Number of String objects created

[Jarle Eide]

Hi,

I'm using the SCJP5 Study Guide by Sierra and Bates (great book by the way) to study for the exam, and I have a question regarding an example in the book on String objects and immutability:

1. String s1 = "spring ";
2. String s2 = s1 + "summer ";
3. s1.concat("fall ");
4. s2.concat(s1);
5. s1 += "winter ";
6. System.out.println(s1 + " " + s2);

Now, the book states that 8 String objects are created in total: "spring ", "summer ", "spring summer ", "fall", "spring fall ", "spring summer spring", "winter", "spring winter". My question is then: would not line 6 also create some String objects, at least the object with value " ", but also (s1 + " ") and (s1 + " " + s2) during the + operations?


jarle

[John Stone]

I think 2 strings on line 6:

ONE in constant pool: ""
SECOND when result of operations is converted to string by calling toString() on StringBuilder

class bum {
public static void main (String[] args) {
String s1 = "spring";
String s2 = "summer";
System.out.println(s1 + " " + s2);
}
}

public static void main(java.lang.String[]);
  Code:
   Stack=4, Locals=3, Args_size=1
   0:ldc#16; //String spring
   2:astore_1
   3:ldc#18; //String summer
   5:astore_2
   6:getstatic#20; //Field java/lang/System.out:Ljava/io/PrintStream;
   9:new#26; //class java/lang/StringBuilder
   12:dup
   13:aload_1
   14:invokestatic#28; //Method java/lang/String.valueOf: (Ljava/lang/Object :wink: Ljava/lang/String;
   17:invokespecial#34; //Method java/lang/StringBuilder."<init>": (Ljava/lang/String :wink: V
   20:ldc#37; //String  
   22:invokevirtual#39; //Method java/lang/StringBuilder.append: (Ljava/lang/String; )Ljava/lang/StringBuilder;
   25:aload_2
   26:invokevirtual#39; //Method java/lang/StringBuilder.append: (Ljava/lang/String; )Ljava/lang/StringBuilder;
   29:invokevirtual#43; //Method java/lang/StringBuilder.toString: ()Ljava/lang/String;
   32:invokevirtual#47; //Method java/io/PrintStream.println: (Ljava/lang/String; )V
   35:return

20: here's empty string ""
29: here's toString()

[John Stone]

You can also search in previous topics. I rememeber at least one such topic. Just search for "spring summer" ;-)

[Michael Raymond Jr.]

System.out.println(s1 + " " + s2);


How do we know it's actually concatenating them? At least for s1 and s2, perhaps println() just references their memory locations, prints the data in those references, and doesn't actually do any string manipulation hence the reason no new objects are created. Dunno about the " " though.

Just a shot in the dark.