Java declaration confusion for datatypes

Question

I have a doubt about null assigning to variable in Java. In my program I have assigned null to String variable as String str_variable = null;. For the learning purpose i assigned null integer variable as int int_variable = null; It shows error Add cast with Integer. So that rewrite the above int declaration as Integer int_variable = null;. This does not shows errors. I do not know the reason of these two kind of declaration.

Please the difference between to me.

String str_variable = null;

int int_variable = null; // error.

Integer int_variable1 = null; // no error.

Primark · Accepted Answer · 2012-02-27 14:02:36Z

1

String and Integer are both classes, in a way they are not native data types that is why it is always okay for you to set null as an initial value, however for int you must always initialize it with a number, one good way to find out their appropriate initialization value is to create variables outside your main(), example String var1; int var2; then use System.out.println(var1); System.out.println(var2); within the main() to see what was placed as an initial value when you run the program.

answered Feb 27, 2012 at 14:02

Primark

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2 Comments

M.A.Murali Over a year ago

thanks i get idea of it. what is wrapper class? what is the different between other class and wrapper class in java?

Primark Over a year ago

when you try to picture a wrapper class a simple analogy can be formed, native data type -> Class -> Wrapper Class, simply put a wrapper class is a class built over another class, or it is an expansion of a class built to serve another purpose

user647772 · Accepted Answer · 2012-02-27 13:56:36Z

1

int is a primitive, Integer is a class.

See http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

answered Feb 27, 2012 at 13:56

user647772

Comments

Juvanis · Accepted Answer · 2012-02-27 14:04:21Z

1

int is a primitive type, Integer is a wrapper class type extending Object class. Non-referencing objects can be null but primitives cannot. That's why you get an error message saying you need casting.

You can use a line like int num = (Integer) null;, this is how casting is done, however you will get NullPointerException when you try to use num anywhere in your code since a non-referencing(null) Integer object doesn't hold / wrap a primitive value.

edited Feb 27, 2012 at 14:04

answered Feb 27, 2012 at 13:56

Juvanis

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2 Comments

M.A.Murali Over a year ago

thanks. what is wrapper class? what is the different between other class and wrapper class in java?

Juvanis Over a year ago

Wrapper classes are used to represent primitive values (int, long, boolean, ..) when you need an object of them.

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