Weve seen that class initialization is a complicated procedure Before I explain the reason lets have a look at the following piece of code: Output: You would get the following error message when you try to run the above java code. static block of parent class thanks a lot for this example i have been asked about it during an interview.
static myclass getObject{ UPDATE: For some reason I missed the recent post Lets consider the below program , Output: That is just an example to demonstrate the purpose of static keyword. I have edited the post to make it more clear. Does someone can tell me if its really useful? return new myclass(); I still think my article complements it with some interesting details.
-XX:+PrintCompilation -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining. In that case it is useful. This is the most common question that is asked during interview so here we are discuss why something is not allowed in java.
When we created the object of child class, it first invoked the constructor of parent class and then the constructor of its own class. If you made constructor as static then the constructor will be called before object creation same like main method. I just added more details to the post to make it easy to understand. Would you expect JDK updates 11.0.2 and 8u202 to fix the problem? sysout(constructor of class) Copyright 2012 2022 BeginnersBook . Constructor definition should not be static because constructor will be called each and every time when object is created. performance degradation Unfortunately, the fix covers only one particular case when
Say you want to read a file and then decide which object you want to create. Lets understand this with the help of an example , Output: method(); // child class is inheriting static method. } I just wanted to know what exactly is the use of a static block? Hi Arkya, all eventually result in deoptimization and a fall back to the interpreter. it happens in real time. of 100M elements in about 540ms. Now lets simply remove static and fill the array in the constructor. Almost 4 times faster! it wont suffer from performance penalty. Hi, failed to inline nextValue method. Static method cannot be inherited in the sub class.
The static method belongs to the class so has nothing to do with those concepts. through the resolution procedure again and again. Would you please give me one example. Just a method of child class. Lets slightly modify the example by moving array update out of : The algorithm hasnt changed, right? which ensures that static initializer executes in a thread-safe manner at most once. I was searching this concept on [Stackoverflow] and in-between i lied up here.Very good explanation.Can you tell me the use of Method hiding ? If you made constructor as static then the constructor will be called before object creation same like main method. This will violate the whole purpose of inheritance concept and that is reason why a constructor cannot be static. the class is not considered initialized while is running. This is the reason why constructor cannot be static Because if we make them static they cannot be called from child class thus object of child class cannot be created. now lasted 60 seconds on JDK 11.0.2! that allowed HotSpot JVM to invoke a static method in violation of JVMS. Modifier m = new Modifier(); You have mentioned as Its actually pretty simple to understand static method cannot be inherited in the sub class. However I did not mention anywhere in the article that constructors are inherited. I have corrected it. The constructor fills the similar array in 138ms. However, there was a zero-day bug JDK-8215634 here should be super not this. class myclass{ Please correct me if I am wrong. Sorry about that. GitHub Now if the class is uninitialized, the resolved invokestatic target is not saved Or, to be precise, more than 20 minutes. The explicit check can be found in the source code. Lets back to the point, since each constructor is being called by its subclass during creation of the object of its subclass, so if you mark constructor as static the subclass will not be able to access the constructor of its parent class because it is marked static and thus belong to the class only. This must be related to JIT compilation, so lets run the test with I hope its clear now. Its too easy to break the precondition if a hot loop moves from StaticDemo Think how we are able to access constructor from static method give. First constructors are not inherited at all does not matter static or non-static. Just a method of child class. by Claes Redestad on the same topic. Java has static blocks which can be treated as static constructor. I hope its clear now. But why? If you put the computation logic in a helper class with no static initializer, LinkedIn. uninitialized class return back. this explanation is not right. Very useful and abstract example .. thanks for this explanation. Hi Chaitanya. Here is a typical example that builds some static table at I guess yes but the fact is that they are not allowed in Java. public static void main (String args[]) Actually one can have static constructor by marking their class final, TO add one more point that constructor is static create static method in class and create object of that class in that staic method ie A good news - the workaround is pretty straightforward: Just dont do heavy computation in an uninitialized class directly. Second if we even go by your concept & treat constructors as methods so the constructors have default(package-private) access specifier.It means it wont be available in the child class. If you want to make something happen before your object has created. I think you got confused because of an ambiguous statement mentioned in the post. You would never find it in the production code or anywhere else because static constructors are not allowed in java and this post is to discuss the reason of it. In fact, this is exactly what static initializers are designed for. In the above example we have used static blocks in both the classes which worked perfectly. uninitialized class may be an overwhelming obstacle for HotSpot compiler. } Just try to run the above example. It happened because the new keyword creates the object and then invokes the constructor for initialization, since every child class constructor by default has super() as first statement which calls its parent classs constructor. let me know I you have any other questions. initialization time: On my laptop with JDK 11.0.1 static initializer fills the array } You probably want to underline that static block is only called only before the first instance of the class is created. /*By default this() is hidden here */ Most of CPU time is spent inside JVM runtime - SharedRuntime::resolve_static_call_C(). Refer static keyword. probably you have a mistake in comment to code Yes, to some extent. The points you mentioned are correct. I explained this problem in detail on Stack Overflow. Compilation log shows desperate attemts to compile the method, but they It turns out that access to a static field from a static method of I just pointed out that they are being invoked during object creation of sub class, thats all. Did you notice? myclass(){ The bug has been fixed in JDK 11.0.2 and 8u201, but at the cost of the terrible Static means something which is the property of a class while constructors are something which are property of an object so the reason is concept of static constructor is basic design violation. It sounds unbelievable, but the above slowdown is not even the worst one. However, the output of -XX:+PrintCompilation was the same as before, Actually you can rewrite a static method in subclasses but this is not called a override because override should be related to polymorphism and dynamic binding. Shouldnt it be static constructor cannot be inherited in the subclass. is the root method of the compilation. Here prepareTable() becomes the compilation root, and all the problems of Thanks for pointing it out. Its actuallypretty simple to understand Everything that is marked static belongs to the class only, for example static methodcannot be inherited in the sub class because they belong to the class in which they have been declared. Since invocation of is a part of class initialization procedure, Oops When compiling static initializer (called in a class file) both C1 and C2 in the constant pool cache, so each invocation of a static method needs to go to some other method called by static initializer. if cant mark constructor as static, why can we mark a method as a static and inherit in child class. We cannot use static constructor so its a good alternative if we want to perform a static task during object creation. Time to engage async-profiler. StaticDemoChild and is addressed in OpenJDK 13 with a possibility to backport later to OpenJDK 11. The statement super() is used to call the parent class(base class) constructor. Its a quite common practice to prepare immutable data during class Also, constructors are not methods because they do not have a return type. The bug is known - JDK-8188133 I never saw a such thing in production code. static member can be inherited but cannot be overridded by subclass. initialization and save the results in staticfinal fields. Except that now it takes forever. Twitter modifier static not allowed here. Another good point mentioned by Prashanth in the comment section: Constructor definition should not be static because constructor will be called each and every time when object is created. { Constructors are not static coz they violate the basic meaning of static. Have you heard of static constructor in Java? By Chaitanya Singh | Filed Under: OOPs Concept. Hi Pravat, You can overload a static method but you cant override a static method. Here we get to the first problem: HotSpot does not inline methods of uninitialized classes. Sitemap. static block of child class The rewrite of static method is more like a shadowing. Privacy Policy . What caused the dramatic slowdown then? A constructorcan not be marked as static in Java. was still compiled. Thanks in advance. What took 540 ms on JDK 11.0.1