Can an int be null in Java?
For example:
int data = check(Node root);
if ( data == null ) {
// do something
} else {
// do something
}
My goal is to write a function which returns an int. Said int is stored in the height of a node, and if the node is not present, it will be null, and I'll need to check that.
I am doing this for homework but this specific part is not part of the homework, it just helps me get through what I am doing.
Thanks for the comments, but it seems very few people have actually read what's under the code, I was asking how else I can accomplish this goal; it was easy to figure out that it doesn't work.
int can't be null, but Integer can. You need to be careful when unboxing null Integers since this can cause a lot of confusion and head scratching!
e.g. this:
int a = object.getA(); // getA returns a null Integer
will give you a NullPointerException, despite object not being null!
To follow up on your question, if you want to indicate the absence of a value, I would investigate java.util.Optional<Integer>
No. Only object references can be null, not primitives.
A great way to find out:
public static void main(String args[]) {
int i = null;
}
Try to compile.
In Java, int is a primitive type and it is not considered an object. Only objects can have a null value. So the answer to your question is no, it can't be null. But it's not that simple, because there are objects that represent most primitive types.
The class Integer represents an int value, but it can hold a null value. Depending on your check method, you could be returning an int or an Integer.
This behavior is different from some more purely object oriented languages like Ruby, where even "primitive" things like ints are considered objects.
Along with all above answer i would like to add this point too.
For primitive types,we have fixed memory size i.e for int we have 4 bytes and char we have 2 bytes. And null is used only for objects because there memory size is not fixed.
So by default we have,
int a=0;
and not
int a=null;
Same with other primitive types and hence null is only used for objects and not for primitive types.
The code won't even compile. Only an fullworthy Object can be null, like Integer. Here's a basic example to show when you can test for null:
Integer data = check(Node root);
if ( data == null ) {
// do something
} else {
// do something
}
On the other hand, if check() is declared to return int, it can never be null and the whole if-else block is then superfluous.
int data = check(Node root);
// do something
Autoboxing problems doesn't apply here as well when check() is declared to return int. If it had returned Integer, then you may risk NullPointerException when assigning it to an int instead of Integer. Assigning it as an Integer and using the if-else block would then indeed have been mandatory.
To learn more about autoboxing, check this Sun guide.
instead of declaring as int i declare it as Integer i then we can do i=null;
Integer i;
i=null;
Integer object would be best. If you must use primitives you can use a value that does not exist in your use case. Negative height does not exist for people, so
public int getHeight(String name){
if(map.containsKey(name)){
return map.get(name);
}else{
return -1;
}
}
No, but int[] can be.
int[] hayhay = null; //: allowed (int[] is reference type)
int hayno = null; //: error (int is primitive type)
//: Message: incompatible types:
//: <null> cannot be converted to int
As #Glen mentioned in a comment, you basically have two ways around this:
use an "out of bound" value. For instance, if "data" can never be negative in normal use, return a negative value to indicate it's invalid.
Use an Integer. Just make sure the "check" method returns an Integer, and you assign it to an Integer not an int. Because if an "int" gets involved along the way, the automatic boxing and unboxing can cause problems.
Check for null in your check() method and return an invalid value such as -1 or zero if null. Then the check would be for that value rather than passing the null along. This would be a normal thing to do in old time 'C'.
Any Primitive data type like int,boolean, or float etc can't store the null(lateral),since java has provided Wrapper class for storing the same like int to Integer,boolean to Boolean.
Eg: Integer i=null;
An int is not null, it may be 0 if not initialized. If you want an integer to be able to be null, you need to use Integer instead of int . primitives don't have null value. default have for an int is 0.
Data Type / Default Value (for fields)
int ------------------ 0
long ---------------- 0L
float ---------------- 0.0f
double ------------- 0.0d
char --------------- '\u0000'
String --------------- null
boolean ------------ false
Since you ask for another way to accomplish your goal, I suggest you use a wrapper class:
new Integer(null);
I'm no expert, but I do believe that the null equivalent for an int is 0.
For example, if you make an int[], each slot contains 0 as opposed to null, unless you set it to something else.
In some situations, this may be of use.
String bob2 = "3";
System.out.println((int)bob2);
I'm unsure of why this causes an exception. Can anyone explain? Pretty sure because of the int on String type, but want to make sure.
Yes you are right its because of typecasting. If u need to convert String to int use below code
Integer.parseInt("3");
You are correct.
You can't just cast a string to an int.
You should convert it using Integer.parseInt()
Use this
Integer.valueOf("3");
or
Integer.parseInt("3");
In Java whenever you are trying to change type of an entity to another, both the types should have some relation. Like if you are trying to caste a sub class object to super class, it will work smoothly. But if you try to compare a Person object with a Lion object, that comparison is meaning less, the same is the logic in casting. We cannot cast a Person object to Lion object.
In your code bob is String type and you are trying to cast it to int and in Java both String and Integer is not having any relation. That's why Java is throwing Exception, Class Cast Exception I guess, this is raised when different types of objects are compared.
But the parseInt(String arg) method in Integer class gives an option to convert numeric String to Integer, given that the argument is a qualified Integer as per Java standards.
Example :-
String numericString = "1234";
int numberConverted = Integer.parseInt(numericString);
System.out.println(numberConverted);
You can also try these which will tell you the precautions before using this method
int numberConverted = Integer.parseInt("1234r");
int numberConverted = Integer.parseInt("1234.56");
int numberConverted = Integer.parseInt("11111111111111111111111111111");
You can't cast String to Integer. Change:
System.out.println((int)bob2);
to:
System.out.println(Integer.parseInt(bob2));
It will create an Integer value from the String provided with bob2 variable. You can also create a reference to int variable like this if you want to store primitive int instead of Integer:
int intBob2 = Integer.parseInt(bob2);
Even though I have done the exact same thing with an integer array, I seem to be getting an 'Cannot cast from object to int' error while passing the integer from a servlet to a jsp
In my JavaBean
public int getNoOfVotes(){
return noOfVotes;
}
In my servlet
int noOfVotes = bean.getNoOfVotes();
request.setAttribute("totalVotes", noOfVotes);
In my jsp
int votes = (int)request.getAttribute("totalVotes");
Its in the jsp I am getting the error
request.getAttribute returns Object. You cannot cast Object to a primitive type int. But you may cast it to Integer.
reality，the obj couldn't cast to int,there are two method to cast.
the one is cast to String,then use Integer.valueof();
the other one is Integer.parsetoInt()
Try using this
int no = Integer.parseInt(request.getAttribute("totalVotes"));
I hope this helps.
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.
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.
int is a primitive, Integer is a class.
See http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
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.
With autounboxing, this statement will automatically work:
int myPrimitive = (Integer) doIt();
But if I want to explicitly convert from an Integer to an int here in a single line, where do I have to put the parentheses?
You could do this :
int myPrimitive = (int) (Integer) doIt();
But as you said, auto-unboxing will get that for you.
A bad example to show that chain casts work (don't ever use this code) :
Map notReallyAMap = (Map) (Object) new String();
The thing with chain casts, is that wherever you use it, either the cast is legit, and you can remove intermediaries; or the cast will simply cause a ClassCastException. So you should never use it.
Either the compiler unboxes the Integer for you, or you do it yourself - this cannot be avoided.
So you need to either do
int myPrimitive = ((Integer) doIt()).intValue();
or more simply, change doIt() to return an int since you seem to want to deal with ints rather than (null-able) Integers.