immutable strings

jjwallaby · October 10th, 2015, 03:19 AM

Page 49. Says Strings are special objects in Java.
'They are immutable' you can't change the value of a String that has been initialized.

Does it mean the compiler will allow it...but unexpected behaviour may occur in run-time.. Why the compiler stop from compiling

Below compile/with allowable changing of String objects :-

public class MyString {

String mystr = "Mary";

public static void main(String args[]) {
MyString mystring = new MyString();

System.out.println( mystring.mystr);
mystring.mystr = "Smith, Mary";

System.out.println( mystring.mystr);

}
}

Output
Mary
Smith,Mary

Cristina · October 11th, 2015, 03:15 AM

Hi,

When you do

Code:

mystring.mystr = "Smith, Mary";

a new String object is created, as (I hope) the following drawing explains:
https://java4kids.moodlecloud.com/pl...24HTrainer.jpg

Cheers,
Cristina

jjwallaby · October 12th, 2015, 05:05 AM

Thanks Cristina.
Well explained.

abenmariem · November 10th, 2015, 07:03 PM

I think that you need to take a look at what means a reference type variable.
A string is immutable in JAVA. This means that you cannot change the text that it holds once it is initialized.

Cristina · January 27th, 2016, 05:26 AM

If you know JavaScript, it may be easier to understand.

The following piece of code

Code:

var myStr = "Mary";
myStr[0] = "S";

leads to TypeError: 0 is read-only.

However, the following is fine.

Code:

myStr = "Smith, Mary";

The contents of myStr cannot be altered, which does not mean that myStr cannot be changed.