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Thread: Java Wrapper Classes

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    Default Java Wrapper Classes

    I understand the reason behind String class being immutable. Would like to know for what reasons the other wrapper classes like Integer,FLOAT,LONG are made immutable?


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    Default Re: Java Wrapper Classes

    Can you think of any reasons? I wouldn't spend too much time worrying about why certain decisions were made about the language. Why is it called a JFrame instead of a JForm? Why is it int and not integer? It's usually best just to accept the fact of the situation and move on.

    I can think of at least one reason for making them immutable though. Say you have a class that contains an Integer that you want other classes to be able to access, but not change. You might do something like this:


    public class IntegerHolder{
     
       private Integer i = 15;
     
       public Integer getI(){
          return i;
       }
    }

    That's all well and good, right? The Integer is private so other classes can't access it directly, and there's only a getter method, so other classes can't change it. But if Integer was not immutable, another class could do this:

    IntegerHolder holder = new IntegerHolder();
    holder.getI().setInt(10000);
    Last edited by KevinWorkman; June 13th, 2012 at 08:07 AM.
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    Default Re: Java Wrapper Classes

    Making a class immutable has advantages for both Garbage Collection and security. Security wise, one cannot alter the value of the object and leave it in an unexpected state. Examples and how this relates to threading is discussed in the following:
    See Immutable Objects (The Java™ Tutorials > Essential Classes > Concurrency)
    For a discussion on the advantages for garbage collection, see
    Java theory and practice: Garbage collection and performance
    If a class has no reason to have its behavior overridden or inherited (as would be the case for Integer, Long, etc...), there's a good chance I will make the class immutable for the above reasons.
    Last edited by copeg; June 13th, 2012 at 12:26 PM. Reason: spelling

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    Default Re: Java Wrapper Classes

    well Immutability offer several advantages :

    1) Immutable object like String and wrapper can be used as key in HashMap (those are most used keys anyway)
    2) Immutability enable them to be reused in application. valueOf() often return same object if its immutable
    3) Immutable are inherently thread-safe

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    Default Re: Java Wrapper Classes

    Quote Originally Posted by javabuddy View Post
    well Immutability offer several advantages :

    1) Immutable object like String and wrapper can be used as key in HashMap (those are most used keys anyway)
    2) Immutability enable them to be reused in application. valueOf() often return same object if its immutable
    3) Immutable are inherently thread-safe
    Be careful in getting Immutability confused with Wrapper classes and caching (points 1 and 2). Immutability does not have much to do with being able to wrap a primitive and place it into a Map (it does from the context of thread safety and the hashCode() method contract - and FWIW it is quite relative to say they are the most used keys), and Immutability has nothing to do with the caching involved with the valueOf method of those classes (and just to note, is incorrect to say it always returns the same object - only a few select values are actually cached, in which case for only a few select values will you get the same object)
    Last edited by copeg; June 15th, 2012 at 09:44 AM.

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