiler will always give an error of the type "modifier final not allowed"
Final, when applied to methods, means that the method cannot be overridden in a subclass.
Constructors are NOT ordinary methods. (different rules apply)
Additionally, Constructors are NEVER inherited. So there is NO SENSE in declaring it final.

Constructors are NOT ordinary methods. (different rules apply)
Additionally, Constructors are NEVER inherited. So there is NO SENSE in declaring it final.
No Constructors can NEVER be declared final. YOur compiler will always give an error of the type "modifer final not allowed"
Check the JLS Section 8.8.3 (The JLS & API docs should be some of your primary sources of information).

JLS section 8 mentions this.
Constructors (§8.8) are similar to methods, but cannot be invoked
directly by a method call; they are used to initialize new class
instances. Like methods, they may be overloaded (§8.8.8).
But constructors per say are not regular methods. They can't be compared as such.

why constructor can not be static and final are well defined in above answers.
Abstract: "Abstract" means no implementation . and it can only be implemented via inheritance. So when we extends some class, all of parent class members are inherited in sub-class(child class) except "Constructor". So, lets suppose, you some how manage to declare constructor "Abstract", than how can you give its implementation in sub class, when constructor does not get inherit in child-class?
that's why constructor can't be
abstract .

lets see first
final public K(){
*above the modifier final is restrict 'cause if it final then some situation where in some other class or same class only we will override it so thats not gonna happen here proximately not final
eg:
we want public void(int i,String name){
//this code not allowed
let static,, static itz all about class level but we create the object based constructor by using 'new' keyword so,,,,,, thatsall
abstract itz worst about here not at 'cause not have any abstract method or any declared method

Unfortunately in PHP the compiler does not raise any issue for both abstract and final constructor.
<?php
abstract class AbstractClass
{
public abstract function __construct();
}
class NormalClass
{
public final function __construct() {
echo "Final constructor in a normal class!";
}
}
In PHP static constructor is not allowed and will raise fatal exception.
Here in AbstractClass obviously a constructor either can be declared as abstract plus not implemented or it can be declared as something among (final, public, private, protected) plus a function body.
Some other related facts on PHP:
In PHP having multiple constructor __construct() is not possible.
In PHP a constructor __construct() can be declared as abstract, final, public, private and protected!
This code was tested and stood true for in PHP versions from 5.6 up to 7.4!

Why is Java.Lang.Math not abstract?

The "abstract" keyword means you cannot create an instance of the class (an object).
Java.Lang.Math is preceded with the following keywords
public final class Math {
...
}
But no "abstract" keyword. The class simply provides a collection of related static variables and methods, like PI and sin().
Static means that those variables/methods can't be unique between different instances of the object (there is only one copy of those variables/methods associated with the class itself). So why even allow the programmer to create an instance of the class? Why not precede with "abstract"?

final and abstract keywords can't be applied together because final class can't be overridden and abstract classes are meant for override.
A class that is declared final cannot be subclassed that is used for creating an immutable class such as String
It's better explained under JSL section - 8.1.1. Class Modifiers
An abstract class is a class that is incomplete, or to be considered incomplete.
A class can be declared final if its definition is complete and no subclasses are desired or required.
both above statements are contradicting each-other.
If you want to read more about then have a look at Java Tutroial on A Strategy for Defining Immutable Objects
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.

java.lang.Math is a Utility class (contains only static utility methods).
Correct way to define utility class is to make it final so that no other class can extend it and to have private no-args constructor so that no one can create an instance of the class.
So you won't be able to create instance of class any how. However if you go by abstract approach, you cannot use final and there is no way you can prevent it from being extended. Hence former approach is better.

If it were abstract, someone could do
public class NewMath extends Math {
which frankly speaking makes no sense.
However, you cannot create a new Math instance anyway because its only constructor is private:
private Math() {}

final means you cannot extend it, the reason you cannot create an instance is because the constructor is private. This is how (in java) you define static classes or factory classes.

The Abstract keyword means the class can't be Instantiable but it can be Extended.
And in case of Utility Classes such Math Class,Extending it has no sense. Declaring the no-args Constructor makes it Non-Instantiable and final makes it non-Extendable...So its the Perfect Match .... And If you use Abstract then you can't use Final...

Java - is it bad practice not to have a class constructor?

I want to make a helper class that deals with formatting (i.e. has methods to remove punctuation and convert between types, as well as reformatting names etc.). This doesn't seem like it will need any fields - its only purpose is to get passed things to convert and return them, reformatted. Is it bad practice to leave out a constructor? If so, what should my constructor be doing? I was looking at this link and noticed that the class it describes lacks a constructor.

Is it bad practice to leave out a constructor?
Yes - because unless you specify any constructors, the Java compiler will give you a constructor with the same visibility as the class itself.
Assuming all your methods are static - which seems likely unless you want polymorphism - you should make your class final and give it a private constructor, so that other developers don't accidentally create an instance of your class, when that would be pointless. When thinking about an API, any time I can remove the ability for developers to do something stupid, I do so :)
So something like:
public final class Helpers {
private Helpers() {
}
public static String formatDate(Date date) {
// etc
}
}
Note that by taking polymorphism out of the equation, you're also removing the possibility of changing this behaviour for tests etc. That may well be fine - I'm not a believer in "no statics, ever" - but it's worth considering.

Any class that has all the methods which do not have or need any state is free to reduce the visibility of constructor by making the constructor private.
Example java.lang.Math class in Java.
As java.lang.Math has all static methods which do similar job as your class they have declared the constructor as private so that nobody can accidentally create the instance of that class.
/**
* Don't let anyone instantiate this class.
*/
private Math() {}

Not bad practice. but the example that you have given doesn't have any member variables that can be used in an Object context. In such situations, it's best to have static methods because then you don't need to allocate memory to create objects for the class before calling the methods.

Compiler will generate a default constructor (with no parameters) for you. If your class has not state and does not extend a class which needs initialization, you can let it without declaring explicit constructor

no its good to leave out a constructor as there aren't any instance variables in your class!
constructors are meant to initialize the instance variables!
still if you skip the constructor, compiler anyways inserts the default constructor which is fair enough!!

You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor.
Java Official Document: Providing Constructors for Your Classes

Usually it is a good coding practice to define your constructor in the class though each class has a default constructor .
But if you do not have any special need to use a oveloaded constructor or to make any singleton pattern then you can remove the constructor .
If you are using static methods in your case then also you dont have any need to define constructor , as you do not need to have object of this class .

why we should not use static and abstract for a single method?

why we should not use static and abstract for a single method?

the static keyword is defined so that a method can be called by a class name rather then an object. that means the method has to have some sort of definition. but abstract means you do not have any details about what the method does, it is as it says **Abstract**. When you inherit or extend a class you can then define the method.
Think of an interface.
If you are asking about having a static method inside of an abstract class, that is a different story. An abstract class is essentially as mentioned an interface and contains just a template of say functions that you must later on implement by inheriting / extending the class. Once you extend that class the static method does not come along with it (that is by default unless the access modifier is public / protected).

A static method is not inherited. Therefore, making it abstract is a nonsense.

The abstract keyword means that child classes must override the method - this is (one of the ways) Java supports polymorphism. If you want to make it so that subclasses cannot override the method you mark it final. So it would be impossible to have an "abstract final" method since they are the exact opposite of each other.
the static keyword implies final as well - all static method are also final. Thus it is impossible to have a method that is both static and abstract since you would be able to make a method that is abstract and final.
The reason for static being final is that it is bound to the class instead of the instance. That means that the compiler looks it up at compile time rather than runtime to determine which method to call. The reason what it is like that? Arbitrary decision that the designers of Java made - they could have allowed static method to be overridden but decided not to. I don't have any particular insight as to why the chose one over the other unfortunately.

As others have said, static+abstract is nonsense in Java. But there have been (rare) occasions where I've wished I could do just that.
The result I was looking for was basically to say that... "all concrete classes that extent this abstract class (or implement this interface) must provide a static method with this signature." This capability would allow these classes to provide meta-information about themselves.
Normally I have ended up with an instance method in these cases. If you stipulate that concrete implementations must support the default (no-arg) constructior, you can do...
MyInterface obj = MyClassThatImplementsMyInterface.newInstance();
obj.invokeTheMethodIWishWasBothStaticAndAbstract();

simulation of static class in java

What do you think of the following way to simulate a static class in java?
You can add non static methods but you wouldn't be able to call them.
/**
* Utility class: this class contains only static methods and behaves as a static class.
*/
// ... prevent instantiation with abstract keyword
public abstract class Utilities
{
// ... prevent inheritance with private constructor
private Utilities() {}
// ... all your static methods here
public static Person convert(String foo) {...}
}

That is the usual way. However, there is not need for the abstract keyword. Using a private constructor is sufficient because
it prevents the creation of objects (from outside the class)
it prevents inheritance
The abstract keyword suggests the user that users of the class might implemented the class what is not the case here.

Item 4 in Effective Java (a very... effective book) says:
// Noninstantiable utility class
public final class Utility {
private Utility() {
throw new AssertionError();
}
}
because the explicit costructor is private:
you cannot instantiate it
you cannot extend it (as if it was declared as final)
The AssertionError isn't required but it provides another small benefit: it prevents that the costructior is accidentally invoked from within the class.
You can also create a specific annotation, like #BagOfFunction, and annotate your class:
#BagOfFunctions
public final class Utility {
private Utility() {
throw new AssertionError();
}
}
basically you trade a comment for a self-documenting annotation.

My FindBugs plugin suggests rather final class instead of abstract class. And I use that in my project. It seems to be a widespread idiom if it became a rule that is checked by FindBugs.

i would say, if you habe already a private constructor
private Utilities() {}
the abstract keyword is not neccessary. rather make it final.
the difference to your version is marginal, for any practical means.

I prefer making such classes final, but not abstract. Though it is just a matter of personal style.
By the way, I suppose it is still possible to call its instance methods if you put some energies. E.g. one can try using objenesis to create instance of class.

I'll have to agree with those above. Use "final" instead of "abstract". Remember, words like "final" and "abstract" are as much a means of communicating with your fellow programmers as they are instructions to the machine. Abstract implies that there will be descendant classes later, whereas final decidedly means that you will not, save through refactoring, see anything descended of this class (which is your intended meaning).
Further, in most standards I've seen, and consistently in my company, it is considered best practice to make the abstract class something which is specifically left unused, save as a parent of other classes. "Abstract" is treated as "blueprint" or "general structure", you would never drive an "abstract" car. On the other hand, final classes are instantiated perpetually, especially with Factory patterns.

My suggestion is: prevent incorrect use (i.e. instantiation) by placing javadocs
Isn't that simpler? I think your teammates are able to read ;)