Why is an Integer variable assigned null by default?

Question

I was going through a test on itester.org and found a question which I don't understand:

public class Runner 
{
    public static Integer i;

    public static void main(String[] args) 
    {
        if (i == 42) {
            System.out.printf("wow");
        }
    }
 }

I read before, that integer variable is assigned by default 0. Why is it assigned null here?

Because it's not a primitive, but an object. The default value for an object is null. When you are doing i == 42, it tries to unbox the int value in i but since i is null, you get a NPE. — Alexis C.
– Alexis C., Commented Nov 30, 2014 at 9:13
Gotta love Java -- it makes so much sense that int and Integer are different data types. — Craig Tullis
– Craig Tullis, Commented Nov 30, 2014 at 9:19

Eran · Accepted Answer · 2014-11-30 09:13:54Z

18

Any reference type (i.e. any variable whose type is Object or a sub-class of Object) has a default value of null. This includes Integer.

The primitive int, on the other hand, has a default value of 0.

answered Nov 30, 2014 at 9:13

Eran

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Maroun · Accepted Answer · 2014-11-30 09:14:03Z

6

Because the JLS 4.12.5. Initial Values of Variables:

For all reference types (§4.3), the default value is null.

And since Integer is a reference type, it gets null:

ReferenceType:
    ClassOrInterfaceType
    TypeVariable
    ArrayType

^{See the link for other types}

answered Nov 30, 2014 at 9:14

Maroun

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Comments

anon · Accepted Answer · 2014-11-30 09:14:18Z

5

The primitive int type is assigned 0 by default, but an Integer reference is assigned null by default. Integer is a wrapper class -- it's an object, not a primitive type.

You may want to read about autoboxing and unboxing in Java, the process by which Java automatically converts between primitive types and wrapper classes.

answered Nov 30, 2014 at 9:14

anon

Comments

S. M. Mohiuddin · Accepted Answer · 2014-11-30 11:09:04Z

2

In Java Integer is a Object Type. In this sample code, you need primitive type which is int. In Java any Object type/Reference type or any Sub type of object type variable which is not initialized automatically initialized by null. Where as primitive type has a default value.

The reason behind of this is, Objects provide facilities for polymorphism, are passed by reference (or more accurately have references passed by value), and are allocated from the heap. Conversely, primitives are immutable types that are passed by value and are often allocated from the stack.

answered Nov 30, 2014 at 11:09

S. M. Mohiuddin

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Eng.Fouad · Accepted Answer · 2014-11-30 09:13:54Z

1

All non-primitive non-local variables are assigned to null, if not assigned explicitly.

answered Nov 30, 2014 at 9:13

Eng.Fouad

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user2623906 · Accepted Answer · 2014-11-30 09:50:29Z

1

Integer is a wrapper class and in this code sample I is reference variable. (In java everything is not object so we use wrapper classes to make object and java perform boxing and unboxing ) All reference variables in java by default null value and all primitive type have its default value (e.g int i -> 0 )

Use:- Private static int; then it have 0 value default .

answered Nov 30, 2014 at 9:50

user2623906

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