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Why are there wrapper classes in Java? [duplicate]
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I know what a wrapper class is, they wrap primitive types (e.g. int, double, etc) to objects of their respective class.
But I want to know with a Java Code Example which can explain me practically.
With wrapper class and with out wrapper class what it will do.
When the Java language was "invented" people thought that having primitive types int, long, ... would avoid performance issues. 15+ years back, there were no efficient JIT compilers; so it made a huge difference if you had to create an array for 10000 ints ... or for 10 000 Integer objects.
On the other hand, Java wants to be a object-oriented language. Therefore those primitive types came with accompanying classes. You are correct in the sense: if there would be no primitive tpyes, just Integer, Long, ... and so on, the Java type system would be more consistent, much simpler ... but back in 1997 this simple type system would have been to "expensive" regarding performance. And unfortunately people didn't think about allowing primitive types in bytecode ... but not having them in the Java language itself (and the compiler converting Integer to int internally).
The main usage nowadays is the fact that the Java compiler does autoboxing (automated conversion between primitive and "object based" types); so you can write stuff like:
Map<Integer, String> someMap = ...
someMap.put(5, "string")
A simple one line answer could be that you can have ArrayList<Integer> but you can not an ArrayList<int>. They are used for collections, polymorphism etc. So I think Java designers wanted to make things simple and thats why they used the concept of wrapper class.
Java is an object-oriented language and as said everything in java is
an object. But what about the primitives? They are sort of left out in
the world of objects, that is, they cannot participate in the object
activities, such as being returned from a method as an object, and
being added to a Collection of objects, etc. . As a solution to this
problem, Java allows you to include the primitives in the family of
objects by using what are called wrapper classes.
Wrapper classes are used to convert any data type into an object. The primitive data types are not objects; they do not belong to any class; they are defined in the language itself. Sometimes, it is required to convert data types into objects in Java language
What is a Wrapper class?
A wrapper class wraps or encloses a data type and gives it an appearance of an object. You can also get the primitive datatype from the object.
Observe the following example.
int x = 100;
Integer iObj = new Integer(x);
The int data type (x) is converted into an object (iObj) with the help of an Integer class. This can be used whever an object is required.
The following code can be used to unwrap the object iObj and obtain the primitive datatype.
int y = iObj.intValue();
System.out.println(y); // prints 100
intValue() is a method of Integer class that returns an int data type.
Why Wrapper classes?
To convert primitive data types into objects and vice versa.
Wrapper types exists in Java primarily for two reasons:
to allow type conversion in an object-oriented fashion
all Collection types used generic java Objects, so if you need a List of, say floating point numbers, you can't use simple float numbers, because they are not objects. Also, they don't inherit from java.lang.Object
So you can have an ArrayList<Integer> because Integer extends from Object (as every other object inside the JVM). But you can't have an ArrayList because int is not an object type: it's just a simple int variable, without methods, instance variables, etc.
Since Java 1.5 you have automatic boxing and unboxing of primitive types into their correspondent wrapper types, which makes everything a little bit more confusing. Consider this listing:
List list = new ArrayList<Integer>()
list.add(new Integer(10))
list.add(20)
Here you see that this list only uses Integer objects. In the last line the compiler is putting the simple value 20 inside a new Integer object to save you some typing. This is a little syntax sugar but the idea remains: list can only use Integer objects.
Java does not allow primitive types to be used in generic data structures. E.g. ArrayList<int> is not allowed. The reason is, primitive types can not be directly converted to Object. However Java 1.5 does support auto-boxing, and wrapper classes work in generic data structures. So why couldn't the compiler auto-box it to ArrayList<Integer>? Are there any other reasons for why this can not work?
So as far as I understand it, your proposed ArrayList<int> would be identical to ArrayList<Integer>. Is that right? (In other words, internally it still stores an Integer; and every time you put something in or get it out, it would automatically box/unbox it, but autoboxing/autounboxing already does that for ArrayList<Integer>.)
If it is the same, then I don't understand what the utility of having a duplicate syntax <int> is when it means the same thing as <Integer>. (In fact it will introduce additional problems, because for example int[] is not the same runtime type as Integer[], so if you have T[], and T is int, what would it mean?)
The generic type information is erased at run time. Check this link. Generics have more to do with compile time checking than run time checking. The autoboxing and unboxing are the run time operations. See the link. This is the reason that autoboxing should not work with Generics.
The problem will be in performance. For every get()/set() method, in the list, the JVM will have to unbox/box the respective value for the mentioned method respectively. Remember, autoboxing take primitive types and wraps them into an Object and vice-versa, as stated on Autoboxing:
Finally, there are performance costs associated with boxing and
unboxing, even if it is done automatically.
I think they wanted a List to do simple operation and alleviating performance all together.
I don't think there's any technical reason it couldn't be done like you say, but there are always interface considerations: e.g., if you automatically converted objects of type ArrayList<int> to be ArrayList<Integer>, you lose some explicitness in terms of the interface specifications: it is less obvious that ArrayList in fact store objects, not primitives.
My understanding is that autoboxing is more for compatibility and flexibility in parameter types than for the ease of being able to say "int" instead of "Integer." Java's not exactly known for it's obsession with conciseness...
A small P.S.: I don't think it would technically be correct to say "autobox ArrayLint<int> to ArrayList<Integer>," because you aren't actually wrapping anything in an object "box" -- you're just actually converting a typename ArrayList<int> to "actual" type ArrayList<Integer>
I'm glad it is impossible, because int use much less memory than Integer and is much faster too. Therefore it forces me to think whether it is acceptable to use Collection<Integer> or not (lot of times in business application it's ok, but in other apps it is not).
I would be much happier if Collection<int> was possible and efficient, but it is not.
I don't think this is any sort of problem - do you have any concrete case where is this limiting you somehow? And btw there is difference between int and Integer while the object can be null and primitive type can't.
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Possible Duplicate:
storing primitive values in a java collection?
My Java textbook says elements of a collection, for example ArrayList, cannot be primitive types. Is there a reason for this? I mean did someone at Sun decide on this or is there some barrier against doing it? I understand my example half-answers my question since ArrayList requires an object and primitives are not objects. But then I think why can't they have primitive types as well?
is there some barrier against doing
it?
You could write near-identical versions of ArrayList that were tailor made to store one of the non-class types, e.g. IntegerArrayList and so on. The barrier against this is that there would be an explosion of such classes, as you'd multiply the number of primitive types by the number of collection types. In order to keep the standard collection framework manageable, this was ruled out.
To solve this more neatly in the language, you'd need generics to allow primitive types to serve as type parameters, and improve the interaction between arrays and generics.
Storing unwrapped primitives would dramatically complicate the collections code. Whereas, with the wrappers (Integer for int, etc.), the code is fairly straight-forward. For several years now, Java has supported "auto-boxing", which means that if you give an int where an Integer is expected, the int is wrapped up in an Integer instance for you (and vice-versa).
There are objects called "wrappers" that represent all of the primitive types. For example, there is a class called Integer that supports int. You can use the primitive wrappers to hold values in a Collection.
The problem with primitive types (at least until Java 5) is that they didn't extend from the base Object class. All of the collections need to specify a class for all the methods they are using - and they specify Object, since Object is the base of all the classes.
As of Java 5, you will find that Java will implicitly switch between a primitive and it's corresponding wrapper class when you need it. This means you can add an int, or a double, etc. to a Collection. The VM will automatically wrap the primitive in a wrapper class for you and place the wrapper in the Collection.
Currently the only way to store primtives directly into a collection, is to have a collection for each primitive type e.g. TIntArrayList.
You are likely to find that even though ArrayList is slower than using primitives, it is fast enough for 90+% of use cases.
Read this article on wikipedia. It might help: http://en.wikipedia.org/wiki/Object_type_(object-oriented_programming)#Autoboxing
In computer science, an object type
(a.k.a. wrapping object) is a datatype
which is used in object-oriented
programming to wrap a non-object type
to make it look like a dynamic object.
Some object-oriented programming
languages make a distinction between
reference and value types, often
referred to as objects and non-objects
on platforms where complex value types
don't exist, for reasons such as
runtime efficiency and syntax or
semantic issues. For example, Java has
primitive wrapper classes
corresponding to each primitive type:
Integer and int, Character and char,
Float and float, etc. Languages like
C++ have little or no notion of
reference type; thus, the use of
object type is of little interest.
Boxing is the process of placing a
primitive type within an object so
that the primitive can be used as a
reference object. For example, lists
may have certain methods which arrays
might not, but the list might also
require that all of its members be
dynamic objects. In this case, the
added functionality of the list might
be unavailable to a simple array of
numbers. For a more concrete example,
in Java, a LinkedList can change its
size, but an array must have a fixed
size. One might desire to have a
LinkedList of ints, but the LinkedList
class only lists references to dynamic
objects — it cannot list primitive
types, which are value types.
To circumvent this, ints can be boxed
into Integers, which are dynamic
objects, and then added to a
LinkedList of Integers. (Using generic
parameterized types introduced in J2SE
5.0, this type is represented as LinkedList.) On the other
hand, C# has no primitive wrapper
classes, but allows boxing of any
value type, returning a generic Object
reference.
The boxed object is always a copy of
the value object, and is usually
immutable. Unboxing the object also
returns a copy of the stored value.
Note that repeated boxing and unboxing
of objects can have a severe
performance impact, since it
dynamically allocates new objects and
then makes them eligible for Garbage
collection.
Performance issue is one of the problems since we need auto-boxing for this to be achieved. Also some of the structures may benefit from having null values also.
On the very high level, I know that we need to "wrap" the primitive data types, such as int and char, by using their respective wrapper classes to use them within Java collections.I would like to understand how Java collections work at the low level by asking:"why do we need to wrap primitive data types as objects to be able to use them in collections?"I thank you in advance for your help.
Because Java collections can only store Object References (so you need to box primitives to store them in collections).
Read this short article on Autoboxing for more info.
If you want the nitty gritty details, it pretty much boils down to the following:
Local Primitives are stored on the Stack. Collections store their values via a reference to an Object's memory location in the Heap. To get that reference for a local primitive, you have to box (take the value on the Stack and wrap it for storage on the Heap) the value.
At the virtual machine level, it's because primitive types are represented very differently in memory compared to reference types like java.lang.Object and its derived types. Primitive int in Java for example is just 4 bytes in memory, whereas an Object takes up at minimum 8 bytes by itself, plus another 4 bytes for referencing it. Such design is a simple reflection of the fact that CPUs can treat primitive types much more efficiently.
So one answer to your question "why wrapper types are needed" is because of performance improvement that it enables.
But for programmers, such distinction adds some undesirable cognitive overhead (e.g., can't use int and float in collections.) In fact, it's quite possible to do a language design by hiding that distinction --- many scripting languages do this, and CLR does that. Starting 1.5, Java does that, too. This is achieved by letting the compiler silently insert necessary conversion between primitive representation and Object representation (which is commonly referred to as boxing/unboxing.)
So another answer to your question is, "no, we don't need it", because the compiler does that automatically for you, and to certain extent you can forget what's going on behind the scene.
Read all of the answers, but none of them really explains it simply in layman terms.
A wrapper class wraps(encloses) around a data type (can be any primitive data type such as int, char, byte, long) and makes it an object.
Here are a few reasons why wrapper classes are needed:
Allows null values.
Can be used in collection such as List, Map, etc.
Can be used in methods which accepts arguments of Object type.
Can be created like Objects using new ClassName() like other objects:
Integer wrapperInt = new Integer("10");
Makes available all the functions that Object class has such as clone(), equals(), hashCode(), toString() etc.
Wrapper classes can be created in two ways:
Using constructor:
Integer i = new Integer("1"); //new object is created
Using valueOf() static method:
Integer i = Integer.valueOf("100"); //100 is stored in variable
It is advised to use the second way of creating wrapper classes as it takes less memory as a new object is not created.
To store the Primitive type values in Collection. We require Wrapper classes.
Primitive data types can't be referenced as memory addresses. That's why we need wrappers which serve as placeholders for primitive values. These values then can be mutated and accessed, reorganized, sorted or randomized.
Collection uses Generics as the bases. The Collection Framework is designed to collect, store and manipulate the data of any class. So it uses generic type. By using Generics it is capable of storing the data of ANY CLASS whose name you specify in its declaration.
Now we have various scenario in which want to store the primitive data in the same manner in which the collection works. We have no way to store primitive data using Collection classes like ArrayList, HashSet etc because Collection classes can store objects only. So for storing primitive types in Collection we are provided with wrapper classes.
Edit:
Another benefit of having wrapper classes is that absence of an object can be treated as "no data". In case of primitive, you will always have a value.
Say we have method signature as
public void foo(String aString, int aNumber)
you can't make aNumber as optional in above method signature.
But if you make signature like:
public void foo(String aString, Integer aNumber)
you have now made aNumber as optional since user can pass null as a value.
See Boxing and unboxing: when does it come up?
It's for C#, but the same concept apply to Java. And John Skeet wrote the answer.
Well, the reason is because Java collections doesn't differentiate between primitive and Object. It processes them all as Object and therefore, it will need a wrapper. You can easily build your own collection class that doesn't need wrapper, but at the end, you will have to build one for each type char, int, float, double, etc multiply by the types of the collections (Set, Map, List, + their implementation).
Can you imagine how boring that is?
And the fact is, the performance it brings by using no wrapper is almost negligible for most applications. Yet if you need very high performance, some libraries for primitive collections are also available (e.g. http://www.joda.org/joda-primitives/)
Wrapper classes provide useful methods related to corresponding data types which you can make use of in certain cases.
One simple example. Consider this,
Integer x=new Integer(10);
//to get the byte value of 10
x.byteValue();
//but you can't do this,
int x=10;
x.byteValue(); //Wrong!
can you get the point?
If a variable is known to either hold a specific bit pattern representing null or else information which can be used to locate a Java Virtual Machine object header, and if the method for reading an object header given a reference will inherently trap if given the bit pattern associated with null, then the JVM can access the object identified by the variable on the assumption that there is one. If a variable could hold something which wasn't a valid reference but wasn't the specific null bit pattern, any code which tried to use that variable would have to first check whether it identified an object. That would greatly slow down the JVM.
If Object derived from Anything, and class objects derived from Object, but primitives inherited from a different class derived from Anything, then in a 64-bit implementation it might be practical to say that about 3/4 of the possible bit patterns would represent double values below 2^512, 1/8 of them to represent long values in the range +/- 1,152,921,504,606,846,975, a few billion to represent any possible value of any other primitve, and the 1/256 to identify objects. Many kinds of operations on things of type Anything would be slower than with type Object, but such operations would not be terribly frequent; most code would end up casting Anything to some more specific type before trying to work with it; the actual type stored in the Anything would need to be checked before the cast, but not after the cast was performed. Absent a distinction between a variable holding a reference to a heap type, however, versus one holding "anything", there would be no way to avoid having the overhead extend considerably further than it otherwise would or should.
Much like the String class, Wrappers provide added functionality and enable the programmer to do a bit more with the process of data storage. So in the same way people use the String class like....
String uglyString = "fUbAr";
String myStr = uglyString.toLower();
so too, they can with the Wrapper. Similar idea.
This is in addition to the typing issue of collections/generics mentioned above by Bharat.
because int does not belongs any class .
we convert datatype(int) to object(Interger)
I am trying to create a nullalble object in Java but no idea how to do this , in C# this would be done like this
int? someTestInt;
This allows me to check for for null , while in certain cases i can use a 0 value ,this isnt always possible since certain execution paths allow 0 values
I'm not entirely sure what you want, but if you want to have an integer value that also can be declared null, you probably want to use the Integer class:
Integer nullableInteger = 1;
nullableInteger = null;
System.out.println(nullableInteger); // "null"
There are corresponding classes for each primitive: Character, Long, Double, Byte, etc. The 'standard library' numeric classes all extend the Number class.
Note that Java autoboxes these objects automatically since JDK 1.5, so you can use and declare them just like the primitives (no need for e.g. "new Integer(1)"). So, although they are technically objects (and, therefore, extend the Object class, which the primitive int type does not), you can do basic arithmetics with them. They are converted to object operations at compile time.
Java does not support nullable primitives. You can use the Integer type if you want the ability to store nulls.
(This is a duplicate of this post:
How to present the nullable primitive type int in Java?)
Perhaps try the Integer type in Java.