Why does this program compile?

Question

public class Fir extends Test 
{
    public static void main(String args[])  
    {
        Fir fir = new Fir();
        System.out.println(fir);
    }

    public String toString(String x) throws RuntimeException
    {
        return "Fir";
    }
}

class Test
{
    protected String toString(String x) throws NullPointerException
    {
        return "Test";
    }
}

Both classes do not override the toString method appropriately.

Question: Are these methods overriden?

If yes, what about this rule: If the superclass method declares an exception, subclass overridden method can declare same, subclass exception or no exception but cannot declare parent exception. RuntimeException is parent of NullPointerException.

@IanMcGrath I misunderstood. I thought you were referring to override the toString() method from Object, not the one from Test. My apologies. — Christian Tapia
– Christian Tapia, Commented Mar 30, 2014 at 8:32

JB Nizet · Accepted Answer · 2014-03-30 08:33:30Z

8

The rule you gave is correct, but it applies to checked exceptions, not to runtime exceptions. Any method is allowed to throw any runtime exception it wants, without declaring it. You may declare them, but it's purely for documentation purpose.

To answer your question, yes, the toString(String) method in Fir overrides the toString(String) method in Test. Add the @Override annotation to Fir.toString(), and the compiler will check that for you: you'll get a compilation error if Fir.toString(String) doesn't override any method.

edited Mar 30, 2014 at 8:33

answered Mar 30, 2014 at 8:27

JB Nizet

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2 Comments

Ian McGrath Over a year ago

oops forgot about this rule. Thanks.

Jesper Over a year ago

JB makes a good point about the @Override annotation. You should always use it when overriding a method, to let the compiler help you catch mistakes.

shem · Accepted Answer · 2014-03-30 08:27:06Z

3

Question: Are these methods overriden?

No. these methods are overloaded

the original method looks like this:

public String toString()

answered Mar 30, 2014 at 8:27

shem

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1 Comment

Jesper Over a year ago

But the toString(String) method in Fir does override the toString(String) method in Test. So, overloaded with respect to Object.toString(), but overridden with respect to each other.

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