I want to declare an ArrayList of type int.
Why does the following give me an error:
ArrayList<int> list1 = new ArrayList<int>(); But the following works:
ArrayList<Integer> list1 = new ArrayList<Integer>(); ?
7 Answers
ArrayList can only reference types, not primitives. Integer is a class, not a primitive.
When you declare ArrayList<Integer> list1 = new ArrayList<Integer>(), you're creating an ArrayList which will store the Integer type, not the int primitive.
If you want to read about the difference between primitive and reference types, check out http://pages.cs.wisc.edu/~hasti/cs302/examples/primitiveVsRef.html
1 Comment
Sartaj Singh Gill
You probably don't need the <Integer> type argument on the right of the new expression.
ArrayList<Integer> list1=new ArrayList(); works too.Because int is a primitive type. Only reference types can be used as generic parameters.
answered Jan 15, 2013 at 23:43
Oliver Charlesworth
274k34 gold badges591 silver badges687 bronze badges
4 Comments
Aravind Yarram
Even before generics (before Java 5) this restriction was there so I doesn't think its the only reason
Oliver Charlesworth
@Pangea: Sure, you can't do
new ArrayList().add(5). But the OP is asking about the error wrt that particular line of code...
Peter Lawrey
@OliCharlesworth You can do that actually. It just will be adding an
Integer not an int value.Oliver Charlesworth
@PeterLawrey: Ah, you're right. I guess this didn't work prior to autoboxing?
The short answer is that generics (like ArrayList<Integer>) do not accept primitive types (int), only objects (Integer).
This is because classes like ArrayList are implemented as using Objects. Since every class inherits from Object, the compiler can just plug in other classes. But primitive types (like int) do not inherit from Object, for they are not classes. So, Sun/Oracle made the Integer class to help with this.
So, in short: int is not an Object.
1 Comment
Hamman Samuel
This is the best answer because it explains this declaration, which works fine:
ArrayList<int[]>All the answers above answer why but the root of this question is frequent auto boxing and unboxing of the primitive data types. This problem is already solved by IntBuffer or ChadBuffer or you name the primitive type it's already there in the nio folder. Next time if you want to use primitive ArrayList don't use List instead use IntBuffer
Comments
int is a primitive data type but Integer is a class so an arrayList array can only take reference types as its parameter not primitive type
Comments
int is not an Object and hence if list type is int, implementations of the list cannot be done.
Comments
int is a primitive. It is not a Object.
Refer this link for further details.
1 Comment
Zach Latta
Link only answers are not encouraged at StackOverflow.
intisn't an object in the same context theIntegerisTIntArrayListif you want something which wrapsint[]intis primitive datatype we can use wrapper class inArrayList