When should we use intern method of String on String literals

Question

According to String#intern(), intern method is supposed to return the String from the String pool if the String is found in String pool, otherwise a new string

Answer 1

Learn Java String Intern - once for all

Strings in java are immutable objects by design. Therefore, two string objects even with same value will be different objects by default. However, if we wish to save memory, we could indicate to use same memory by a concept called string intern.

The below rules would help you understand the concept in clear terms:

1. String class maintains an intern-pool which is initially empty. This pool must guarantee to contain string objects with only unique values.
2. All string literals having same value must be considered same memory-location object because they have otherwise no notion of distinction. Therefore, all such literals with same value will make a single entry in the intern-pool and will refer to same memory location.
3. Concatenation of two or more literals is also a literal. (Therefore rule #2 will be applicable for them)
4. Each string created as object (i.e. by any other method except as literal) will have different memory locations and will not make any entry in the intern-pool
5. Concatenation of literals with non-literals will make a non-literal. Thus, the resultant object will have a new memory location and will NOT make an entry in the intern-pool.
6. Invoking intern method on a string object, either creates a new object that enters the intern-pool or return an existing object from the pool that has same value. The invocation on any object which is not in the intern-pool, does NOT move the object to the pool. It rather creates another object that enters the pool.

Example:

String s1=new String (“abc”);
String s2=new String (“abc”);
If (s1==s2)  //would return false  by rule #4
If (“abc” == “a”+”bc” )  //would return true by rules #2 and #3
If (“abc” == s1 )  //would return false  by rules #1,2 and #4
If (“abc” == s1.intern() )  //would return true  by rules #1,2,4 and #6
If ( s1 == s2.intern() )      //wound return false by rules #1,4, and #6

Note: The motivational cases for string intern are not discussed here. However, saving of memory will definitely be one of the primary objectives.