Example for Certification book doesnt work

[Vaibhav B Sharma]

Hi,

I am newbie to java, started learning after completing 7 years in Cobol development. I am preparing for Certification exam, and need your help in following.

I copied the sample code from SCJP 6 guide to play around with it, and it doesn't run in my machine. Can someone assist. The code is taken from Chapter 2, question 8 from section Q&A:Self Test.

Error that I get in eclipse is

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
Dogshow cannot be resolved to a type
at TestQ.DogShow.main(DogShow.java:5)

Thanks !

Code -

package TestQ;

public class DogShow {
	public static void main(String args[]){
		new Dogshow.go();
	}
	void go(){
			new Hound.bark();
		}
}

class Dog {
	public void bark(){
		System.out.println("bark");
	}
}
class Hound extends Dog{
	public void bark(){
		System.out.println("bark Hound");
	}
	public void sniff(){
		System.out.println("Sniff");
	}
}

[Mohamed Sanaulla]

Hi Vaibhav,
Welcome to JavaRanch
You have a class declared as DogShow but in the main method you are referring to it as Dogshow. So the compiler is not able to find class of Dogshow type.

[Mohamed Sanaulla]

And apart from that, the way the objects are created is- you specify the new operator followed by a call to the constructor which initializes the object. By default if you dont specify any constructor Java provides a default no-arg constructor like:

public class ClassName
{
  ClassName()
  {
  }
}

the above is equivalent to:

public class ClassName
{
}

because the compiler adds the no-arg (no argument) constructor to the class.

Now you create instance of your class by:

ClassName myInstance = new ClassName();

in case of methods which are static, you would not need an instance to invoke them and can use:

ClassName.staticMethod();

otherwise you would require an instance to invoke non-static methods, something like

myInstance.someInstanceMethod();

You would have to edit your code at 2 places where you are instantiating a class.

[Vaibhav B Sharma]

Hi,

Thanks for quick response, and sorry, that was my typo. But it still doesn't work for anything that I declare as an object without assigning to reference type. There are two such errors that I am getting -

C:\JAVA\Packone\Source>javac DogShow.java
DogShow.java:5: error: cannot find symbol
new DogShow.go();
^
symbol: class go
location: class DogShow
DogShow.java:8: error: cannot find symbol
new Hound.bark();
^
symbol: class bark
location: class Hound
2 errors

Code as per below -

public class DogShow {
	public static void main(String args[]){
		new DogShow.go();
	}
	void go(){
			new Hound.bark();
		}
}

class Dog {
	public void bark(){
		System.out.println("bark");
	}
}
class Hound extends Dog{
	public void bark(){
		System.out.println("bark Hound");
	}
	public void sniff(){
		System.out.println("Sniff");
	}
}

[Mohamed Sanaulla]

Vaibhav B Sharma wrote:Hi,

Thanks for quick response, and sorry, that was my typo. But it still doesn't work for anything that I declare as an object without assigning to reference type. There are two such errors that I am getting -

I think I pointed out this issue in my second post. As I overlooked the way you were creating the objects in my first post.

[Mohamed Sanaulla]

So the way you are creating the instance would be:

new ClassName().someMethod();

[Vaibhav B Sharma]

Sorry Mohamed, didn't see your complete reply before posting my query. I still have a few questions -

1. Since I need an instance to invoke non-static method, I am creating one already -

new Hound.bark();

It works if I create the object, assign it to reference variable, and then use that reference variable to invoke bark() -

Hound h = new Hound();
h.bark();

but why doesn't it work if I just create and provide an instance of Hound on which I want to run bark() on ? I don't want to use reference variable, since I don't care if object is lost after method is complete.


2. This coding technique is used in many examples in SCJP 6 certification guide. So am I missing something here ?

Thanks again for your prompt response.

[Mohamed Sanaulla]

Vaibhav B Sharma wrote:
1. Since I need an instance to invoke non-static method, I am creating one already -

new Hound.bark();

It works if I create the object, assign it to reference variable, and then use that reference variable to invoke bark() -

Hound h = new Hound();
h.bark();

You are missing the (). new Hound().bark(); is the correct syntax for creating the object.

[Jeff Verdegan]

Vaibhav B Sharma wrote:
It works if I create the object, assign it to reference variable, and then use that reference variable to invoke bark() -

Hound h = new Hound();
h.bark();

but why doesn't it work if I just create and provide an instance of Hound on which I want to run bark() on ?

You mean new Hound().bark()? If the above works, with the "h" variable, then this will also work. If you're saying it doesn't, then you're doing something else different.

[Vaibhav B Sharma]

That was a silly, silly error on my part !!

Thanks for your inputs.. Have a nce weeked !