Java define exception type

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User defined exception in java
Example of User defined exception in Java
Another Example of Custom Exception
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Java define exception type
Java define exception type
Java define exception type

User defined exception in java

In java we have already defined, exception classes such as ArithmeticException, NullPointerException etc. These exceptions are already set to trigger on pre-defined conditions such as when you divide a number by zero it triggers ArithmeticException, In the last tutorial we learnt how to throw these exceptions explicitly based on your conditions using throw keyword.

In java we can create our own exception class and throw that exception using throw keyword. These exceptions are known as user-defined or custom exceptions. In this tutorial we will see how to create your own custom exception and throw it on a particular condition.

To understand this tutorial you should have the basic knowledge of try-catch block and throw in java.

Example of User defined exception in Java

/* This is my Exception class, I have named it MyException * you can give any name, just remember that it should * extend Exception class */ class MyException extends Exception < String str1; /* Constructor of custom exception class * here I am copying the message that we are passing while * throwing the exception to a string and then displaying * that string along with the message. */ MyException(String str2) < str1=str2; >public String toString() < return ("MyException Occurred: "+str1) ; >> class Example1 < public static void main(String args[])< try< System.out.println("Starting of try block"); // I'm throwing the custom exception using throw throw new MyException("This is My error Message"); >catch(MyException exp) < System.out.println("Catch Block") ; System.out.println(exp) ; >> >

Starting of try block Catch Block MyException Occurred: This is My error Message

Explanation:
You can see that while throwing custom exception I gave a string in parenthesis ( throw new MyException(«This is My error Message»); ). That’s why we have a parameterized constructor (with a String parameter) in my custom exception class.

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Notes:
1. User-defined exception must extend Exception class.
2. The exception is thrown using throw keyword.

Another Example of Custom Exception

In this example we are throwing an exception from a method. In this case we should use throws clause in the method signature otherwise you will get compilation error saying that “unhandled exception in method”. To understand how throws clause works, refer this guide: throws keyword in java.

class InvalidProductException extends Exception < public InvalidProductException(String s) < // Call constructor of parent Exception super(s); >> public class Example1 < void productCheck(int weight) throws InvalidProductException< if(weight<100)< throw new InvalidProductException("Product Invalid"); >> public static void main(String args[]) < Example1 obj = new Example1(); try < obj.productCheck(60); >catch (InvalidProductException ex) < System.out.println("Caught the exception"); System.out.println(ex.getMessage()); >> >

Caught the exception Product Invalid

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Comments

Hi Sir,
In the first example
public String toString() return (“Output String = “+str1) ;
>
when this block of code is called. I used breakpoint to check when this block is executed. but couldnt really understand what is happening. Please explain this.

Gopinath actually when
System.out.println(exp) ; is executed as we are passing Object type to println() toString() method is called, here they have overrided the toString() so we get the output: Output String = Custom Hope this helps

ya we can do it. if you want to write user define exception you need to catch that exception and your class should extends from RuntimeException and you need to write cause for exception

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