Exception Propagation and throw-throws keyword

Exception Propagation:

Sending exception from one method to another method is called exception propagation

Here propagation is nothing but sending.

So guys what is the situation when we are going to implement this propagation concept?

Answer to this question is when we throw an exception, if it is not caught in that method it is propagated to another method and interesting thing is that we don’t need to do anything in this process we just need to throw an exception that’s it.

Rule:

If the propagated exception is checked exception then not only current method developer but also method’s caller method developer should caught or report that checked exception else it leads to same compiler exception….CE: Unreported Exception

So according to that basic rule is “If checked exception is raised the method either directly by using throw keyword or indirectly by a method call it must be caught or reported.

void m1() throws ClassNotFoundException{

throw new ClassNotFoundException();  // direct raising

}

void m2(){

                m1();     // Indirect raising

}

Output: Compiler Error

void m3() throws ClassNotFoundException{

m1();                     //indirect raising

}

Output: Compiled successfully.

Differences between throw and throws keyword

throw	throws
It is used to raise and throw exception manually or explicitly by developer	It is used to report that exception to this method caller
throw should follow exception object	throws should follow exception class name
It can raise and throw only one exception at a time	It can report multiple exceptions those raising or throwing from current method with “,” separator
It is appeared or used only in a method logic	It is appeared or used only in method prototype after method signature

Rules:

1.       After throw and throws keyword only Throwable type class object and name is allowed. If we pass non-throwable object it leads to CE: incompatible types exception.

2.       If we interchange these keywords places it leads to CE: illegal start of expression

e.g.

void method1(){

                throw  “abc”;

}

Output: CE: Incompatible Types

void method2() throws Integer{

}

Output: Incompatible Types

void method3(){

                throws new Exception();

}

Output: CE: Illegal start of Expression

Rule on throw keyword:

After throw statement we are not allowed to place any java statement directly, it leads to CE: Unreachable statement because “throw” keyword transfers the flow of execution and gives output as exception. So if we write any code after throw statement it is obviously doesn’t make any sense.

e.g.:

void method1(){

                System.out.println(“Hello”);

throw new ArithmeticException();

System.out.println(“Hi”);

}

Output: CE: Unreachable Statement

So If needed how can we place statement after throw keyword?

We must place throw keyword in if condition

e.g.

void method1( int a ){

                System.out.println(“Hi);

If(a == 10){

                                Throw new ArithmeticException();

}

System.out.println(“Input Value=”+a);

}

Thank You….!!!