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?
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Nominalista
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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). – JoeG Sep 29 '15 at 12:48
2 Answers
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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.
jsheeran
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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.
SatyaTNV
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@Nominalista immutable means every change done to the Object, `Integer` in this case, return a new Object – SomeJavaGuy Sep 29 '15 at 12:27
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1@Nominalista you can create `MyClass`, and sure, it can be mutable if you want, but you can't overload the `+=` operator for it. – Andy Turner Sep 29 '15 at 12:31
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@Nominalista You *can* create a new wrapper class for `int` that allows updates, but you will lose the auto-boxing/unboxing features that are required for the `+` operator to work. – Phylogenesis Sep 29 '15 at 12:31
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class is `mutable` by default. It you make the class as `final` (having `final` properties) it will be `immutable`. – SatyaTNV Sep 29 '15 at 12:34
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1The new class is mutable if you have a method that updates its internal state. – Phylogenesis Sep 29 '15 at 12:34
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1@Nominalista [check this for immutable](http://stackoverflow.com/questions/18194139/make-immutable-java-object) – SomeJavaGuy Sep 29 '15 at 12:34