SCJP static import question

[Brian K Smith]

Here is a question I missed on a SCJP practice test:

Given two files:

package xcom;
public class Stuff{

  static int MY_CONSTANT = 5;
  static int doStuff(int x) { return (x++)*5; }
}

import static xcom.Stuff.*;
import static java.lang.System.out;
class User {
  public static void main(String[] args) {
   new User().go();
}
void go() { out.println(doStuff(MY_CONSTANT));}

What is the result?

The reason I missed it was I thought these two files were in the same xcom package. If they were, this compiles and runs as User can find the static imports. If they are not, it obviously can not. How many of you would've read the question the same way? I thought with only one package listed at the top, it implied that both files were in the same package. Apparently this is not what the test writers had in mind. What are the conventions you've come across? Thanks.

Brian.

[Shaikh Ali]

Any file that is in a package must have the mandatory package statement at the top. That means the 2nd file can't be in the same package since there is no package statement (its not in any package at all).

[Brian K Smith]

Of course, it said 2 files.

[Shaikh Ali]

In questions such as these you should assume that 2 classes are defined in separate files, and there are no shared code/statements between them. So in this question you can assume that lines 1-6 constitute the contents of the first file and 8-14 strictly the second file. In this instance the import statements marking the start of the second file make the separation a little easier.

[Bert Bates]

Geez, I hope that wasn't one of my questions... was it?

In any case, on the real exam (and mock exam creators SHOULD follow this rule although they probably don't):

- when a code listing starts at line 1, assume you're seeing the entire file
- if there are multiple files, the question will either say that, or each file will start at line 1.

So the mock question you showed, if you copied it exactly correctly, doesn't follow standards - the second program should restart at line 1.

hth,

Bert

[Brian K Smith]

Geez, I hope that wasn't one of my questions... was it?

In any case, on the real exam (and mock exam creators SHOULD follow this rule although they probably don't):

- when a code listing starts at line 1, assume you're seeing the entire file
- if there are multiple files, the question will either say that, or each file will start at line 1.

So the mock question you showed, if you copied it exactly correctly, doesn't follow standards - the second program should restart at line 1.

Exactly as described would've been no numbers. The text editor automatically put those in when I used code tags. Exactly would've been the same with the following line,

Given two files:

package xcom;
public class Stuff{

static int MY_CONSTANT = 5;
static int doStuff(int x) { return (x++)*5; }
}

import static xcom.Stuff.*;
import static java.lang.System.out;
class User {
public static void main(String[] args) {
new User().go();
}
void go() { out.println(doStuff(MY_CONSTANT));}

I'm not using code tags now to avoid confusion.

Also the following questions were on the quiz in the CD from your book:

Given:

class Father{
protected Father() {System.out.println("Created a Father");}
}
public class Oedipus extends Father {
private Oedipus() {
System.out.println("Well that's all right then");
}
public static void main(String[] args) {
new Oedipus();
}
}

What's the output?

The given correct answer is:

Created a Father Well, that's all right then.

Wrong.

It would show as two lines give that println was used both times.

And a more substantial example:

Given:

class SortOf {
String name;'
int bal;
String code;
short rate;
public int hashCode() {
return (code.length()*bal);
}
public boolean equals(Object o){
//insert code here
}
}

Which of the following will fulfill the equals() and hashCode() contracts for this class? (Choose all the apply)

return ((SortOf)o).code.length()*((SortOf)o).bal*((SortOf)o).rate== this.code.length()*this.bal*this.rate;

is given as a correct answer.

It's not and the reason given is even worse: Te equals() algorithm must be at least as precise in defining what "meaningfully equivalent" means as the hashCode() method is.

?

One of the basic rules of equals() and hashCode() is that if two objects return true on equals() they must have the same hash code.

Here's a trivially easy example of this problem.


foo.code = "yes";
foo.bal = 2;
foo.rate = 2;

bar.code = "not";
bar.bal = 1;
bar.rate = 4;

Both equals products ==12 so they are "equal" but their hash values are 6 and 3, so, they won't be in the same hash bucket, and they will never be found on a search.

[Sami Devine]

Hi Brian, what is the answer for the first question?

When I compile the second file, I get "cannot find symbol" for MY_CONSTANT. I think it is because of default access for this constant?

[Brian K Smith]

Sami Devine wrote:Hi Brian, what is the answer for the first question?

When I compile the second file, I get "cannot find symbol" for MY_CONSTANT. I think it is because of default access for this constant?

Yup. The basic point we went over in the thread, was should I have realized as written that these two files were in different packages. I assumed they were both in the xcom package.

They were not, and was not sure if I was misreading the files, or was the question poorly written. I misread it. It said two files, and without a package statement in both files, I should not have assumed they were both under the same package.

What's your take on the problems I found with the other two practice exam questions?

[Krish Khan]

As the both classes are in different package, we cannot access either static method or static variable from one package to another. Its Complie time error.

Please let me know am i correct.

[Amir Ashfaq]

@Krish Khan..

both constant and function dostuff are static thats OK ,,,and reason why will not compile here is because of there assess level.. default is package level access... has nothing to do with static