ID:13 SCJP Question of the Day

[wbrian radley]

class Question {

	private	static interface ia {void a();};

	static class a implements ia {

		a(){;};

		public void a() {

			java.util.Set <? super CharSequence> set = new java.util.HashSet<Object>();

			CharSequence cs = new String("for");
			String [] strArray = {new String("boo"),new String("who"), "are", new String("boo"), "you"};
			int indx = 0;
			while(set.add(strArray[indx]) == true)System.out.printf(" nonce ", indx++);;
		}	
	};


	public static void main (String ... args){

		Question.ia ia = new a();
		ia.a();
	};

};

the usual questions:
does this compile? does it throw exceptions? what is the output (if any)? is there an infinite loop?

[Prasad Kharkar]

 
class Question {

	private	static interface ia {void a();};

	static class a implements ia {

		a(){;}; // semi colon in the method body does no affect 

		public void a() {

			java.util.Set <? super CharSequence> set = new java.util.HashSet<Object>(); // will compile because we can instantiate using 
                                                                                                                                          //any type that is super type of CharSequence

			CharSequence cs = new String("for"); // referring to subclass object using super class reference i.e. polymorphism
			String [] strArray = {new String("boo"),new String("who"), "are", new String("boo"), "you"}; //line 1
			int indx = 0;
			while(set.add(strArray[indx]) == true)System.out.printf(" nonce ", indx++ + " " + strArray[indx]);;//line 2  // extra semicolons do not cause any problems 
		}	
	};


	public static void main (String ... args){

		Question.ia ia = new a(); // static interface type referring to static class object 
		ia.a();
	};

}; // adding semi-colons does not have ill effects 

   

The explanation for lines line 1 and line 2 is as follows

now we are looping till the while condition returns true
the add method in set returns true if the element is added to the set
now
the set contains only the unique elements and not duplicate ones
in our example
we add "boo" twice
first time the array index 0 refers to the boo String
but when we add it again then the array index 3 refers to it
the loop execution is as follows

for the first element
i.e. new String("boo");
the add method returns true as there is no duplicate element and loop executes
for the second element
i.e. new String("who")
same operation happens
now for the "are" again same thing happens

and now again we are adding new String("boo")
now this object already exists in set and duplicates are not allowed and the add method returns false
and hence the loop terminates and we do not get infinite loop
here is the output

C:\Users\Prasad\Desktop>java Question
nonce 0 who nonce 1 are nonce 2 boo

[Sahil Kapoor]

Set : Duplicates are not allowed
String : Overrides equals() .

boo who and are are successfully added after which loop condition becomes false.

I am just a bit confused about ; (Semicolon) after main method().

If it is allowed then output should be nonce nonce nonce
otherwise
Complie time error!!!

[wbrian radley]

yeah, the extra semi-colons were added to make the code look odd and
thus a temptation to choose compile error, but they have no effect at all (at least in this example).

the key to a correct evaluation of the output is to consider the difference between:

printf(" nonce ", indx++);

and

printf(" nonce %d", indx++ );

in the both cases indx++ is computed but only in the second is the value of indx
(before the addition) printed.

a rule with printf seems (please correct is your experience is different) to be that while
there must be a matching arg for each conversion there is no requirement that each arg be
matched with a conversion.

if the arg or its result is to be output then there must be a matching conversion.

see also Question 12 for another trick with printf.

also note:

System.out.printf(" boo ", null);  // warning but compiles and runs
System.out.printf(null)              // warning and NPE
System.out.printf(cs.toString());   // cs in original question; explicit String conversion is required 

[wbrian radley]

in order to see better what rules the compiler is using when adding to a set consider
the question as originally posted and add the following code right
after the while loop; what is the output ?

CharSequence cs1 = new String("boo");
Object o = new String("for");
CharSequence cs2 = new String("for");
StringBuffer sb1 = new StringBuffer("who");
CharSequence cs3 = new StringBuffer("who");

set.add(cs);
set.add(cs1);
set.add(cs2);
set.add(cs3);
set.add(sb1)
set.add(o); 

System.out.printf("%n%-10s%-10d", "size=", set.size());
System.out.printf("%n%-10s%-10s", "set=", set);