Consider below Java program.
// 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; } } |
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:
- Unbox i to an int value
- Add 1 to that value
- Box the result into another Integer object
- Assign the resulting Integer to i (thus changing what object i references)
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.
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