How to throw exception in java
10:52:00
In java we
have already defined exception classes such as
ArithmeticException, ArrayIndexOutOfBoundsException, NullPointerException etc. There are certain conditions
defined for these exceptions and on the occurrence of those conditions they are
implicitly thrown by JVM(java virtual machine).
Do you know that a programmer can create
a new exception and throw it explicitly? These exceptions are known as user-defined exceptions. In order to throw
user defined exceptions, throw keywordis being used. In this
tutorial, we will see how to create a new exception and throw it in a program
using throw keyword.
You can also
throw an already defined exception like
ArithmeticException, IOException etc.
Syntax of throw statement
throw AnyThrowableInstance;
Example:
//A void method
public void sample()
{
//Statements
//if (somethingWrong) then
IOException e = new IOException();
throw e;
//More Statements
}
Note:
·
A call to the above mentioned sample method should be always
placed in a try block as it is throwing a checked exception –
IOException.
This is how it the call to above method should be done:· MyClass obj = new MyClass();
· try{
· obj.sample();
· }catch(IOException ioe)
· {
· //Your error Message here
· System.out.println(ioe);
}
·
Exceptions in java are compulsorily of type Throwable. If
you attempt to throw an object that is not throwable, the compiler
refuses to compile your program and it would show a compilation error.
Flow of execution while throwing an
exception using throw keyword
Whenever a throw
statement is encountered in a program the next statement doesn’t execute.
Control immediately transferred to catch block to see if the thrown exception
is handled there. If the exception is not handled there then next catch block
is being checked for exception and so on. If none of the catch block is handling the thrown exception then a
system generated exception message is being populated on screen, same what we
get for un-handled exceptions.
E.g.
E.g.
class ThrowDemo{
public static void main(String args[]){
try{
char array[] = {'a','b','g','j'};
/*I'm displaying the value which does not
* exist so this should throw an exception
*/
System.out.println(array[78]);
}catch(ArithmeticException e){
System.out.println("Arithmetic Exception!!");
}
}
}
Output:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException:
78 at beginnersbook.com.ThrowDemo.main(Details.java:9)
Since the
exception thrown was not handled in the catch blocks the system generated
exception message got displayed for that particular exception.
Few examples of throw exception in Java
Example 1: How to throw your own
exception explicitly using throw keyword
package beginnersbook.com;
class MyOwnException extends Exception {
public MyOwnException(String msg){
super(msg);
}
}
class EmployeeTest {
static void employeeAge(int age) throws MyOwnException{
if(age < 0)
throw new MyOwnException("Age can't be less than zero");
else
System.out.println("Input is valid!!");
}
public static void main(String[] args) {
try {
employeeAge(-2);
}
catch (MyOwnException e) {
e.printStackTrace();
}
}
}
Output:
beginnersbook.com.MyOwnException: Age can't be less than zero
Points to Note: Method call should be in try block as it
is throwing an exception.
Example2: How to throw an already
defined exception using throw keyword
package beginnersbook.com;
class Exception2{
static int sum(int num1, int num2){
if (num1 == 0)
throw new ArithmeticException("First parameter is not valid");
else
System.out.println("Both parameters are correct!!");
return num1+num2;
}
public static void main(String args[]){
int res=sum(0,12);
System.out.println(res);
System.out.println("Continue Next statements");
}
}
Output:
Exception in thread main java.lang.ArithmeticException: First parameter is not valid
Similarly other
exceptions, such as
NullPointerException, ArrayIndexOutOfBoundsException etc. can be thrown. That’s all for the
topic how to throw exception in java. Let me know your feedback on this.
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