ID:2 SCJP Question of the day : Thursday June 10 2010

[Sahil Kapoor]

After discussion with some savvy guys , i am taking an Initiative to start a thread which has following features :-

1) A question would be asked by an Author of the thread.

2) You are required to answer it with reasoning, and without actually executing it in your compiler because it is for your help.

3) At the end of the day , i request SCJP cleared and other savvy java guys to answer it in their words so that it could help SCJP preparing guys like me.

NOTE:- If you ask a question, i request you to follow the following :-

1) Start New topic and do not post your question here.

2) Please use the format for the Subject line of the Topic or thread as follows:-

i.e. ID: <Your Question number> SCJP Question of the day : <TIME> -----<Your Name>

like , it is my second question and my name is Sahil Rally so in accordance with the above format i used the following Subject line for my topic.

ID:2 SCJP Question of the day : Wednesday June 10 2010 ---Sahil Rally .


NOTE 2: - Make as challenging Questions as you can while reading the topics and i really hope that this Idea would Rock !!!

Thanks !!!

So here goes my Question :-

[Sahil Kapoor]

  class SCJPPrepration2 {
  public static void main(String[] args) {
     
    String s1 ="SCJP";
    String s2 ="SCJP";
    
    System.out.println(s1==s2);
    
    String s3="SCJ";
    final String s4="P";
    String s5=s3+s4;
    
    System.out.println(s1==s5);
    
    final String s6="SCJ";
    final String s7="P";
    String s8=s6+s7;
    
    System.out.println(s1==s8);
    
    
    String s9="SCJ";
    String s10="P";
    final String s11=s9+s10;
    
    System.out.println(s1==s11);
    
    
    String s12="SCJ";
    final String s13;
    s13="P";
    String s14=s12+s13;
    
    System.out.println(s1==s14);
  }
}  

What is the output , tell with the reason ???

[Sahil Kapoor]

NO one answered yet !!!
Is it so easy !!!

[Martin Vanyavchich]

I does not compile

final String s13;  
s13="P"; 

That shouldn't work. You can't change object reference if it's marked final.

[Ulf Dittmer]

Please leave the name out of the subject - it's long enough as it is. What's more, the name is visible wherever the topic title is visible, so it's redundant to begin with.

[Sahil Kapoor]

Martin Vanyavchich wrote:I does not compile

final String s13;  
s13="P"; 

That shouldn't work. You can't change object reference if it's marked final.

No dear, you can always initialize once the final variables, yes after

 s13 ="P"

you cannot assign anything to s13.

It would compile !!! Now try !!!

[Peter Taucher]

First comparison should be true (s1 and s2 reference the same object).
Other comparisons should be false (s5, s8, s11, s15 refrence different objects, concatenation invokes String constructor)

I fear it's not that simple though ; - )

[Martin Vanyavchich]

The first one is true, that is sure The second and the last one are basicaly the same (whith the exception of the trick I have fallen for , you got me there Sahil) and are both false. The third and fourth ... I'll go with final references beeing as good as string literals, therefore true. As Peter mentioned, concatenation returns a new object, therefore the fourth is false.

To sum up:
true
false
true
false
false

[Lalit Mehra]

good one ... I almost fell in the trap with final members playing it the rough way ...

anyways ... the answer as per i think is

1. true // for both are literals
2. false // concatenation invokes constructor
3. true // both are final hence can be treated as constants or literals
4. false
5. false

[Sandra Bachan]

Lalit Mehra wrote:

good one ... I almost fell in the trap with final members playing it the rough way ...

anyways ... the answer as per i think is

1. true // for both are literals
2. false // concatenation invokes constructor
3. true // both are final hence can be treated as constants or literals
4. false
5. false

Please elaborate on the explanation for number 3. Why would s8 point to the same object as s1 if s6 and s7 are both final?

[Sahil Kapoor]

Its all about "Compile time constants"

Since s6 and s7 are both final, hence both s6 and s7 are compile time constants. The sum of two compile time constants is also compile time constant.

Hence s1 and s8 would point to same String literal.

Cheers!!!

[Moguluri Ravi Kiran]

Hi All,

According to me s1 & s2 pointing to the same String objects in String pool.
as Strings are immutable for s1 & s2 , there may not be two String objects that containing "SCJP".
s1 & s2 would refer a same location.

so s1==s2 is true.

in other cases it may not be true.

[Prasad Kharkar]

answer is as follows and with the following reasons
1. true // both strings refer the to String literal pool object "SCJP"
2. false// this is because when one of the operand is non final, then concatenation is done at run time and hence the + operator creates a new string and hence s1 and s5 refer to different objects
3.true // here both are final hence compile time constants hence concatenation is done at compile time and hence the variable s8 refers to the string in the constant pool referred by s1
4.false// here both the strings are non final and hence not the compile time constants hence the concatenation is done at the run time and hence the "+" operator creates the new string
5.false // the same reason that is for the third S.O.P.

[Tumi Mathibedi]

1 - 5. false // Strings are immutable in order to compare their values you need to use a.equals(b).

[Unmesh Chowdhury]

I comply with the result and reasoning which is submitted by Prasad Kharkar for this excellent question. More Details