String intern()

[Ram Reddy]

class MyClass6
{
public static void main (String args[])
{
String str = new String("SCJP");

String str2 = ""; //blank string

System.out.println(str == (str + str2).intern());
}
}

the out put is "false"

please explain me in detail.

And also when it will return true. please give me example

[Srinivasan thoyyeti]

Hi Rami,

Good One.

Here we go...

1) String str = new String("SCJP");

When you create a String with "new" key word, Two String Objects will be created. One in Heap and other in Constant Pool. Both Object references are different.

2)(str + str2).intern()

(str + str2) ---> "SCJP", which will be found in pool.
So pool Object reference is taken here.

3) str == (str + str2).intern()

Here str on left hand side represents Heap Object.
(str + str2).intern() on RHS represents Pool Object.

As both the references are different.
You got false.

All the Best Rami.

[Bert Bates]

FYI, the intern() method is not on any current version of the SCJP exam.

[Chandra Bhatt]

class MyClass6
{
public static void main (String args[])
{
String str = new String("SCJP");

String str2 = ""; //blank string

System.out.println(str == (str + str2).intern());
}
}

the out put is "false"

please explain me in detail.

And also when it will return true. please give me example

You little misinterpreted the use of intern()

What intern does: if you intern you call the intern() method on a String reference, if the object referenced by it already exists in the memory, the reference to the existing String object will be returned otherwise it will return the same object reference that is new.

    String s1 = "SCJP ";
          String s2 = "cerify";
 
          String s3 = "SCJP certify";
          s1+=s2;
          s1 = s1.intern(); //object already exits so it will return the
//reference to the object referenced by s3 as we see
 
System.out.println(s1==s3);//prints true 
 

try this out:

 public class Demointern {
        
    public static void main(String... args) {
       String s1 = new String("SCJP ");
       String s2 = new String("certify");
       String s3 = "SCJP certify";
       /* if you would have done like
       String s3 = new String("SCJP certify");
(s1==s3) //returns false
/*
it is only because the s3 will refer to an object that will be created at run time. So at run time two different references will be compared and as a result returns false. While String s3 = "SCJP certify"; is compile time constant, goes to String constant pool.
 
*/
       s1+=s2;
       s1=s1.intern();
       
       System.out.println(s1==s3);
       
    }
}   

[Srinivasan thoyyeti]

Hi Rami,

On Basic point:

How to know new String("xxx") will create different Heap and Pool Objects.

String str = new String("Hello");
if ( str != str.intern()){
System.out.println("Heap Object reference and Pool Object reference are different");
}

Hope this will make good foundation to understand your doubt.

[Ram Reddy]

I got it . Thanks to all.