Java Checked Exception

Question

I'm struggling figuring out something about checked Exception in Java. What I've learned (I may be wrong) is that if you throw a checked Exception you have two options.

1. you have to catch it with try catch block
2. you delegate the caller with the keyword throws

In the code down below I'm going with the first option, I'm trying to catch the exception in the Main class but I get the "exception PersonaException is never thrown in body of corresponding try statement" as if I never try to catch it. What am I missing?

public class Main{
public static void main(String args[]){
    Persona p = new Persona("Michele", "Bianchi");
    Scanner s = new Scanner(System.in);
    System.out.println("Inserisci un numero di telefono: ");
    String tel = s.nextLine();
    
    try{
        p.setTel(tel);
    }
    catch(PersonaException e){
        System.out.println(e.toString());
    }

    System.out.println("Ciao");
    
    System.out.println(p.toString());

}

}

public class Persona{
private String nome;
private String cognome;
private String tel;

public Persona(String nome, String cognome){
    this.nome = nome;
    this.cognome = cognome;
}

public void setTel(String tel){
    if(tel.length()!=10){
        throw new PersonaException("Il numero di telefono è errato");
    }
    else
        this.tel = tel;
    
}

public String getTel(){
    return tel;
}

public String toString(){
    return nome+" "+cognome+": "+tel;
}

}

public class PersonaException extends Exception{
public PersonaException(String msg){
    super(msg);
}

}

Answer 1

What you're missing is that you wrote invalid java code.

Java's linking phase doesn't care about the contents of any method anywhere. Intentionally. Code works one way today; perhaps tomorrow somebody wants you to change things a bit, or you need to fix a bug. Thus, analysis of your code is irrelevant, it's all about the signatures.

Thus, when compiling:

try {
   bunchOfStatementsHere();
} catch (CheckedException e) { .. }

javac looks at all signatures of all the things you invoke in that try block, but not at the code of all the things you invoke there.

In other words, if bunchOfStatementsHere is setTel("someTel");, java looks at the signature of the setTel method and not at the code body!

The signature of your setTel method indicates it doesn't throw anything. It's defined as public void setTel(String tel) - that part is the signature, all the stuff in the {} that follows is the body.

No throws clause here.

Of course, this also means your Persona.java file doesn't compile - you can't throw new X(); where X is a checked exception, unless you catch it (which you don't), or put it in your throws clause (which you didn't).

Answer 2

You indeed must declare or handle any Exception, unless it's a RuntimeException. but you must also do this where you create the exception:

public void setTel(String tel) throws PersonaException{
        if(tel.length()!=10){
            throw new PersonaException("Il numero di telefono è errato");
        }
        else
            this.tel = tel;

    }
````

if you dont declare in the signature of setTel, you must handle it here (in a try-catch), which would of course be pointless here.