Why when I add the same object to the list:
Integer a = new Integer(0);
testList.add(a);
a += 1;
testList.add(a);
The first didin't change?
asked Sep 29, 2015 at 12:24
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In the future, include all your relevant code when asking questions such as this. As shown, this is not even close to being valid Java. Also, read up on "Immutable" types (such as Integer).– JoeGCommented Sep 29, 2015 at 12:48
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2 Answers
Because an Integer is immutable. When you modify the value of the one referenced by a, you're creating a new object and updating the reference to it. testList holds a reference to both objects.
Since Integer wrapper class is immutable in java. Not only Integer, all wrapper classes and String is immutable.
a is a reference which points to an object. When you run a += 1, that reassigns a to reference a new Integer object, with a different value.
You never modified the original object.
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@Nominalista immutable means every change done to the Object,
Integerin this case, return a new Object Commented Sep 29, 2015 at 12:27 -
1@Nominalista you can create
MyClass, and sure, it can be mutable if you want, but you can't overload the+=operator for it. Commented Sep 29, 2015 at 12:31 -
1The new class is mutable if you have a method that updates its internal state. Commented Sep 29, 2015 at 12:34
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