Final object's value could be changed?

[Larry Olson]

Hi,

A final object's value could be changed, right? i.e. not the reference itself could be re-assigned, but the contents of the object the reference is pointing to could be changed.
But how does this work out in the case of wrapper objects. For example given,

final Integer x = 5;

How could I change the value of x?

Thanks.

[Michael Angstadt]

Hi Larry,

When you can change the contents of an object, the object is mutable. The ArrayList class is mutable because you can add and remove elements from it. Wrapper objects, however, are immutable, meaning you cannot alter the contents of the object. The String class is also immutable. Methods like substring() and replace() return a new String instance, leaving the original one untouched. So because wrapper objects are immutable, there's no way to change the value of an Integer object. And, in your case, since the variable is final, you can't even reassign the reference.

[Henry Wong]

Immutable objects are... immutable. The objects value does not change.

[EDIT: beaten again. And with a much more detailed answer]

Henry

[Larry Olson]

Thanks for the detailed explanation. That helps. I just have one more questoin. In that case i.e. wrapper objects being immutable and particular final wrapper objects being very much so, why aren't they being allowed inside switch statements as case values?

i.e.

(1)
final Integer x = 5;
case x: //isn't allowed

even though logically x is a constant and it is immutable

whereas

(2)
final int x = 5;
case x: //is allowed

When I compile (1), the compiler gives the following error:
"constant expression required", even though as you very well explained "final Integer x = 5"; is immutable, which means it is a constant. But still the compiler insists that it requires a constant expression. What gives? Please, I don't want to hear that the JLS is the reason.

[Henry Wong]

 

Larry Olson wrote:Please, I don't want to hear that the JLS is the reason.

Then ignore this answer.... When the compiler is complaining about it not being a constant. It is referring to a compile time constant.

And the definition of compile time constant is very specific. Wrapper objects are not candidates of being compile time constants. The operations needed to box and unbox the values are not allowed of compile time constants.

Henry

[Michael Angstadt]

I think this is because what switch cases do is check for equality using ==. So this switch statement:

switch (input){
case 1: //...
case 2: //...
}

is equivalent to:

if (input == 1){
//...
} else if (input == 2){
//...
}

This doesn't jive well with objects, which use the equals() method to check for equality. For example:

Integer i1 = 1000;
Integer i2 = 1000;
assert i1 == i2; //false
assert i1.equals(i2) //true

(== works when Integers are within a specific range, due to the way Java interns Number objects, but for larger numbers it does not work)

Also, there's no way for Java to know if an object is immutable or not.

[Simon Boyle]

Careful! I think I remember seeing somewhere that Integer wrappers < 128 are likely to return true when using an == comparison, anyhting else is likely to return false.

A quick test reveals this:

public class eq {
public static void main(String... args) {
Integer i1 = 127;
Integer i2 = 127;
Integer i3 = 128;
Integer i4 = 128;

System.out.println(i1 == i2);
System.out.println(i3 == i4);
}
}

prints:

true
false

Take care if you're going to do this (I wouldn't recommend it...). I didn't get a question like this on my SCJP but that's not to say there aren't any!

Simon.

[Yogesh Gnanapraksam]

 

== works when Integers are within a specific range, due to the way Java interns Number objects, but for larger numbers it does not work)

I do not understand .I remember something related to this context was discussed earlier here but I am not able to find it.

Can somebody please explain..

[Michael Angstadt]

Like Simon alluded to, the values from -128 to 127 are interned.

Integer i1 = -128;
Integer i2 = -128;
System.out.println(i1 == i2); //true
Integer i3 = -129;
Integer i4 = -129;
System.out.println(i3 == i4); //false
Integer i5 = 127;
Integer i6 = 127;
System.out.println(i5 == i6); //true
Integer i7 = 128;
Integer i8 = 128;
System.out.println(i7 == i8); //false

This also applies to Bytes, Shorts, and Longs.

[wbrian radley]

addendum: be very careful with interned values as they apply only in one very specific case:
(and, yes, the correct boolean values are being printed even tho' the flag is s)

Integer i122 = new Integer(10);
Integer i123 = new Integer(10);
Integer i220 = 10;
Integer i222 = 10;
Integer i224 = 210;
Integer i225 = 210;
System.out.printf("%n43 %s%n", i122 == i123); // false
System.out.printf("%n45 %s%n", i122.equals(i220)); //true
System.out.printf("%n46 %s%n", i122 == i220); // false
System.out.printf("%n47 %s%n", i220 == i222); // true
System.out.printf("%n48 %s%n", i224 == i225); //false
System.out.printf("%n49 %s%n", i122 == i123); // false
int i500 = new Integer(10);
int i501 = new Integer(10);
int i502 = new Integer(200);
int i503 = new Integer(200);
System.out.printf("%n57%s%n", i500 == i501); // true
System.out.printf("%n58%s%n", i502 == i503); // true

[Sahil Kapoor]

Please explain what is the Logic going on !!!

[Yogesh Gnanapraksam]

Sahil,
This is the same logic that is implemented by the intern() method in String Class.
You can look here https://coderanch.com/t/488922/java/java/new-keyword-objects-Java#2198278 for details.
As per the discussion here the same applies for Integers,Short and Byte.
But I do not understand why intern is not done for numbers greater than 127.

[Henry Wong]

 

Yogesh Gnanapraksam wrote:
I remember something related to this context was discussed earlier here but I am not able to find it.

Just do a search on "integer cache". This topic comes up often.

Henry

[Yogesh Gnanapraksam]

Thanks Henry.
I got an understanding of this logic now.
JSL spec

[Martin Vanyavchich]

As for switch statements and wrapper classes ... keep in mind that an Integer object can point to null. How would you squeeze that into a switch statement