String Concatenation and Autoboxing in Java

Question

When you concatenate a String with a primitive such as int, does it autobox the value first.

ex.

String string = "Four" + 4;

How does it convert the value to a string in Java?

Answer 1

To see what the Java compiler produces it is always useful to use javap -c to show the actual bytecode produced:

For example the following Java code:

String s1 = "Four" + 4;
int i = 4;
String s2 = "Four" + i;

would produce the following bytecode:

   0:   ldc     #2; //String Four4
   2:   astore_1
   3:   iconst_4
   4:   istore_2
   5:   new     #3; //class java/lang/StringBuilder
   8:   dup
   9:   invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V
   12:  ldc     #5; //String Four
   14:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/
String;)Ljava/lang/StringBuilder;
   17:  iload_2
   18:  invokevirtual   #7; //Method java/lang/StringBuilder.append:(I)Ljava/lan
g/StringBuilder;
   21:  invokevirtual   #8; //Method java/lang/StringBuilder.toString:()Ljava/la
ng/String;
   24:  astore_3
   25:  return

From this we can see:

• In the case of "Four" + 4, the Java compiler (I was using JDK 6) was clever enough to deduce that this is a constant, so there is no computational effort at runtime, as the string is concatenated at compile time
• In the case of "Four" + i, the equivalent code is new StringBuilder().append("Four").append(i).toString()
• Autoboxing is not involved here as there is an StringBuilder.append(int) method which according to the docs is using String.valueOf(int) to create the string representation of the integer.