If String concatenation using + is implemented using StringBuilder then why are extra objects created during concatenation?

Question

If the following code:

String s = "a" + 1 + "b";// 1.

Is implemented using using StringBuilder equivalent to

String s = new StringBuilder().append("a").append(1).append("b");

then will extra objects "a" and "b" be created in 1 and why?

Answer 1

Your example will not actually use a StringBuilder because none of the elements are variables. Because "a", 1, and "b" are all literals, the compiler will make a single String for you! If, however, you included a variable in that String concatenation, then it would use a StringBuilder and would need separate Strings for the concatenated elements.

For your example the compiler would create a single String literal:

const #2 = String       #21;    //  a1b

public void foo();
  Code:
   Stack=1, Locals=2, Args_size=1
   0:   ldc     #2; //String a1b
   2:   astore_1
   3:   return
  LineNumberTable: 
   line 7: 0
   line 8: 3

Let's say we had instead written

public void bar(String c)
{
    String s = "a" + c + "b";// 1.
}

Now the compiler will need to create a StringBuilder, and it will use the a and b constant ASCII literals with the StringBuilder.

const #20 = Asciz       a;
const #22 = Asciz       b;

public void bar(java.lang.String);
  Code:
   Stack=2, Locals=3, Args_size=2
   0:   new     #2; //class java/lang/StringBuilder
   3:   dup
   4:   invokespecial   #3; //Method java/lang/StringBuilder."<init>":()V
   7:   ldc     #4; //String a
   9:   invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava    /lang/String;)Ljava/lang/StringBuilder;
   12:  aload_1
   13:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   16:  ldc     #6; //String b
   18:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   21:  invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   24:  astore_2
   25:  return
  LineNumberTable: 
   line 7: 0
   line 8: 25