Why is RuntimeException legal here?

Question

This is my code:

 import java.io.*;
 import java.sql.*;
 import java.io.*;
 import java.sql.*;
 class Vehicle{
    public void park()throws IOException {//compiletime checked     
    }
 }
 class Car extends Vehicle{
     public void park()throws RuntimeException{//compile time unchecked
         //
     }  
 }

Why compile time unchecked exceptions is legal here?

See https://stackoverflow.com/questions/11547914/why-runtime-exception-is-unchecked-exception — Ori Marko, Jul 08 '17 at 09:08
Filler characters and seems that you made no effort to research in advance... — Arthur Ceccotti, Jul 08 '17 at 09:49

GhostCat · Accepted Answer · 2017-07-08T12:27:22.780

It is legal because:

restricting derived methods is OK (if you are prepared to catch an io exception you have no problem if no such exception is thrown).
putting up RuntimeException is always possible because it is not a checked exception - so the compiler does not care anyway.

So your subclass simply overrides a method dropping a checked exception from the signature.

This is all rooted in the Liskov substitution principle: the idea is that you must be able to perform any call that is possible on a super class method also on the derived class.

When you think about this requirement it becomes clear quickly that it is perfectly fine when a derived method returns or throws "less" than the super class method.

score 1 · Answer 2 · answered Jul 08 '17 at 09:46

If superclass method throws/declare checked/compileTime exception, then--

overridden method of subclass can declare/throw narrower (subclass of) checked exception

overridden method of subclass cannot declare/throw broader (superclass of) checked exception

overridden method of subclass can declare/throw any unchecked /RuntimeException

overridden method of subclass can declare/throw same exception

overridden method of subclass may not declare/throw any exception.

Why is RuntimeException legal here?

2 Answers2