String can't change. But int, char can change

Question

I've read that in Java an object of type String can't change. But int and char variables can. Why is it? Can you give me an example?

Thank you. (I am a newer -_- )

Answer 1

As bzabhi said, strings are immutable in Java. This means that a string object will never change. This does not mean you can not change string variables, just that you cannot change the underlying memory representation of the string. for an example:

String str = "Hello";
str += " World!";

Following the execution of these lines, str will point to a new string in memory. The original "Hello" string still exists in memory, but most likely it will not be there for long. Assuming that there are no extenuating circumstances, nothing will be pointing at the original string, so it will be garbage collected.

I guess the best way to put this would be to say that when line 2 of the example executes, a new string in memory is created from the concatenation of the original string and the string being added to it. The str variable, which is just a reference to a memory location, is then changed to point at the new variable that was just created.

I am not particularly knowledgeable on the point, but, as I understand it, this is what happens with all "non-primitive" values. Anything that at some point derives from Object follows these rules. Primitive values, such as ints, bools, chars, floats and doubles allow the actual value in memory to be changed. So, from this:

int num = 5;
num += 2;

the actual value in memory changes. Rather than creating a new object and changing the reference, this code sample will simply change the value in memory for the num variable.

As for why this is true, it is simply a design decision by the makers of Java. I'm sure someone will comment on why this was made, but that isn't something I know.