Why is "throws Exception" necessary when calling a function?

class throwseg1
{
    void show() throws Exception
    {
        throw new Exception("my.own.Exception");
    }

    void show2() throws Exception  // Why throws is necessary here ?
    {
        show();
    }

    void show3() throws Exception  // Why throws is necessary here ?
    {
        show2();
    }

    public static void main(String s[]) throws Exception  // Why throws is necessary here ?
    {
        throwseg1 o1 = new throwseg1();
        o1.show3();
    }
}

Why compiler reports that methods show2(), show3(), and main() have

unreported exception Exception that must be caught or declared to be thrown

when I remove throws Exception from these methods?

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Java requires that you handle or declare all exceptions. If you are not handling an Exception using a try/catch block then it must be declared in the method's signature.

For example:

class throwseg1 {
    void show() throws Exception {
        throw new Exception();
    }
}

Should be written as:

class throwseg1 {
    void show() {
        try {
            throw new Exception();
        } catch(Exception e) {
            // code to handle the exception
        }
    }
}

This way you can get rid of the "throws Exception" declaration in the method declaration.

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The throws Exception declaration is an automated way of keeping track of methods that might throw an exception for anticipated but unavoidable reasons. The declaration is typically specific about the type or types of exceptions that may be thrown such as throws IOException or throws IOException, MyException.

We all have or will eventually write code that stops unexpectedly and reports an exception due to something we did not anticipate before running the program, like division by zero or index out of bounds. Since the errors were not expected by the method, they could not be "caught" and handled with a try catch clause. Any unsuspecting users of the method would also not know of this possibility and their programs would also stop.

When the programmer knows certain types of errors may occur but would like to handle these exceptions outside of the method, the method can "throw" one or more types of exceptions to the calling method instead of handling them. If the programmer did not declare that the method (might) throw an exception (or if Java did not have the ability to declare it), the compiler could not know and it would be up to the future user of the method to know about, catch and handle any exceptions the method might throw. Since programs can have many layers of methods written by many different programs, it becomes difficult (impossible) to keep track of which methods might throw exceptions.

Even though Java has the ability to declare exceptions, you can still write a new method with unhandled and undeclared exceptions, and Java will compile it and you can run it and hope for the best. What Java won't let you do is compile your new method if it uses a method that has been declared as throwing exception(s), unless you either handle the declared exception(s) in your method or declare your method as throwing the same exception(s) or if there are multiple exceptions, you can handle some and throw the rest.

When a programmer declares that the method throws a specific type of exception, it is just an automated way of warning other programmers using the method that an exception is possible. The programmer can then decide to handled the exception or pass on the warning by declaring the calling method as also throwing the same exception. Since the compiler has been warned the exception is possible in this new method, it can automatically check if future callers of the new method handle the exception or declare it and enforcing one or the other to happen.

The nice thing about this type of solution is that when the compiler reports Error: Unhandled exception type java.io.IOException it gives the file and line number of the method that was declared to throw the exception. You can then choose to simply pass the buck and declare your method also "throws IOException". This can be done all the way up to main method where it would then cause the program to stop and report the exception to the user. However, it is better to catch the exception and deal with it in a nice way such as explaining to the user what has happened and how to fix it. When a method does catch and handle the exception, it no longer has to declare the exception. The buck stops there so to speak.

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Exception is a checked exception class. Therefore, any code that calls a method that declares that it throws Exception must handle or declare it.

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package javaexception;


public class JavaException {
   void show() throws Exception
    {
        throw new Exception("my.own.Exception");
    }

void show2() throws Exception  // Why throws is necessary here ?
{
    show();
}

void show3() throws Exception  // Why throws is necessary here ?
{
    show2();
}
public static void main(String[] args) {

   JavaException a = new JavaException();

   try{
   a.show3();
   }catch(Exception e){
       System.out.println(e.getMessage());
   }
}

Only small changes in your program. What It seems to be misunderstood by many regarding the main issue, is whenever you throw exception you need to handle it, not necessary in the same place ( ex. show1,2,3 method in your program) but you must at first caller method inside the 'main'. in one word, there is 'throw', there must be 'catch/try', even if not same method where exception happens.

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Comments
  • @PaulTomblin main certainly can be declared to throw Exception. If it does, the JVM will shut down. This is as close to ignoring it as the compiler will allow.
  • When the called method (Methdod1) throws Exception, we have to define the calling method (Method2) with throws Exception; if we are not handing that exception in the calling method. The purpose of this is to give heads up to the calling method (Method3) of Method2 that an Exception may be thrown by Method2 and you should handle it here, else it may interrupt your program.
  • Similarly, if Method3 is not handling the exception in its body then, it had to define throws Exception in its method definition to give heads up its calling method. extension of the previous comment
  • what i got from your answer is that the Error class and its sublaclasses and RuntimeException class and its sub classes , they come under unchecked exception (like System.out.println(5/0); there is no need to throw as its is a runtime exception but still we can apply try catch) and Exception class is checked so we need to declare throws clause in that method and every method that calls it
  • One more questions: if the exceptions are categorized into unchecked and checked, specially the unchecked ones(runtime errors) to avoid more number of errors during compile time ?
  • @Jomoos - Say that the code inside a method throws IOException. Then, is it okay to put "throws Exception" in the method's declaration ?
  • oh. I get it now. there's an exception to the rules of the Exception hierarchy. Makes. Perfect. Sense. Thanks Java.
  • All exceptions which are not subclasses of RuntimeException, that is.
  • if there is any other class like Exception that is checked ?
  • What do you mean? Since all exception objects have "Exception" as their base class, if you catch an "Exception" object (like in my example) it will catch any exception that is thrown. To be more specific (since different exceptions will probably require different ways of handling things) you should have multiple catch blocks.
  • if I were to say that checked exceptions need to be handled @ compile time and caught at runtime . Am i right ? if yes then phrasing the same sentence for Unchecked exception would be ?
  • Every method in the chain is declared to throw Exception, including main. So where's the problem?
  • @PaulTomblin i m asking that that why is it necessary to write throws exception in the calling functions, calling a function that throws an exception
  • Ok, I didn't understand why you were asking about a compiler error that you weren't actually getting from the code you posted. That's an odd way of asking.
  • This comment should be added to the comments section as it does not provide a verifiable answer or an example of one. - stackoverflow.com/help/how-to-answer