String.intern() returns a string that has the same contents as this string, but is guaranteed to be from a pool of unique strings.
String.intern() 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.
All literal strings and string-valued constant expressions are interned. String literals are defined in �3.10.5 of the
Java Language Specification.
In one way we can say that s3 and s4 will contains the same type of data i.e. the reference of the string s1. Where as s1 and s3 are not contains the same data.