Fluent Essays

Identifying Errors in Code 1. ( b2 == b1 ) And ( a1 <= a2 ) 2. if ( a1 == 4 ); System.out.println( a1 equals 4 ); 3 if ( b2 == true ) System.out.println( b2 is true ); 4. if ( b1 == true ) System.out.println( b1 is true ); else System.out.println( b1 is false ); else if ( a1 < 100 ) System.out.println( a1 is <=100 ); 5. if ( b2 ) System.out.println( b2 is true ); else if ( a1 50 ) ) System.out.println( a1 50 ); ) else System.out.println( none of the above ); explanation i am attaching a file for an idea how you have to explain 10/8/21, 9:26 PM inheritance - java.lang.ClassException: A cannot be cast into B - Stack Overflow https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b 1/8 java.lang.ClassException: A cannot be cast into B Asked 8 years, 3 months ago Active 1 year, 11 months ago 20k timesViewed 15 4 I implemented this code: { } { } { { (B) (); (B) (); } } class A //some code class B extends A // some code class C public static void main(String []args) B b1 = new A A a1 = new A Both of these lines, when compiled separately, compile fine,but give runtime error with .java.lang.ClassException: A cannot be cast into B Why they compile well, but give a runtime error? java inheritance Share Improve this question Follow edited Jun 20 13 at 15:47 blackpanther 10k 10 43 76 asked Jun 20 13 at 15:40 Ozil 925 3 11 28 Because an instance of has nothing to do with . And thats a runtime check.A B – Brian Roach Jun 20 13 at 15:43 4 Seriously, this is basic OOP. But looks like people dont care about it and prefer to post answers and upvote when this is widely explained in the net. – Luiggi Mendoza Jun 20 13 at 15:54 7 Consider the classic example: . By the cast, youre telling the compiler to trust you that youre not going to do mistakes. But, in runtime, your program will crash since youre not really know what youre doing. Is dog an animal? . Is animal a Dog? . But not alwaysYes Maybe – Maroun Jun 20 13 at 15:55 13 Answers Active Oldest Variables of type can store references to objects of type or its subtypes like in your case class . A A B   Votes https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b?lastactivity https://stackoverflow.com/posts/17217965/timeline https://stackoverflow.com/questions/tagged/java https://stackoverflow.com/questions/tagged/inheritance https://stackoverflow.com/q/17217965 https://stackoverflow.com/posts/17217965/edit https://stackoverflow.com/posts/17217965/revisions https://stackoverflow.com/users/1618135/blackpanther https://stackoverflow.com/users/2459789/ozil https://stackoverflow.com/users/302916/brian-roach https://stackoverflow.com/users/1065197/luiggi-mendoza https://stackoverflow.com/users/1735406/maroun https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b?answertab=active#tab-top https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b?answertab=oldest#tab-top https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b?answertab=votes#tab-top 10/8/21, 9:26 PM inheritance - java.lang.ClassException: A cannot be cast into B - Stack Overflow https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b 2/8 10 So it is possible to have code like: (); A a = new B Variable is of type so it have only access to API of that class, it cant access methods added in class B which object it refers to. But sometimes we want to be able to access those methods so it should be possible to somehow store reference from in some variable of more accurate type (here ) via which we would be able to access those additional methods from class B. BUT HOW CAN WE DO THAT? a A a B Lets try to achieve it this way: a; B b = //WRONG!!! Type mismatch error Such code gives compile time error. It happens to save us from situation like this:Type mismatch class B1 extends A class B2 extends A and we have .A a = new B1(); Now lets try to assign . Remember that so it needs to generate code which will be safe for all possible values. If compiler wouldnt complain about it should also allow to compile . So just to be safe it doesnt let us do it. B1 b = a; compiler doesnt know what actually is held under variable a B1 b = a; B2 b = a; So what should we do to assign reference from to ? We need to tell compiler that we are aware of potential type mismatch issue here, but we are sure that reference held in can be safely assigned in variable of type . We do so by value from to type via . a B1 explicitly a B casting a B (B)a (B)a; B b = But lets go back to example from your question (B) (); (B) (); B b1 = new A A a1 = new A operator returns reference of the same type as created object, so returns reference of the type so new new A() A (B) (); B b1 = new A can be seen as https://stackoverflow.com/posts/17218727/timeline 10/8/21, 9:26 PM inheritance - java.lang.ClassException: A cannot be cast into B - Stack Overflow https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b 3/8 (); (B) tmp; A tmp = new A B b1 = Problem here is that . Why such limitation exist? Lets say that derived class adds some new methods that supertype doesnt have like you cant store reference to object of superclass in variable of its derived type { } { i; { .i=i; } } class A // some code class B extends A private int public void setI( i)int this If this would be allowed (B) (); B b = new A you could later end up with invoking . But will it be correct? No because instance of class A doesnt have method nor which that method uses. b.setI(42); setI field i So to prevent such situation at throws .(B)new A(); runtime java.lang.ClassCastException Share Improve this answer Follow edited Oct 17 19 at 18:54 answered Jun 20 13 at 16:18 Pshemo 115k 22 170 244 Awesome answer! Thanks a lot. –  Ozil Jun 20 13 at 16:38 7 The reason it fails at runtime is that the object isnt a B. Its an A. So while As can be casts as Bs, yours cannot. some The compilier just cant analyze everything that happened to your A object. For example. (); (); (B) a1; (B) a2; A a1 = new B A a2 = new A B b1 = // Ok B b2 = // Fails https://stackoverflow.com/a/17218727 https://stackoverflow.com/posts/17218727/edit https://stackoverflow.com/posts/17218727/revisions https://stackoverflow.com/users/1393766/pshemo https://stackoverflow.com/users/2459789/ozil https://stackoverflow.com/posts/17218074/timeline 10/8/21, 9:26 PM inheritance - java.lang.ClassException: A cannot be cast into B - Stack Overflow https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b 4/8 So the compilier isnt sure whether your A object is actually castable to a B. So in the above, it would think that the last 2 lines were ok. But when you actually run the program, it realizes that is not a B, its only an A.a2 Share Improve this answer Follow edited Jun 20 13 at 15:56 answered Jun 20 13 at 15:45 Snowy Coder Girl 5,160 10 38 67 2 (B) (); A a1 = new A Because is NOT .A B Compile time works because you are casting and explicitly guaranteeing compiler that you are sure at runtime will be .A B Share Improve this answer Follow answered Jun 20 13 at 15:41 kosa 64.2k 12 119 160 2 When B extends A, it means all methods and properties of A are also present in B. So you can ever cast B to A, but you CANNOT cast A to B. You have to be really care about casting in your application. Share Improve this answer Follow answered Jun 20 13 at 15:44 Martin Strejc 3,956 2 19 35 2 when you say B extends A, A becomes father of B now technically B has all charecteristics of A while A has charecteristics of itself onlyplus its own if you say convert A into B and assign to B, that is ok but if you say cast A into B and assign to A, thats not possible as A here does not know extra charecteristics present in B. and these things happens at runtime so it will give you a runtime error. Share Improve this answer Follow answered Jun 20 13 at 15:48 dev2d 4,031 2 26 51 https://stackoverflow.com/a/17218074 https://stackoverflow.com/posts/17218074/edit https://stackoverflow.com/posts/17218074/revisions https://stackoverflow.com/users/815749/snowy-coder-girl https://stackoverflow.com/posts/17217990/timeline https://stackoverflow.com/a/17217990 https://stackoverflow.com/posts/17217990/edit https://stackoverflow.com/users/1094597/kosa https://stackoverflow.com/posts/17218063/timeline https://stackoverflow.com/a/17218063 https://stackoverflow.com/posts/17218063/edit https://stackoverflow.com/users/2018247/martin-strejc https://stackoverflow.com/posts/17218165/timeline https://stackoverflow.com/a/17218165 https://stackoverflow.com/posts/17218165/edit https://stackoverflow.com/users/2381006/dev2d 10/8/21, 9:26 PM inheritance - java.lang.ClassException: A cannot be cast into B - Stack Overflow https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b 5/8 2 The name it self implies the will just looks at compile-time type of the .compiler expression It does not do assumptions on the runtime type of the .expression http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.5.1 Coming to real problem You cast A to B.You cast B to A.When you have a Mango,You have Fruit.But when you have a Fruit,It not mean that You have a Mango. cannot can Share Improve this answer Follow edited Jun 20 13 at 15:52 answered Jun 20 13 at 15:43 Suresh Atta 116k 37 181 287 1 Im not sure about the compile part, but I can explain the runtime error. B extends A, which means that every object of class B, is also an object of type A. The other way around is not true. Compare A with Mammal, and B with Cow. A Cow is always a Mammal, but not every Mammal is a Cow. Share Improve this answer Follow answered Jun 20 13 at 15:43 Johanneke 4,461 3 16 32 1 Has to do with when casting is done. You are telling the compiler: Hey, dont worry about it, this is what I say it is, if you have a problem, take it up with me at runtime. Basically, the compiler is letting you do your thing. When you explicitly cast something, the compiler doesnt do checks. When you run, and the program tries to cast but fails, thats when you will see the error. Share Improve this answer Follow answered Jun 20 13 at 15:43 Captain Skyhawk 3,445 2 21 37 1 Because, all the compiler sees is that an A is cast into a B. Since some As can actually be Bs this may work for those As. By writing the explicit cast, you ensure that this particular A is actually a valid B. However, this is not the case. (); (); A justA = new A A anAThatIsAlsoAValidB = new B // implicit cast to supertype https://stackoverflow.com/posts/17218027/timeline http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.5.1 https://stackoverflow.com/a/17218027 https://stackoverflow.com/posts/17218027/edit https://stackoverflow.com/posts/17218027/revisions https://stackoverflow.com/users/1927832/suresh-atta https://stackoverflow.com/posts/17218039/timeline https://stackoverflow.com/a/17218039 https://stackoverflow.com/posts/17218039/edit https://stackoverflow.com/users/2151700/johanneke https://stackoverflow.com/posts/17218045/timeline https://stackoverflow.com/a/17218045 https://stackoverflow.com/posts/17218045/edit https://stackoverflow.com/users/2283678/captain-skyhawk https://stackoverflow.com/posts/17218207/timeline 10/8/21, 9:26 PM inheritance - java.lang.ClassException: A cannot be cast into B - Stack Overflow https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b 6/8 (A) anAThatIsAlsoAValidB ; (A) justA; B b1 = // Cast an A into a B. At runtime, this will work fine! Compiler allows casting A into B. B b2 = // Cast an A into a B. At runtime, this wont work. Compiler has/uses no more info than above. Heres why the compiler does not really know about the type: com.example. .example.ThridPartyType(); (B) obj.getSomeA(); ThridPartyType obj = new com B b = // getSomeA() returns A and that is all the compiler knows. // Depeding on the implementation of ThridPartyType::getSomeA() the A returned may or may not actually also be a valid B. // Hence, if the cast works or not will only be known at runtime. If it doesnt, the Exception is thrown. Share Improve this answer Follow edited Jun 20 13 at 16:04 answered Jun 20 13 at 15:50 b.buchhold 3,807 2 19 33 1 Following is a compiletime casting - (); A a = new B Such static castings are implicitly performed by the compiler, because the compiler is aware of the fact that B is-a A. Following doesnt compile - (); B b = new A No compiletime casting here because the compiler knows that A is not B. Following compiles - (B) (); B b = new A It is a dynamic casting. With you are telling the compiler explicitly that you want the casting to happen at runtime. And you get a CCE at runtime, when the runtime tries to perform the cast but finds out that it cannot be done and throws a CCE. (B) When you do (or have to do) something like this, the responsibility falls upon you (not the compiler) to make sure that a CCE doesnt occur at runtime. Share Improve this answer Follow edited Feb 23 14 at 2:13 answered Jun 20 13 at 15:53 Bhesh Gurung https://stackoverflow.com/posts/17218207/timeline https://stackoverflow.com/a/17218207 https://stackoverflow.com/posts/17218207/edit https://stackoverflow.com/posts/17218207/revisions https://stackoverflow.com/users/762567/b-buchhold https://stackoverflow.com/posts/17218271/timeline https://stackoverflow.com/a/17218271 https://stackoverflow.com/posts/17218271/edit https://stackoverflow.com/posts/17218271/revisions https://stackoverflow.com/users/738746/bhesh-gurung 10/8/21, 9:26 PM inheritance - java.lang.ClassException: A cannot be cast into B - Stack Overflow https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b 7/8 49k 20 87 139 0 Its simple. Think that when you are extending you have to use is a B `is a` A A `is not` B A more realistic example { } { } { } class Animal class Dog extends Animal class Cat extends Animal A DOG Animal AN ANIMAL a DOG necessary (Example : a cat is not a dog, and a cat is an animal) IS A IS NOT You are getting cause , in runtime realize that that animal is not a dog, this is call and is not safe what are you trying to do. runtime exception downcasting Share Improve this answer Follow answered Jun 20 13 at 15:42 nachokk 14.1k 4 22 49 0 The fact that means that , i.e. B is like A but probably add some other stuff.B extends A B is A The opposite is wrong. . Therefore you cannot cast to .A is not B A B Think this way. is . is . Is animal? Yes it is. Is any animal Bee? Not it is not. For example Dog is animal but definitely not a Bee. A Animal B Bee Bee Share Improve this answer Follow answered Jun 20 13 at 15:44 AlexR 110k 14 120 197 0 Because the is a parent of . the functionality of the but keeps the original functionality of . So can in fact be cast to , but not vice versa. A B B extends A A B A What if added a new method, lets say . If you would try to call that method through variable on the instance (imagine that the cast worked) what would you expect? Well, you would definitely get an error because that method does not exist in . B newMethodInB() B A A https://stackoverflow.com/posts/17218005/timeline https://stackoverflow.com/a/17218005 https://stackoverflow.com/posts/17218005/edit https://stackoverflow.com/users/2415194/nachokk https://stackoverflow.com/posts/17218070/timeline https://stackoverflow.com/a/17218070 https://stackoverflow.com/posts/17218070/edit https://stackoverflow.com/users/478399/alexr https://stackoverflow.com/posts/17218149/timeline 10/8/21, 9:26 PM inheritance - java.lang.ClassException: A cannot be cast into B - Stack Overflow https://stackoverflow.com/questions/17217965/java-lang-classexception-a-cannot-be-cast-into-b 8/8 Share Improve this answer Follow answered Jun 20 13 at 15:48 darijan 9,509 23 37 https://stackoverflow.com/posts/17218149/timeline https://stackoverflow.com/a/17218149 https://stackoverflow.com/posts/17218149/edit https://stackoverflow.com/users/1256583/darijan