Can't understand how Java string literals are implemented

Eitanos30 :

I'm learning about the StringBuider class. I have read in this site and many other books that when the compiler meets a '+' operator of literal, it automatically uses StringBuilder append's method to concatenate them.

It seems a little bit problematic since a StringBuilder object will be created at run time but the String reference is supposed to get the address of the concatenate String object already at compilation time.

String s1 = "hello";  
String s2 ="bc"; 
int value = 22;

When the compiler "meets" this code:

String s = s1+s2+22;

it "changes" it to:

String s = new StringBuilder().append("hello").append("bc").append(22).toString();

Maybe I am misunderstanding something?

Andrew Tobilko :

15.18.1. String Concatenation Operator +

An implementation may choose to perform conversion and concatenation in one step to avoid creating and then discarding an intermediate String object. To increase the performance of repeated string concatenation, a Java compiler may use the StringBuffer class or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression.

In your case,

String s1 = "hello";
String s2 ="bc";
int value = 22;

String r = s1 + s2 + value;

you will get

INVOKESPECIAL java/lang/StringBuilder.<init> ()V
ALOAD 1
INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
ALOAD 2
INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
ILOAD 3
INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;

While concatinating constant objects

String r = "hello" + "bc" + 22;

you will get

LDC "hellobc22"
ASTORE 2

---

When the compiler "meets" this code:

String s = s1+s2+22;

it "changes" it to:

String s = new StringBuilder().append("hello").append("bc").append(22).toString();

No. It may optimise it to

String s = new StringBuilder().append(s1).append(s2).append(value).toString();

but it can't replace s1 with "hello" because there is no guarantee the variable will keep referring to "hello". The variable is not final and thus open to reassignment.