The SCJP Tip Line
Strings, Literally
by Corey McGlone
This month, I'm going to tackle String literals and how they're handled in Java. If you read last month's SCJP Tip Line article, you'll find that this article is a nice follow up all about Strings. If you didn't read it, no big deal - I plan to cover enough information here to get you up to speed. String literals are a little "special" in Java in the way that they are treated. For the most part, the information contained in this post is not on the SCJP exam, but I see it on mock exams all the time and it comes up in the SCJP/OCPJP forum quite often. Besides, it's just darned interesting stuff.

Let's start off by asking a simple question. What is a String Literal? A String literal is a sequence of characters between quotation marks, such as "string" or "literal". You've probably used String literals hundreds of times in your own applications. You might not, however, have realized just how special String literals are in Java.

Strings are Immutable

So what makes String literals so special? Well, first of all, it's very important to remember that String objects are immutable. That means that, once created, a String object cannot be changed (short of using something like reflection to get at private data). Immutable, you say? Unchangable? What about this code?

Source Code 
            
public class ImmutableStrings
{
    public static void main(String[] args)
    {
        String start = "Hello";
        String end = start.concat(" World!");
        System.out.println(end);
    }
}

// Output

Hello World!

Well look at that, the String changed...or did it? In that code, no String object was ever changed. We start by assigning "Hello" to our variable, start. That causes a new String object to be created on the heap and a reference to that object is stored in start. Next, we invoke the method concat(String) on that object. Well, here's the trick, if we look at the API Spec for String, you'll see this in the description of the concat(String) method:
Concatenates the specified string to the end of this string.
If the length of the argument string is 0, then this String object is returned. Otherwise, a new String object is created, representing a character sequence that is the concatenation of the character sequence represented by this String object and the character sequence represented by the argument string.

Examples:

    "cares".concat("s") returns "caress"
    "to".concat("get").concat("her") returns "together"

Parameters:
    str - the String that is concatenated to the end of this String.
Returns:
    a string that represents the concatenation of this object's characters followed by the string argument's characters.
Notice the part I've highlighted in bold. When you concatenate one String to another, it doesn't actually change the String object, it simply creates a new one that contains the contents of both of the original Strings, one after the other. That's exactly what we did above. The String object referenced by the local variable start never changed. In fact, if you added the statement System.out.println(start); after you invoked the concat method, you would see that start still referenced a String object that contained just "Hello". And just in case you were wondering, the '+' operator does the exact same thing as the concat() method.

Strings really are immutable.

Storage of Strings - The String Literal Pool

If you've done any preparation for the SCJP exam (and quite possibly even if you haven't), you've probably heard of the "String Literal Pool." What is the String Literal Pool? Most often, I hear people say that it is a collection of String objects. Although that's close, it's not exactly correct. Really, it's a collection of references to String objects. Strings, even though they are immutable, are still objects like any other in Java. Objects are created on the heap and Strings are no exception. So, Strings that are part of the "String Literal Pool" still live on the heap, but they have references to them from the String Literal Pool.

Yeah, so that doesn't really explain what the pool is, or what it's for, does it? Well, because String objects are immutable, it's safe for multiple references to "share" the same String object. Take a look at this example:

Source Code 
            
public class ImmutableStrings
{
    public static void main(String[] args)
    {
        String one = "someString";
        String two = "someString";
        
        System.out.println(one.equals(two));
        System.out.println(one == two);
    }
}

// Output

true
true

In such a case, there is really no need to make two instances of an identical String object. If a String object could be changed, as a StringBuffer can be changed, we would be forced to create two separate objects. But, as we know that String objects cannot change, we can safely share a String object among the two String references, one and two. This is done through the String literal pool. Here's how it is accomplished:

When a .java file is compiled into a .class file, any String literals are noted in a special way, just as all constants are. When a class is loaded (note that loading happens prior to initialization), the JVM goes through the code for the class and looks for String literals. When it finds one, it checks to see if an equivalent String is already referenced from the heap. If not, it creates a String instance on the heap and stores a reference to that object in the constant table. Once a reference is made to that String object, any references to that String literal throughout your program are simply replaced with the reference to the object referenced from the String Literal Pool.

So, in the example shown above, there would be only one entry in the String Literal Pool, which would refer to a String object that contained the word "someString". Both of the local variables, one and two, would be assigned a reference to that single String object. You can see that this is true by looking at the output of the above program. While the equals() method checks to see if the String objects contain the same data ("someString"), the == operator, when used on objects, checks for referential equality - that means that it will return true if and only if the two reference variables refer to the exact same object. In such a case, the references are equal. From the above output, you can see that the local variables, one and two, not only refer to Strings that contain the same data, they refer to the same object.

Graphically, our objects and references would look something like this:


Note, however, that this is a special behavior for String Literals. Constructing Strings using the "new" keyword implies a different sort of behavior. Let's look at an example:

Source Code 
            
public class ImmutableStrings
{
    public static void main(String[] args)
    {
        String one = "someString";
        String two = new String("someString");
        
        System.out.println(one.equals(two));
        System.out.println(one == two);
    }
}

// Output

true
false

In this case, we actually end up with a slightly different behavior because of the keyword "new." In such a case, references to the two String literals are still put into the constant table (the String Literal Pool), but, when you come to the keyword "new," the JVM is obliged to create a new String object at run-time, rather than using the one from the constant table.

In such a case, although the two String references refer to String objects that contain the same data, "someString", they do not refer to the same object. That can be seen from the output of the program. While the equals() method returns true, the == operator, which checks for referential equality, returns false, indicating that the two variables refer to distinct String objects.

Once again, if you'd like to see this graphically, it would look something like this. Note that the String object referenced from the String Literal Pool is created when the class is loaded while the other String object is created at runtime, when the "new String..." line is executed.


If you'd like to get both of these local variables to refer to the same object, you can use the intern() method defined in String. Invoking two.intern() will look for a String object referenced from the String Literal Pool that has the same value as the one you invoked the intern method upon. If one is found, a reference to that String is returned and can be assigned to your local variable. If you did so, you'd have a picture that looks just like the one above, with both local variables, one and two, referring to the same String object, which is also referenced from the String Literal Pool. At that point, the second String object, which was created at run-time, would be eligible for garbage collection.

Garbage Collection

What makes an object eligible for garbage collection? If you're preparing for the SCJP exam (or even if you're not), the answer to that question should roll right off your tongue. An object is eligible for garbage collection when it is no longer referenced from an active part of the application. Anyone see what is special about garbage collection for String literals? Let's look at an example and see if you can see where this is going.

Source Code 
            
public class ImmutableStrings
{
    public static void main(String[] args)
    {
        String one = "someString";
        String two = new String("someString");
        
        one = two = null;
    }
}

Just before the main method ends, how many objects are available for garbage collection? 0? 1? 2?

The answer is 1. Unlike most objects, String literals always have a reference to them from the String Literal Pool. That means that they always have a reference to them and are, therefore, not eligible for garbage collection. This is the same example as I used above so you can see what our picture looked liked originally there. Once we assign our variables, one and two, to null, we end up with a picture that looks like this:


As you can see, even though neither of our local variables, one or two, refer to our String object, there is still a reference to it from the String Literal Pool. Therefore, the object is not elgible for garbage collection. The object is always reachable through use of the intern() method, as referred to earlier.

Conclusion

Like I said at the outset of this article, virtually none of this information is included on the SCJP exam. However, I constantly see this question coming up in the SCJP forum and on various mock exams. These are a few of the highlights you can keep in mind when it comes to String literals: Be sure to check out these resources, as well:

API Spec for String
JLS, ยง3.10.5 String Literals

Until next time,
Corey