Java runtime error что это
Это первая часть статьи, посвященной такому языковому механизму Java как исключения (вторая (checked/unchecked) вот). Она имеет вводный характер и рассчитана на начинающих разработчиков или тех, кто только приступает к изучению языка.
1. Ключевые слова: try, catch, finally, throw, throws
- try
- catch
- finally
- throw
- throws
catch:
Годится
throw:
Годится
Кроме того, throw требуется не-null аргумент, иначе NullPointerException в момент выполнения
throw и new — это две независимых операции. В следующем коде мы независимо создаем объект исключения и «бросаем» его
Однако, попробуйте проанализировать вот это
2. Почему используем System.err, а не System.out
System.out — buffered-поток вывода, а System.err — нет. Таким образом вывод может быть как таким
Так и вот таким (err обогнало out при выводе в консоль)
Давайте это нарисуем
3. Компилятор требует вернуть результат (или требует молчать)
Если в объявлении метода сказано, что он возвращает НЕ void, то компилятор зорко следит, что бы мы вернули экземпляр требуемого типа или экземпляр типа, который можно неявно привести к требуемому
вот так не пройдет (другой тип)
Вот так не выйдет — нет возврата
и вот так не пройдет (компилятор не может удостовериться, что возврат будет)
Компилятор отслеживает, что бы мы что-то вернули, так как иначе непонятно, что должна была бы напечатать данная программа
Из-забавного, можно ничего не возвращать, а «повесить метод»
Тут в d никогда ничего не будет присвоено, так как метод sqr повисает
Компилятор пропустит «вилку» (таки берем в квадрат ИЛИ висим)
Но механизм исключений позволяет НИЧЕГО НЕ ВОЗВРАЩАТЬ!
Итак, у нас есть ТРИ варианта для компилятора
Но КАКОЙ ЖЕ double вернет функция, бросающая RuntimeException?
А НИКАКОЙ!
Подытожим: бросаемое исключение — это дополнительный возвращаемый тип. Если ваш метод объявил, что возвращает double, но у вас нет double — можете бросить исключение. Если ваш метод объявил, что ничего не возвращает (void), но у вам таки есть что сказать — можете бросить исключение.
Давайте рассмотрим некоторый пример из практики.
Задача: реализовать функцию, вычисляющую площадь прямоугольника
важно, что задание звучит именно так, в терминах предметной области — «вычислить площадь прямоугольника», а не в терминах решения «перемножить два числа»:
Вопрос: что делать, если мы обнаружили, что хотя бы один из аргументов — отрицательное число?
Если просто умножить, то мы пропустили ошибочные данные дальше. Что еще хуже, возможно, мы «исправили ситуацию» — сказали что площадь прямоугольника с двумя отрицательными сторонами -10 и -20 = 200.
Мы не можем ничего не вернуть
Можно, конечно, отписаться в консоль, но кто ее будет читать и как определить где была поломка. При чем, вычисление то продолжится с неправильными данными
Можно вернуть специальное значение, показывающее, что что-то не так (error code), но кто гарантирует, что его прочитают, а не просто воспользуются им?
Можем, конечно, целиком остановить виртуальную машину
и другие операторы.
- вызов метода: создаем новый фрейм, помещаем его на верхушку стека и переходим в него
- выход из метода: возвращаемся к предыдущему фрейму (через return или просто кончились инструкции в методе)
throw — выходим из ВСЕХ фреймов
Останавливаем через 1 фрейм (фактически аналог return, просто покинули фрейм «другим образом»)
Итак, давайте сведем все на одну картинку
5. try + catch (catch — полиморфен)
Напомним иерархию исключений
То, что исключения являются объектами важно для нас в двух моментах
1. Они образуют иерархию с корнем java.lang.Throwable (java.lang.Object — предок java.lang.Throwable, но Object — уже не исключение)
2. Они могут иметь поля и методы (в этой статье это не будем использовать)
По первому пункту: catch — полиморфная конструкция, т.е. catch по типу Parent перехватывает летящие экземпляры любого типа, который является Parent-ом (т.е. экземпляры непосредственно Parent-а или любого потомка Parent-а)
Даже так: в блоке catch мы будем иметь ссылку типа Exception на объект типа RuntimeException
catch по потомку не может поймать предка
catch по одному «брату» не может поймать другого «брата» (Error и Exception не находятся в отношении предок-потомок, они из параллельных веток наследования от Throwable)
По предыдущим примерам — надеюсь вы обратили внимание, что если исключение перехвачено, то JVM выполняет операторы идущие ПОСЛЕ последних скобок try+catch.
Но если не перехвачено, то мы
1. не заходим в блок catch
2. покидаем фрейм метода с летящим исключением
А что будет, если мы зашли в catch, и потом бросили исключение ИЗ catch?
В таком случае выполнение метода тоже прерывается (не печатаем «3»). Новое исключение не имеет никакого отношения к try-catch
Мы можем даже кинуть тот объект, что у нас есть «на руках»
И мы не попадем в другие секции catch, если они есть
Обратите внимание, мы не напечатали «3», хотя у нас летит Error а «ниже» расположен catch по Error. Но важный момент в том, что catch имеет отношение исключительно к try-секции, но не к другим catch-секциям.
Как покажем ниже — можно строить вложенные конструкции, но вот пример, «исправляющий» эту ситуацию
6. try + catch + catch + .
Как вы видели, мы можем расположить несколько catch после одного try.
Но есть такое правило — нельзя ставить потомка после предка! (RuntimeException после Exception)
Ставить брата после брата — можно (RuntimeException после Error)
Как происходит выбор «правильного» catch? Да очень просто — JVM идет сверху-вниз до тех пор, пока не найдет такой catch что в нем указано ваше исключение или его предок — туда и заходит. Ниже — не идет.
Выбор catch осуществляется в runtime (а не в compile-time), значит учитывается не тип ССЫЛКИ (Throwable), а тип ССЫЛАЕМОГО (Exception)
7. try + finally
finally-секция получает управление, если try-блок завершился успешно
finally-секция получает управление, даже если try-блок завершился исключением
finally-секция получает управление, даже если try-блок завершился директивой выхода из метода
finally-секция НЕ вызывается только если мы «прибили» JVM
System.exit(42) и Runtime.getRuntime().exit(42) — это синонимы
И при Runtime.getRuntime().halt(42) — тоже не успевает зайти в finally
Однако finally-секция не может «починить» try-блок завершившийся исключение (заметьте, «more» — не выводится в консоль)
Трюк с «if (true) <. >» требуется, так как иначе компилятор обнаруживает недостижимый код (последняя строка) и отказывается его компилировать
И finally-секция не может «предотвратить» выход из метода, если try-блок вызвал return («more» — не выводится в консоль)
Однако finally-секция может «перебить» throw/return при помощи другого throw/return
finally-секция может быть использована для завершающего действия, которое гарантированно будет вызвано (даже если было брошено исключение или автор использовал return) по окончании работы
Например для освобождения захваченной блокировки
Или для закрытия открытого файлового потока
Специально для этих целей в Java 7 появилась конструкция try-with-resources, ее мы изучим позже.
Вообще говоря, в finally-секция нельзя стандартно узнать было ли исключение.
Конечно, можно постараться написать свой «велосипед»
Не рекомендуемые практики
— return из finally-секции (можем затереть исключение из try-блока)
— действия в finally-секции, которые могут бросить исключение (можем затереть исключение из try-блока)
8. try + catch + finally
Не заходим в catch, заходим в finally, продолжаем после оператора
Есть исключение и есть подходящий catch
Заходим в catch, заходим в finally, продолжаем после оператора
Есть исключение но нет подходящего catch
Не заходим в catch, заходим в finally, не продолжаем после оператора — вылетаем с неперехваченным исключением
9. Вложенные try + catch + finally
Операторы обычно допускают неограниченное вложение.
Пример с if
Суть в том, что try-cacth-finally тоже допускает неограниченное вложение.
Например вот так
Или даже вот так
Ну что же, давайте исследуем как это работает.
Вложенный try-catch-finally без исключения
Мы НЕ заходим в обе catch-секции (нет исключения), заходим в обе finally-секции и выполняем обе строки ПОСЛЕ finally.
Вложенный try-catch-finally с исключением, которое ПЕРЕХВАТИТ ВНУТРЕННИЙ catch
Мы заходим в ПЕРВУЮ catch-секцию (печатаем «3»), но НЕ заходим во ВТОРУЮ catch-секцию (НЕ печатаем «6», так как исключение УЖЕ перехвачено первым catch), заходим в обе finally-секции (печатаем «4» и «7»), в обоих случаях выполняем код после finally (печатаем «5»и «8», так как исключение остановлено еще первым catch).
Вложенный try-catch-finally с исключением, которое ПЕРЕХВАТИТ ВНЕШНИЙ catch
Мы НЕ заходим в ПЕРВУЮ catch-секцию (не печатаем «3»), но заходим в ВТОРУЮ catch-секцию (печатаем «6»), заходим в обе finally-секции (печатаем «4» и «7»), в ПЕРВОМ случае НЕ выполняем код ПОСЛЕ finally (не печатаем «5», так как исключение НЕ остановлено), во ВТОРОМ случае выполняем код после finally (печатаем «8», так как исключение остановлено).
Вложенный try-catch-finally с исключением, которое НИКТО НЕ ПЕРЕХВАТИТ
Мы НЕ заходим в ОБЕ catch-секции (не печатаем «3» и «6»), заходим в обе finally-секции (печатаем «4» и «7») и в обоих случаях НЕ выполняем код ПОСЛЕ finally (не печатаем «5» и «8», так как исключение НЕ остановлено), выполнение метода прерывается по исключению.
Контакты
Я занимаюсь онлайн обучением Java (вот курсы программирования) и публикую часть учебных материалов в рамках переработки курса Java Core. Видеозаписи лекций в аудитории Вы можете увидеть на youtube-канале, возможно, видео канала лучше систематизировано в этой статье.
In one of my prof slides on ploymorphism, I see this piece of code with a couple of comments:
As you can see, it says in the first casting statement that it'll produce run-time error and in the other one it says it'll produce compiler error.
What makes these errors? and how they differ from each other?
Doesn't matter if it has addressed my example or not, what really matters is that I got the answer clearly. That guy explained it in a simple way that can be easy to understand to a naive programmer like me.
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Errors in Java occur when a programmer violates the rules of Java programming language.
It might be due to programmer’s typing mistakes while developing a program. It may produce incorrect output or may terminate the execution of the program abnormally.
For example, if you use the right parenthesis in a Java program where a right brace is needed, you have made a syntax error. You have violated the rules of Java language.
Therefore, it is important to detect and fix properly all errors occurring in a program so that the program will not terminate during execution.
2 Answers 2
I am unable to follow where exactly RuntimeException should be used
That's probably because you are looking at an argument, i.e. people are disagreeing about exactly this point.
and how it is different from normal Exceptions and its subclasses.
Very simple: All subclasses of Exception (except for RuntimeException and its subclasses) are checked i.e. the compiler will reject the code unelss you catch or declare them in the method signature. However, subclasses of RuntimeException are unchecked.
Googling gave me a complex answer, that is, it should be used to deal with programming logic errors and should be thrown when no Exception should normally occur, such as in the default block of switch-case construct.
This is the conventional wisdom, which says that for everything that a program can usefully deal with, you should use checked exceptions because then the compiler will force you to deal with them. Conversely, programs can typically not deal usefully with programmer errors, thus they don't have to be checked. This is how the Java Standard API uses RuntimeException .
The discussion you linked to is sparked by the view of some people (this includes me) who think that checked exceptions lead to bad code and should therefore not be used. Since you can't disable exception checking in the compiler, the only way to do this is to use only RuntimeException and its subclasses.
One observation that IMO supports this view is that the conventional wisdom of "use unchecked exceptions only for programmer error" is in fact mainly a rationalization of backwards-reasoning: there is no code safety reason why the compiler should not force you to deal with programmer errors. However, something like NullPointerException and ArrayIndexOutOfBoundsException can crop up almost anywhere, and if those were checked, nobody would ever want to program in Java. Thus, the language designers had to make a, huh, exception for those, and make them unchecked. To explain this, they came up with the "unchecked exceptions are for programmer errors" story.
A runtime error in Java is an application error that occurs during the execution of a program. A runtime error occurs when a program is syntactically correct but contains an issue that is only detected during program execution. These issues cannot be caught at compile-time by the Java compiler and are only detected by the Java Virtual Machine (JVM) when the application is running.
- IO errors
- Division by zero errors
- Out of range errors
- Undefined object errors
Accessing an out of range value in an array
Here is an example of a java.lang.ArrayIndexOutOfBoundsException thrown due to an attempt to access an element in an array that is out of bounds:
In this example, an array is initialized with 5 elements. An element at position 5 is later attempted to be accessed in the array, which does not exist, leading to a java.lang.ArrayIndexOutOfBoundsException runtime error:
Types of Errors in Java Programming
When we write a program for the first time, it usually contains errors. These errors are mainly divided into three types:
1. Compile-time errors (Syntax errors)
2. Runtime errors
3. Logical errors
What Causes Runtime Errors in Java
- Dividing a number by zero.
- Accessing an element in an array that is out of range.
- Attempting to store an incompatible type value to a collection.
- Passing an invalid argument to a method.
- Attempting to convert an invalid string to a number.
- Insufficient space in memory for thread data.
When any such errors are encountered, the Java compiler generates an error message and terminates the program abnormally. Runtime errors don’t need to be explicitly caught and handled in code. However, it may be useful to catch them and continue program execution.
To handle a runtime error, the code can be placed within a try-catch block and the error can be caught inside the catch block.
Division by zero error
Here is an example of a java.lang.ArithmeticException , a type of runtime exception, thrown due to division by zero:
In this example, an integer a is attempted to be divided by another integer b , whose value is zero, leading to a java.lang.ArithmeticException :
Runtime Error Examples
Runtime Errors in Java
Runtime errors occur when a program is successfully compiled creating the .class file but does not run properly. It is detected at run time (i.e. during the execution of the program).
Java compiler has no technique to detect runtime errors during compilation because a compiler does not have all of the runtime information available to it. JVM is responsible to detect runtime errors while the program is running.
Such a program that contains runtime errors, may produce wrong results due to wrong logic or terminate the program. These runtime errors are usually known as exceptions.
For example, if a user inputs a value of string type in a program but the computer is expecting an integer value, a runtime error will be generated.
The most common runtime errors are as follows:
1. Dividing an integer by zero.
2. Accessing an element that is out of range of the array.
3. Trying to store a value into an array that is not compatible type.
4. Passing an argument that is not in a valid range or valid value for a method.
5. Striving to use a negative size for an array.
6. Attempting to convert an invalid string into a number.
7. and many more.
When such errors are encountered in a program, Java generates an error message and terminates the program abnormally. To handle these kinds of errors during the runtime, we use exception handling technique in java program.
Let’s take different kinds of example programs to understand better. In this program, we will divide an integer number by zero. Java compiler cannot detect it.
Program source code 5:
The above program is syntactically correct. There is no syntax error and therefore, does not cause any problem during compilation. While executing, runtime error occurred that is not detected by compiler.
The error is detected by JVM only in runtime. Default exception handler displays an error message and terminates the program abnormally without executing further statements in the program.
Let’s take an example program where we will try to retrieve a value from an array using an index that is out of range of the array.
Program source code 6:
Java Compile Time Errors
Compile-time errors occur when syntactical problems occur in a java program due to incorrect use of Java syntax.
These syntactical problems may be missing semicolons, missing brackets, misspelled keywords, use of undeclared variables, class not found, missing double-quote in Strings, and so on.
These problems occurring in a program are called syntax errors in Java.
Since all syntax errors are detected by Java compiler, therefore, these errors are also known as compile time errors in Java.
When Java compiler finds syntax errors in a program, it prevents the code from compile successfully and will not create a .class file until errors are not corrected. An error message will be displayed on the console screen.
These errors must be removed by debugging before successfully compile and run the program. Let’s take an example program where we will get a syntax error.
Program source code 1:
When you will try to compile the above program, Java compiler will tell you where errors are in the program. Then you can go to the appropriate line, correct error, and recompile the program.
Let’s create a program where we will try to call the undeclared variable. In this case, we will get unresolved compilation problem.
Program source code 2:
Program source code 3: Missing parenthesis in for statement.
Program source code 4: Missing double quote in String literal.
We may also face another error related to the directory path. An error such as
means that you have not set the path correctly.
Runtime Errors vs Compile-Time Errors
Compile-time errors occur when there are syntactical issues present in application code, for example, missing semicolons or parentheses, misspelled keywords or usage of undeclared variables.
These syntax errors are detected by the Java compiler at compile-time and an error message is displayed on the screen. The compiler prevents the code from being executed until the error is fixed. Therefore, these errors must be addressed by debugging before the program can be successfully run.
On the other hand, runtime errors occur during program execution (the interpretation phase), after compilation has taken place. Any code that throws a runtime error is therefore syntactically correct.
10 Answers 10
A run time error will only occur when the code is actually running. These are the most difficult - and lead to program crashes and bugs in your code which can be hard to track down.
An example might be trying to convert a string: "hello" into an integer:
The compiler may not see this as a problem but when run an error will be thrown.
Compiler errors are due to inaccuracies in code, where the compiler throws an error to alert you to something which will not compile, and therefore cannot be run.
An example of a compiler error would be:
Hope that helps.
A run time error will only occur when the code is actually running. These are the most difficult - and lead to program crashes and bugs in your code which can be hard to track down. This is true relative to compile-time errors but runtime errors are much easier to debug than a program that produces invalid results silently, which is typically the most difficult class of bugs to identify and resolve.
A runtime error happens during the running of the program. A compiler error happens when you try to compile the code.
If you are unable to compile your code, that is a compiler error.
If you compile and run your code, but then it fails during execution, that is runtime.
In your example, discountVariable is of declared type DiscountSale and saleVariable is of another type. When you try to assign the value of one to the other with the line discountVariable = saleVariable , this causes a compiler error. If you put the cast in place (DiscountSale) , this tells the compiler that it's okay. It then breaks when you run it because saleVariable is not actually of the DiscountSale type.
+1 because this is the simplest best answer to the question: What is the difference between a runtime error and a compiler error? I would be much happier if the answer actually addressed the specific example offered by the OP, however.
Compile time errors refers to syntax and semantics. For example, if you do operations that involves different types. Ex: adding a string with an int, or dividing a string by a real. (read the last paragraph thou. )
Run Time errors are those that are detected when the program execute. For example, division by zero. The compiler can not know if the operation x/a-b will leads to division by zero until the execution.
This is a very broad explanation. There are many smart compilers, and, also, is possible to do internal casting among different types that leads to operations that make sense. It it possible to pre-compile code and see some run time errors even if the code is not executed.
Compile time errors are errors of syntax and semantics.
Run time errors are errors of logic primarily. Due to something the programmer has overlooked, the program crashes e.g. division by 0, accessing a variable without initializing it first etc.
Compile Time error means that the Compiler knows that discountVariable = saleVariable must be end with a semi colon as below discountVariable = saleVariable; so it will throw an error when you compile the code.
Run Time error means that the error will occur at run time, because even though you are casting saleVariable into discountVariable, the cast cannot take because they differ in type.
think you've already got the general desc of what's the difference. Specifically in the code you have shown in the OP,
- In second statement, compiler compares the types on LHS and RHS and finds no implicit cast possible so it gives the error.
- first statement is seen by compiler as the same, but here programmer explicitly casts the type, which is as good as telling compiler that I know what I'm doing and of course the compiler trusts you and gives you no errors.
Its because the compiler doesn't know the object type of "saleVariable" until that value has actually been set when the program is running.
Your are forcing whatever is in salesVariable into the type DiscountSale this is considered unsafe and cannot be evaluated until runtime.
Not a +1 because "the compiler doesn't know the object type of saleVariable". That's not true. The compiler knows the type - it's just incompatable with DiscountSale.
Compilation/Compile time/Syntax/Semantic errors: Compilation or compile time errors are error occurred due to typing mistake, if we do not follow the proper syntax and semantics of any programming language then compile time errors are thrown by the compiler. They wont let your program to execute a single line until you remove all the syntax errors or until you debug the compile time errors.
Example: Missing a semicolon in C or mistyping int as Int .
Runtime errors: Runtime errors are the errors that are generated when the program is in running state. These types of errors will cause your program to behave unexpectedly or may even kill your program. They are often referred as Exceptions.
Example: Suppose you are reading a file that doesn't exist, will result in a runtime error.
Read more about all programming errors here
If you'd use Google, you'd get this:
Compile time error is any type of error that prevent a java program compile like a syntax error, a class not found, a bad file name for the defined class, a possible loss of precision when you are mixing different java data types and so on.
A runtime error means an error which happens, while the program is running. To deal with this kind of errors java define Exceptions. Exceptions are objects represents an abnormal condition in the flow of the program. It can be either checked or unchecked.
Compiler errors are due to inaccuracies in code, where the compiler throws an error to alert you to something which will not compile, and therefore cannot be run.
Run Time errors are those that are detected when the program execute. For example, division by zero. The compiler can not know if the operation x/a-b will leads to division by zero until the execution
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Runtime Errors vs Logical Errors
A runtime error could potentially be a legitimate issue in code, for example, incorrectly formatted input data or lack of resources (e.g. insufficient memory or disk space). When a runtime error occurs in Java, the compiler specifies the lines of code where the error is encountered. This information can be used to trace back where the problem originated.
On the other hand, a logical error is always the symptom of a bug in application code leading to incorrect output e.g. subtracting two variables instead of adding them. In case of a logical error, the program operates incorrectly but does not terminate abnormally. Each statement may need to be checked to identify a logical error, which makes it generally harder to debug than a runtime error.
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I am following this great discussion at SO, titled: The case against checked exceptions , but I am unable to follow where exactly RuntimeException should be used and how it is different from normal Exceptions and its subclasses. Googling gave me a complex answer, that is, it should be used to deal with programming logic errors and should be thrown when no Exception should normally occur, such as in the default block of switch-case construct.
Can you please explain RuntimeException in greater detail here. Thanks.
How to Solve Runtime Errors
Runtime errors can be handled in Java using try-catch blocks with the following steps:
- Surround the statements that can throw a runtime error in try-catch blocks.
- Catch the error.
- Depending on the requirements of the application, take necessary action. For example, log the exception with an appropriate message.
To illustrate this, the code in the earlier ArithmeticException example can be updated with the above steps:
Surrounding the code in try-catch blocks like the above allows the program to continue execution after the exception is encountered:
Runtime errors can be avoided where possible by paying attention to detail and making sure all statements in code are mathematically and logically correct.
Logical Errors in Java Program
Logical errors in Java are the most critical errors in a program and they are difficult to detect. These errors occur when the programmer uses incorrect logic or wrong formula in the coding.
The program will be compiled and executed successfully but does not return the expected output.
Logical errors are not detected either by Java compiler or JVM (Java runtime system). The programmer is entirely responsible for them. They can be detected by application testers when they compare the actual result with its expected result.
For example, a programmer wants to print even numbers from an array but he uses division (/) operator instead of modulus (%) operator to get the remainder of each number. Due to which he got the wrong results.
Let’s see the following source code related to this problem.
Program source code 7:
As you can see the program is successfully compiled and executed but the programmer got the wrong output due to logical errors in the program.
Seldom does a program run successfully at its first attempt. A software engineer in a company also commits several errors while designing the project or developing code.
These errors in a program are also called bugs and the process of fixing these bugs is called debugging.
All modern integrated development environments (IDEs) such as Eclipse, NetBeans, JBuilder, etc provide a tool known as debugger that helps to run the program step by step to detect bugs.
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In this tutorial, we have familiarized different types of errors in java that may possibly occur in a program.
Thanks for reading.
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