boxing confuses

[Senthil Kumaran]

hi, somebody explain me this program, especially line 9, this question from Inquisition scjp6 trail set mock exam,

public class testboxing
1. {
2. public static void main(String []as)
3. {
4. String []words = new String[]{"aaa","bbb","ccc","aaa"};
5. Map<String,Integer>m = new Treemap<String,Integer>();
6. for(String word:words)
7. {
8. Integer freq= m.get(word);
9. m.put(word,freq==null? 1:freq+1);
10. }
11. system.out.println(m);
12. }
13. }

the answer is {aaa=2,bbb=1,ccc=1}

is that question mark in line 9 is printing mistake, if it is, i cant understand that boxing & unboxing clearly,

[Campbell Ritchie]

No, the ? is not a mistake. There isn't a lot about it in the Java Tutorials, but it is half of the conditional operator (also called the ternary operator). It is a bit like having a tiny if-else inside a statement. I found some other links SAMS and Wikipedia.

Go through that program with a pencil and paper and write down what it does. You should be able to work it out easily. Try changing words to args, then you can pass several words in via the command-line arguments.

What boxing/unboxing means is that you can swap Integers and ints around, like this:

Integer i = 123;
int j = i;
System.out.println(j * 2);
i = j;
System.out.println(i + 456);

Not a very useful piece of code, but it shows boxing in action.

[Senthil Kumaran]

are ternary operators in scjp 6, tell me the links explain me more clearly about boxing,

[Vishal Srivastav]

The program is simple...

Line 4: Creates ...
Line 5: Creates ...
Line 6: Starts ...
Line 8: Since ...
Line 9: String .


This keeps on . . . and the following steps take place...

1: value from freq
2: now ...
3: since it is treemap ...

Line 11: Produces the output
Hope you are satisfied with my solution if it is not too complex...

Do give feedback...

[Campbell Ritchie]

Please, Vishal Srivastav, don't simply give out answers and explanations like that. It does not help the poster at all. Have a look at the contents page for the Beginners' forum.

I have felt obliged to delete the solution.

CR

[Mumtaz Khan]

Its not only boxing and unboxing, its about put(key, value) method of map too. If the map previously contained a mapping for any key, the old value is replaced. This is happening while putting "aaa" string again to the map.

[Campbell Ritchie]

Don't know about the exam, but the ternary operator is a basic operator which any programmer ought to know about.

[Jesper de Jong]

Do not post the same question in more than one forum; carefully choose one forum. I'm closing this one. Continue in the copy in the SCJP forum.