Java: Why are wrapper classes needed?

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 c开发者_开发问答lasses 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:

1. Allows null values.

2. Can be used in collection such as List, Map, etc.

3. Can be used in methods which accepts arguments of Object type.

4. Can be created like Objects using new ClassName() like other objects:

   Integer wrapperInt = new Integer("10");
   

5. Makes available all the functions that Object class has such as clone(), equals(), hashCode(), toString() etc.

Wrapper classes can be created in two ways:

1. Using constructor:

   Integer i = new Integer("1"); //new object is created
   

2. 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)