Question

I am using the OpenJDK Java compiler under Ubuntu. I wanted to convert a character array to a string and when that seemed to have ended up giving ambiguous results, I tried to write a toString method of my own. In the process, I wrote a test program wherein (out of the fun of it) I tried to compile the following code.

class toString{
    public static void main(String[] args){
        string = "abc";
        string = string + "bcd";
        System.out.println(string);
    }
}

Now, I know that String objects in Java are immutable and the code should have in fact generated an error but to my surprise, it printed abcbcd to the console. Does this mean that String objects in Java are mutable or is there something wrong with the implementation of OpenJDK compiler in this case?

Answer 1

The difference is between References to an object and the object itself.

String XXX = "xxx";

Means:
Make a new variable and assign the reference to an instance of object String that contains the literal string “xxx”.

XXX = XXX + "yyy";

Means:

Get the reference to the object we have in variable named XXX.
Make a new object of type String that contains the string literal “yyy”.
Add them together executing the string + operator.
This operation will create a new String object that contains the literal string “xxxyyy”.
After all this, we put the reference to the new object again in the variable XXX.

The old referenced object containing “xxx” is not used anymore but the content of it was never modified.

As a counter proof, there is an example:

String a = "abc";
String b = "def";
String c = a;

a = a + b;

System.out.println(a); // will print "abcdef".
System.out.println(b); // will print "def".
System.out.println(c); // will print "abc".

// Now we compare references, in java == operator compare references, not the content of objects.

System.out.println(a == a); // Will print true
System.out.println(a == c); // Will print false, objects are not the same!

a = c;

System.out.println(a == c); // Will print true, now a and b points on the same instance.

Object instance are something “abstract” that live in a portion of memory of your program. A reference variable is a reference to that portion of memory.
You can access objects only through variables (or return values).

A single object can have more than one variable pointing at it.

String is immutable means that you cannot modify the content of that area of memory used by the String object, but of course, you are free to change and exchange references to it as you prefer.