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...
Related
What are the classes in Java which:
all its methods are static
Does not contain any instance methods
Lombok define it as UtilityClass:
A utility class is a class that is just a namespace for functions. No instances of it can exist, and all its members are static. For example, java.lang.Math and java.util.Collections are well known utility classes. This annotation automatically turns the annotated class into one.
A utility class cannot be instantiated. By marking your class with #UtilityClass, lombok will automatically generate a private constructor that throws an exception, flags as error any explicit constructors you add, and marks the class final. If the class is an inner class, the class is also marked static.
All members of a utility class are automatically marked as static. Even fields and inner classes.
It sounds like you're describing a static class.
While Java does not allow you to explicitly declare a (non-nested) class as static, it is still possible to implement this paradigm.
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!
In an Interface by default the data member are static and final.
Static it is because it can not be instantiated,but why it is final?
and the other question is even the abstract classes can not be instantiated,then why it can have a non static data member??
and the other question is even the abstract classes can not be instantiated,
then why it can have a non static data member??
Because in abstract classes, you can define as much functionality you want, which can be then used by its childs.
Interface variables are static because Java interfaces cannot be instantiated in their own right; the value of the variable must be assigned in a static context in which no instance exists. The final modifier ensures the value assigned to the interface variable is a true constant that cannot be re-assigned by program code.
I wouldn't advise on putting variables in an interface since the interface mainly serves as a blueprint for it's classes.
Why can an abstract class have a non-static data member?
A static variable is one that’s associated with a class, not objects of that class. Generally speaking, the purpose of an abstract class is to provide a skeleton with some non-abstract behavior, but other bits still to be filled in by subclasses which can also define functionality and use the abstract class' variables.
In Java what is the purpose of using private constructor in an abstract class?
In a review I got this question, and I am curious, for what situation we need to use the constructor in such way?
I think it can be used in pair with another constructor in abstract class, but this is very trivial. Also it can be used for constructing static inner classes which will excend abstract class.
Maybe there is more elegant usage?
If the private constructor is the only constructor of the class, then the reason is clear: to prevent subclassing. Some classes serve only as holders for static fields/methods and do not want to be either instantiated or subclassed. Note that the abstract modifier is in this case redundant—with or without it there would be no instantiation possible. As #JB Nizet notes below, the abstract modifier is also bad practice because it sends wrong signals to the class's clients. The class should in fact have been final.
There is another use case, quite rare though: you can have an abstract class with only private constructors that contains its own subclasses as nested classes. This idiom makes sure those nested classes are the only subclasses. In fact, enums in Java use just this idiom.
If there are other constructors around, well then there's really nothing special about the private constructor. It gets used in an abstract class just as in any other.
Only thing I can think of is reusing common code shared by the other (protected) constructors. They could then call the private constructor in their first line.
Sometimes, the default no-arg constructor is made private, and another constructor which accepts arguments is provided. This constructor might then invoke other private constructor(s) . This forces implementations to supply these arguments, which might ensure some variable is always initialized, although this is not common practice (in my experience). If this is the requirement, you would be better off checking your variables and throwing an IllegalArgumentExeption, explaining why the variable needs to be initialized.
If you create an abstract class with only private constructors, the class is practically useless as no instances can ever be created. If the intention is to create a utility class with only static methods (like the Math class in the java.lang package), private constructors are acceptable, however the class should be marked final instead, as marking the class as abstract implies the class is to be extended.
As mentioned, to be used as a common, internal-use only constructor.
Abstract or not abstract, it's not uncommon to declare a private default constructor on a class containing only static public methods [helper methods] to prevent instantiating the class.
no other elegant use is possible
A private constructor in an abstract class can also serve the purpose of sealed classes (like in Scala or Kotlin etc.). Since you can still provide subclasses from within the abstract class, but outsiders cannot extend/implement (as #Marko Topolnik answered).
It does look like we will be getting sealed interface to more cleanly support this. See https://openjdk.java.net/jeps/8222777
A final class with only private constructors is a design used by singletons and multitons.
An abstract class which has only private constructors is the only way I've seen to prevent a class from being instantiated. I have seen it used to create utility classes (which only have static methods and/or members).
As for setting up user expectations I see that https://docs.oracle.com/javase/tutorial/java/IandI/abstract.html states "Abstract classes cannot be instantiated, but they can be subclassed." I note that it does not state any intention that they are expected to be subclassed.
I also note however that viewing some Java source code I find the following designs are used (none of which are abstract classes with only private constructors):
Final utility classes with private constructors
http://developer.classpath.org/doc/java/lang/Math-source.html
http://developer.classpath.org/doc/java/lang/System-source.html
Final utility classes with private constructors which
throw exceptions
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Objects.java
Neither abstract nor final utility classes with private constructors
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/ArrayPrefixHelpers.java
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Arrays.java
https://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Collections.java
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/FormattableFlags.java
Looks like a utility, but apparently can be instantiated (no private
constructors)
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/ArraysParallelSortHelpers.java
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();