Primitive Wrapper Classes are Immutable in Java

Consider below Java program.

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// Java program to demonstrate that prmitive
// wrapper classes are immutable
class Demo
{
    public static void main(String[] args)
    {
        Integer i = new Integer(12);
        System.out.println(i);
        modify(i);
        System.out.println(i);
    }
  
    private static void modify(Integer i)
    {
        i = i + 1;
    }
}

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Output : 


12
12


The parameter i is reference in modify and refers to same object as i in main(), but changes made to i are not reflected in main(), why?


Explanation:

All primitive wrapper classes (Integer, Byte, Long, Float, Double, Character, Boolean and Short) are immutable in Java, so operations like addition and subtraction create a new object and not modify the old. 


The below line of code in the modify method is operating on wrapper class Integer, not an int




i = i + 1;


It does the following:


    

  1.  Unbox i to an int value
    2. 

  2.  Add 1 to that value
    3. 

  3.  Box the result into another Integer object
    4. 

  4.  Assign the resulting Integer to i (thus changing what object i references)
    5. 



Since object references are passed by value, the action taken in the modify method does not change i that was used as an argument in the call to modify. Thus the main routine still prints 12 after the method returns.


This article is contributed by Yogesh D Doshi. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above


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