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java string equality

Posted on 2013-02-01
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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.
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Question by:javaagile
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Expert Comment

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

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zzynx earned 500 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();

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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.
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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".
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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?
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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();

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then b==c would be true.
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Author Closing Comment

by:javaagile
ID: 38843486
Thanks this clarifies my understanding!
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Expert Comment

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