Java Strings: "String s = new String("silly");"

Question

I'm a C++ guy learning Java. I'm reading Effective Java and something confused me. It says never to write code like this:

String s = new String("silly"); 

Because it creates unnecessary String objects. But instead it should be written like this:

String s = "No longer silly"; 

Ok fine so far...However, given this class:

public final class CaseInsensitiveString {     private String s;     public CaseInsensitiveString(String s) {         if (s == null) {             throw new NullPointerException();         }         this.s = s;     }     :     : }  CaseInsensitiveString cis = new CaseInsensitiveString("Polish"); String s = "polish"; 

1. Why is the first statement ok? Shouldn't it be

   CaseInsensitiveString cis = "Polish";

2. How do I make CaseInsensitiveString behave like String so the above statement is OK (with and without extending String)? What is it about String that makes it OK to just be able to pass it a literal like that? From my understanding there is no "copy constructor" concept in Java?

Answer 1

String is a special built-in class of the language. It is for the String class only in which you should avoid saying

String s = new String("Polish"); 

Because the literal "Polish" is already of type String, and you're creating an extra unnecessary object. For any other class, saying

CaseInsensitiveString cis = new CaseInsensitiveString("Polish"); 

is the correct (and only, in this case) thing to do.