Question

I was having a discussion about usage of Strings and StringBuffers in Java. How many objects are created in each of these two examples?

Ex 1:

String s = "a";
s = s + "b";
s = s + "c";        

Ex 2:

StringBuilder sb = new StringBuilder("a");
sb.append("b");
sb.append("c");

In my opinion, Ex 1 will create 5 and Ex 2 will create 4 objects.

Answer 1

You can determine the answer by analyzing the java bytecode (use javap -c). Example 1 creates two StringBuilder objects (see line #4) and two String objects (see line #7), while example 2 creates one StringBuilder object (see line #2).

Note that you must also take the char[] objects into account (since arrays are objects in Java). String and StringBuilder objects are both implemented using an underlying char[]. Thus, example 1 creates eight objects and example 2 creates two objects.

Example 1:

public static void main(java.lang.String[]);
  Code:
   0:   ldc             #2; //String a
   2:   astore_1
   3:   new             #3; //class java/lang/StringBuilder
   6:   dup
   7:   invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V
   10:  aload_1
   11:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   14:  ldc             #6; //String b
   16:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   19:  invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   22:  astore_1
   23:  new             #3; //class java/lang/StringBuilder
   26:  dup
   27:  invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V
   30:  aload_1
   31:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   34:  ldc             #8; //String c
   36:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   39:  invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   42:  astore_1
   43:  return   
}

Example 2:

public static void main(java.lang.String[]);
  Code:
   0:   new             #2; //class java/lang/StringBuilder
   3:   dup
   4:   ldc             #3; //String a
   6:   invokespecial   #4; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   9:   astore_1
   10:  aload_1
   11:  ldc             #5; //String b
   13:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   16:  pop
   17:  aload_1
   18:  ldc             #7; //String c
   20:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   23:  pop
   24:  return  
}