Behavior of String literals is confusing

Question

Every compile-time constant expression that is of type String will be put into the String pool.

Essentially that means: if the compiler can (easily) "calculate" the value of the String without running the program, then it will be put into the pool (the rules are slightly more complicated than that and have a few corner cases, see the link above for all the details).

That's true for all the Strings in lines 1-3.

"Hel"+lo is not a compile-time constant expression, because lo is a non-constant variable.

The hash codes are the same, because the hashCode of a String depends only on its content. That's required by the contract of equals() and hashCode().

Strings computed by concatenation at runtime are newly created and therefore distinct

here is a link to read: http://docs.oracle.com/javase/specs/jls/se7/html/jls-3.html#jls-3.10.5

String object can be created in the following ways:

String str = new String("abcd");  // Using the new operator 
                                  // str is assigned with "abcd" value at compile time.

String str="abcd";         // Using string literal
                           // str is assigned with "abcd" value at compile time.

String str="ab" + "cd";    // Using string constant expression.
                           // str is assigned with "abcd" value at compile time.
String str1 = "cd";
String str = "ab"+str1;    // Using string expression.
                           // str is assigned with "abcd" value at run time only.

and Hashcode will be calculated only at runtime based on the contents of the String objects.

It's because the comipler in this instance is not smart enough to work out that it can burn in the same string literal.

Hashcode needs to always return the same value for strings that are equivelent (calling .equals on it returns true) so will return the same result.