Improve company productivity with a Business Account.Sign Up

x
?
Solved

java string equality

Posted on 2013-02-01
9
Medium Priority
?
1,067 Views
1 Endorsement
Last Modified: 2013-02-01
The result of executing this code will print '2' but why?
String a = "foo";
String b = "food".substring(0, 3);
String c = b.intern();

if (a.equals(b)) {
    if (a == b) {
        System.out.println("1");
    } else if (a == c) {
        System.out.println("2");
    } else {
        System.out.println("3");
    }
} else {
    System.out.println("4");
}

a) Because "foo" is not equal to "food".substring(0,3) using equals().
b) Because the intern method returns a canonical reference to the string, which just so      happens to be the same as variable a, because a is a constant.
c) Because you must use the .equals function to compare Strings in Java.
d) Because the compiler can tell that the strings will be equal and works it out for you at compile time.
1
Comment
Question by:javaagile
  • 5
  • 4
9 Comments
 
LVL 37

Expert Comment

by:zzynx
ID: 38843276
We're not allowed/here to do your homework. Sorry.
0
 

Author Comment

by:javaagile
ID: 38843293
this is not homework. I need to understand how intern() method on string object works? value of a is "foo" and value of b is "foo" and value of c is also "foo". But why a==c is false and a==b is true?
0
 
LVL 37

Accepted Solution

by:
zzynx earned 2000 total points
ID: 38843304
The answer is b.

The java docs of intern() say:
Returns a canonical representation for the string object.
A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.

That's why a==c.
For a==b to be true, b should have been defined as:

String b = "food".substring(0, 3).intern();

Open in new window

0
Free Tool: ZipGrep

ZipGrep is a utility that can list and search zip (.war, .ear, .jar, etc) archives for text patterns, without the need to extract the archive's contents.

One of a set of tools we're offering as a way to say thank you for being a part of the community.

 

Author Comment

by:javaagile
ID: 38843379
Thanks for responding. I also had a look at javadoc but still could not understand because in code it makes c=b.intern() then why a==c is true? I understand that if c=a.intern() then a==c can be true.
0
 
LVL 37

Expert Comment

by:zzynx
ID: 38843428
Order matters.

>> String a = "foo";
A string "foo" in the pool is created.
>> String b = "food".substring(0, 3);
Another string "foo" in the pool is created.
>> String c = b.intern();
The pool is being searched for a string "foo". One is found. The same as variable 'a' is refering to.

Hence a==c. They're equal since they both "point" to the same pool string containing "foo".
0
 

Author Comment

by:javaagile
ID: 38843445
Thanks for adding more explanation. So does this means that String of pools contains a["foo"] and b["foo} in the order they are added. so when b.intern() is called "foo" is searched and a["foo"] is returned so a==c becomes true?
0
 
LVL 37

Expert Comment

by:zzynx
ID: 38843468
>> so when b.intern() is called "foo" is searched and a["foo"] is returned so a==c becomes true?
That's right.

That's why if b would have been defined as:
String b = "food".substring(0, 3).intern();

Open in new window

then b==c would be true.
0
 

Author Closing Comment

by:javaagile
ID: 38843486
Thanks this clarifies my understanding!
0
 
LVL 37

Expert Comment

by:zzynx
ID: 38843510
You're welcome
Thanx 4 axxepting
0

Featured Post

Get your problem seen by more experts

Be seen. Boost your question’s priority for more expert views and faster solutions

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

Java Flight Recorder and Java Mission Control together create a complete tool chain to continuously collect low level and detailed runtime information enabling after-the-fact incident analysis. Java Flight Recorder is a profiling and event collectio…
Java functions are among the best things for programmers to work with as Java sites can be very easy to read and prepare. Java especially simplifies many processes in the coding industry as it helps integrate many forms of technology and different d…
Viewers learn about the third conditional statement “else if” and use it in an example program. Then additional information about conditional statements is provided, covering the topic thoroughly. Viewers learn about the third conditional statement …
Viewers learn about the “for” loop and how it works in Java. By comparing it to the while loop learned before, viewers can make the transition easily. You will learn about the formatting of the for loop as we write a program that prints even numbers…

606 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question