Make an exception class in java

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Creating Exception Classes

When faced with choosing the type of exception to throw, you can either use one written by someone else the Java platform provides a lot of exception classes you can use or you can write one of your own. You should write your own exception classes if you answer yes to any of the following questions; otherwise, you can probably use someone else’s.

Do you need an exception type that isn’t represented by those in the Java platform?
Would it help users if they could differentiate your exceptions from those thrown by classes written by other vendors?
Does your code throw more than one related exception?
If you use someone else’s exceptions, will users have access to those exceptions? A similar question is, should your package be independent and self-contained?

An Example

Suppose you are writing a linked list class. The class supports the following methods, among others:

objectAt(int n) Returns the object in the n th position in the list. Throws an exception if the argument is less than 0 or more than the number of objects currently in the list.
firstObject() Returns the first object in the list. Throws an exception if the list contains no objects.
indexOf(Object o) Searches the list for the specified Object and returns its position in the list. Throws an exception if the object passed into the method is not in the list.

The linked list class can throw multiple exceptions, and it would be convenient to be able to catch all exceptions thrown by the linked list with one exception handler. Also, if you plan to distribute your linked list in a package, all related code should be packaged together. Thus, the linked list should provide its own set of exception classes.

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The next figure illustrates one possible class hierarchy for the exceptions thrown by the linked list.

Example exception class hierarchy.

Choosing a Superclass

Any Exception subclass can be used as the parent class of LinkedListException . However, a quick perusal of those subclasses shows that they are inappropriate because they are either too specialized or completely unrelated to LinkedListException . Therefore, the parent class of LinkedListException should be Exception .

Most applets and applications you write will throw objects that are Exception s. Error s are normally used for serious, hard errors in the system, such as those that prevent the JVM from running.

Note: For readable code, it’s good practice to append the string Exception to the names of all classes that inherit (directly or indirectly) from the Exception class.

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Make an exception class in java

Хотя имеющиеся в стандартной библиотеке классов Java классы исключений описывают большинство исключительных ситуаций, которые могут возникнуть при выполнении программы, все таки иногда требуется создать свои собственные классы исключений со своей логикой.

Чтобы создать свой класс исключений, надо унаследовать его от класса Exception. Например, у нас есть класс, вычисляющий факториал, и нам надо выбрасывать специальное исключение, если число, передаваемое в метод, меньше 1:

class Factorial < public static int getFactorial(int num) throws FactorialException< int result=1; if(num<1) throw new FactorialException("The number is less than 1", num); for(int i=1; i<=num;i++)< result*=i; >return result; > > class FactorialException extends Exception < private int number; public int getNumber()public FactorialException(String message, int num) < super(message); number=num; >>

Здесь для определения ошибки, связанной с вычислением факториала, определен класс FactorialException , который наследуется от Exception и который содержит всю информацию о вычислении. В конструкторе FactorialException в конструктор базового класса Exception передается сообщение об ошибке: super(message) . Кроме того, отдельное поле предназначено для хранения числа, факториал которого вычисляется.

Для генерации исключения в методе вычисления факториала выбрасывается исключение с помощью оператора throw: throw new FactorialException(«Число не может быть меньше 1», num) . Кроме того, так как это исключение не обрабатывается с помощью try..catch, то мы передаем обработку вызывающему методу, используя оператор throws: public static int getFactorial(int num) throws FactorialException

Теперь используем класс в методе main:

public static void main(String[] args) < try< int result = Factorial.getFactorial(6); System.out.println(result); >catch(FactorialException ex) < System.out.println(ex.getMessage()); System.out.println(ex.getNumber()); >>

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Class Exception

The class Exception and its subclasses are a form of Throwable that indicates conditions that a reasonable application might want to catch.

The class Exception and any subclasses that are not also subclasses of RuntimeException are checked exceptions. Checked exceptions need to be declared in a method or constructor’s throws clause if they can be thrown by the execution of the method or constructor and propagate outside the method or constructor boundary.

Constructor Summary

Constructs a new exception with the specified detail message, cause, suppression enabled or disabled, and writable stack trace enabled or disabled.

Constructs a new exception with the specified cause and a detail message of (cause==null ? null : cause.toString()) (which typically contains the class and detail message of cause ).

Method Summary

Methods declared in class java.lang.Throwable

Methods declared in class java.lang.Object

Constructor Details

Exception

Constructs a new exception with null as its detail message. The cause is not initialized, and may subsequently be initialized by a call to Throwable.initCause(java.lang.Throwable) .

Exception

Constructs a new exception with the specified detail message. The cause is not initialized, and may subsequently be initialized by a call to Throwable.initCause(java.lang.Throwable) .

Exception

Constructs a new exception with the specified detail message and cause. Note that the detail message associated with cause is not automatically incorporated in this exception’s detail message.

Exception

Constructs a new exception with the specified cause and a detail message of (cause==null ? null : cause.toString()) (which typically contains the class and detail message of cause ). This constructor is useful for exceptions that are little more than wrappers for other throwables (for example, PrivilegedActionException ).

Exception

protected Exception (String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace)

Constructs a new exception with the specified detail message, cause, suppression enabled or disabled, and writable stack trace enabled or disabled.

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How to Create an Exception Class in Java

Java has many built-in exception classes, such as NullPointerException and IllegalArgumentException . At times however, you might want to create your own exception class. For example, as opposed to throwing IllegalArgumentException when a 0 is detected as a divisor during a division operation, you might wish to throw a DivideByZeroException . This exception class does not exist in the Java core API, but you can create one yourself. The seven steps below will show you how to create an exception class in Java.

First, you will create the custom exception class. Open your text editor and type in the following Java statements:The class extends the Exception class that is defined in the Java core API (in the package is java.lang ). When extending Exception you are defining a «checked» exception, i.e., an exception that must be caught or thrown. A constructor is provided that passes the message argument to the super class Exception . The Exception class supports a message property.
Save your file as DivideByZeroException.java .
Open a command prompt and navigate to the directory containing your Java program. Then type in the command to compile the source and hit Enter .
Now you will create the program to test your new exception class. Open your text editor and type in the following Java statements:Notice that the divideInt method must provide a throw clause because the method potentially throws the DivideByZeroException . To create the exception object, the program uses the throw keyword followed by the instantiation of the exception object. At runtime, the throw clause will terminate execution of the method and pass the exception to the calling method.
Save your file as TestDivideByZeroException.java .
Open a command prompt and navigate to the directory containing your Java program. Then type in the command to compile the source and hit Enter .
Type in the command to run your program and hit Enter .The first call to the divideInt method is successful. The second call, using a divisor of 0, causes the DivideByZeroException to be thrown in the divideInt method. The message passed to the constructor is displayed in the output.