Quick java question on classes..(Exam Question)

techguy

Hi,

This is a question from a past exam paper but I can't figure out the answer.

When the following classes are compiled, line 10 (indicated by a comment) generates a compile error. Why is this? Modify one of the classes so that it will compile without error. Hint: there are two ways to achieve this.

class CD
{
     String title;
     CD(String s)
     {
          title = s;
     }
}

class Music_CD extends CD
{
     int num_tracks;
     Music_CD()
     {
           num_tracks = 0;
     }
}

I'm getting an error about a CD constructor in the CD class??

I simply don't know the answer to this one

Webmonkey

Havn't done java in ages but it looks like your constructor for CD is private, hince it cannot be constructed. Or the the class should be defined as public class {

techdiver

Just create a default constructor in "class CD", like this:

class CD
{
String title;
CD(String s)
{
title = s;
}
CD()
{
}
}
class Music_CD extends CD
{
int num_tracks;
Music_CD()
{
num_tracks = 0;
}
}

geuro

techguy wrote: »

Hi,

This is a question from a past exam paper but I can't figure out the answer.

When the following classes are compiled, line 10 (indicated by a comment) generates a compile error. Why is this? Modify one of the classes so that it will compile without error. Hint: there are two ways to achieve this.

class CD
{
     String title;
     CD(String s)
     {
          title = s;
     }
}

class Music_CD extends CD
{
     int num_tracks;
     Music_CD()
     {
           num_tracks = 0;
     }
}

I'm getting an error about a CD constructor in the CD class??

I simply don't know the answer to this one

You need a no-args constructor in class CD. eg

CD(){}

If a superclass doesnt contain a no-args constructor (as it doesnt in this case because you have supplied a constructor with arguments), well then you cannot have a no-args constructor in a subclass.

techguy

Thanks Guys..some good responses there, all cleared up now!

Thanks.

Anima

Another way is to have:

[PHP]
class CD
{
String title;
CD(String s)
{
title = s;
}
}

class Music_CD extends CD
{
int num_tracks;
Music_CD()
{
super("title");
num_tracks = 0;
}
}
[/PHP]