String object creation in java [duplicate]

Question

Possible Duplicate:
Questions about Java’s String pool

I have a doubt in java Strings object creation.

String s1 = "Hello"+"world";
String s2 = s1+"Java";

in this program how many String objects will be created and how ?please explain it. Thanks.

Answer 1

The answer is 3

Two String objects will be created once per JVM start:

• "Helloworld"
• "Java"

Both will be interned, because they are constants (known at compile time).

They will be reused every time this code runs. A StringBuilder will be created to concatenate the two String above. References to them will be assigned to s1 and s2.

Here's the bytecode for the code:

   0:   ldc #37; //String Helloworld
   2:   astore_1
   3:   new #39; //class java/lang/StringBuilder
   6:   dup
   7:   aload_1
   8:   invokestatic    #41; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
   11:  invokespecial   #47; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   14:  ldc #50; //String Java
   16:  invokevirtual   #52; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   19:  invokevirtual   #56; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   22:  astore_2
   23:  return

Answer 2

You can't really say, how many Strings are created, since there's several differences due to the different implementations of the JVM.

As String is an immutable class, the naive answer is 5. But with some optimization (e.g. using a StringBuffer/ StringBuilder there would only be 2 Strings.

As concats would be summarized via append()-calls.

Edit: As the're some different answers here an explanation why I said 5:

1. "Hello"
   
2. "world"
   
3. (s1) "Helloworld"
   
4. "Java"
   
5. (s2) "HelloworldJava"
   

Answer 3

if you look at the compiled code, you can easily guess:

String s1 = "Helloworld";
String s2 = (new StringBuilder(String.valueOf(s1))).append("Java").toString();

We can't accurately know by just looking at source code as many optimizations are done by the compiler before execution.

Here we see that 1 String object is created for s1, and another String object for s2. Here 2 string literals are there in the string pool: "Helloworld" and "Java"