Understanding String as an Object

[Michael Farinha]

I understand the difference between primitive and reference datatypes but I'm somewhat baffled by the "String" datatype.

According to the java docs String is an object:
java.lang.String
but why is it that you can declare a string like a primitive?

String myString;
int myInt;

I've discovered that you can treat a string like an object:

String myString = new String("Hello");
System.out.println(myString);

But how is it that you can treat it like a primitive with out declaring "new String()?"

also since you can say:

String myString;
myString = "Hello";

does that mean a string of characters is defined as an object?

Sorry if this is a bit too simplistic but its something that has boggled me since I leared of the differences between primitives and references.

Thanks!

-Mike

[Dani Atrei]

Because they are used so commonly, strings can be created like primitives, yet they really are objects, thus you can write things like myStringVar.length(). It just makes it easier for the programmer. You have similar things in Ruby for arrays etc...

[Michael Farinha]

Sorry, I'm still not understanding.
Is the String class implementing an OO feature to enable it to act like a primitive or is the compiler doing some trick here that enables this special case?

Can I create a class that behaves like String?

Thanks!

[Ernest Friedman-Hill]

It's not true to say that a String is like a primitive in any way, shape or form. Saying

String aString;

gives you an uninitialized variable that points to nothing, just as

Object anObject;

does. Until you assign an object to the variable, it can't be used for anything.

Now, there is one single thing that's special about the String class: there's a special notation for creating them. If you say

String aString = "Some text";

that's a special shorthand for something which you could imagine like this:

String aString = Java.getStringFromPoolOrCreateANewOne("Some text");

There's no real function by that name, of course; instead Java takes a special action when it sees a quoted String literal. But those special actions are as if this made-up Java.getStringFromPoolOrCreateANewOne() function did something like:

public static String getStringFromPoolOrCreateANewOne(characters c) {
    if (stringHasAlreadyBeenCreated(c))
        return lookUpStringsIAlreadyCreated(c);
    else
        return new String(characters);
}

So you see that Strings are always created with "new", just like any other object. It's just that there's a shorthand which hides this from you. It doesn't make Strings anything like primitives, though.

[Dani Atrei]

Sorry, I just realised my answer wasnt very clear... Thanks Ernest for clearing things up

[Michael Farinha]

Ok, I think I understand.
Strings are basically a special case, nothing really to understand with how java implements it other than "its the way it is."

Also, I have a basic understanding of the string pool so I think your example makes more sense to me than if I didn't know about the string pool.

I just have a hard time dealing with 'special cases.' I like to know why its a special case and how the special case was implemented.

But I think I grasp whats going on.

Thanks!

[Bear Bibeault]

Originally posted by Michael Farinha:

Strings are basically a special case

Not really. The String object acts like any other.

What is the special case is the ability to use the string literal notation in Java code. That's a compiler issue, not a run-time issue.