Comparing string references

Question

I'm trying to understand reference comparing in Java. Let's assume we have the following main code:

public static void main (String args[]) {
   String str1 = "Love!";  
   String str2 = "Love!";
   String str3 = new String("Love!");  
   String str4 = new String("Love!");
   String str5 = "Lov"+ "e!";  
   String str6 = "Lo" + "ve!";  
   String s = "e!";
   String str7 = "Lov"+ s;  
   String str8 = "Lo" + "ve!";
   String str9 = str1;
}

I understand that str1 == str2 == str5 == str6 == str8 == str9 and all of them are the same reference to the common pool. (value "Love!"). s is a reference to the common pool as well, but it refers the value "e!"

I understand also that str1 != s.

I know that str3, str4 are references to the HEAP, and each of them is a different Object. str3 != str4.

I do NOT understand why str1 != str7, and I would love to get an explanation.

Answer 1

In

String s = "e!";
String str7 = "Lov"+ s;

While "e!" is a constant expression, s is not a constant variable (JLS §4.12.4); therefore, "Lov" + s, which references s, cannot be a constant expression (JLS §15.28). In order for a variable like s to be a constant variable, it needs to be both final and initialized from a constant expression.

If you had written

final String s = "e!";
String str7 = "Lov" + s;

then str1 == str7 would have been true.