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Java The Thing About Strings

Diego Marrs
Diego Marrs
8,243 Points
Posted by Diego Marrs
Diego Marrs
8,243 Points

Definitions.

Could someone explain to me what primitive, literal, and string interning is in java? I'm pretty sure I have never covered this before...maybe I have...I dunno.

Thanks!

1 Answer

Jennifer Nordell
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STAFF
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Jennifer Nordell
Treehouse Teacher
Jennifer Nordell
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Jennifer Nordell
Treehouse Teacher

Wow you ask the tough ones, don't you? :smiley: Well let's see if I can clarify some of it. A primitive data type is a data type that is predefined by the programming language. This is true of every programming language I've come across. You can find the java predefined data types here:

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

A string literal is simply a string in quotation marks. For example:

String name = "Diego";

The "Diego" part is the string literal. The type is String which happens to be a predefined type in Java thus making it a... you guessed it... primitive data type.

Now the string interning is interesting and it has to do with the heap or memory allocation. Let's say "Diego" appears 50 times through your code. As I understand it, what internalizing does is make it so that string has one memory location and every other reference of it points to that memory location instead of having 50 separate memory locations containg the same thing. But someone please tell me if I've misunderstood this!

If you read here and do a search on the page for "intern" the third hit you get will tell you a bit about the changes made in JDK 7 involving this. And they do mention the memory heap etc. http://www.oracle.com/technetwork/java/javase/jdk7-relnotes-418459.html

Hope this helps and that I didn't muddle it all up! :smiley:

markhunter4
markhunter4
5,564 Points

Hi Jennifer,

Good response.

To clarify your response, Strings are not primitive type data! They are a class of data defined as an array of char (which is why char is lowercased and String is uppercase).

Intern, as described in the video is most useful for when you are dynamically instantiating a String, such as in

String mark = new String("mark")

or when you are taking user input (i.e. using a scanner). Interning adds the string into the string literal pool.

When declaring a String the normal way such as

String mark1 = "mark"

The Java automatically adds/references "mark" to the string pool at compile time (see 5:40 in video). Here's an example, 

String markDynamic = new String("mark"); // java allocates memory at runtime, not in the string pool  
String markStatic = "mark"; // compiler automatically adds to string pool reference 

markDynamic == markStatic //==>false because the two strings are in two different memory locations 

markDynamic = markDynamic.intern() // re-asign the string as a referance to the string already in the string  pool 
markDynamic == markStatic //==>true as they both point to the same string in memory

It is confusing, but it makes sense when you understand memory allocation.