For example if you have an integer:
int i = 9;
How can it do that? I mean the full syntax is:
int i = new Integer(9);
How does it skip the whole new Integer() part and still work?
Thanks.
It doesn't skip it, because primitives aren't objects.
Your second line of code involves auto-unboxing, which was a later addition to the Java language.
new Integer() is not a primitive; it's a boxed primitive.
Actual primitives (int, etc) are not objects and cannot be instantiated.
Note that you can also write Integer x = 9, and the Java compiler will implicitly insert new Integer().
This is called auto-boxing.
Maybe you wanna have a look into AutoBoxing
Primitive : Reference Mapping
byte : Byte
short : Short
int : Integer
long : Long
float : Float
double : Double
bool : Boolean
char : Character
Autoboxing / unboxing is the automated under the covers conversion
between primitive types and their equivalent object types. For
example, the conversion between an int primitive and an Integer object
or between a boolean primitive and a Boolean object. This was
introduced in Java 5.
Primitives and object are two different things. Without primitive you wouldn't be able to create Integer object like new Integer(9) (Integer uses primitive 9 inside constructor).
Your question would have made a lot more sense if it asked why
Integer i = 9;
works without new and then the answer would be "due to auto-boxing of primitives introduced in Java 5". So maybe that's what you really wanted to ask :)
Related
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What is the difference between an int and an Integer in Java and C#?
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I have seen many times in code that people use int or Integer to declare variable in beans. I know int is datatype and Integer is wrapper class.
My question is, in which condition int or Integer should be used and is there any advantage of either of them over another?
My question is, in which condition int or Integer should be used and is there any advantage of either of them over another?
Well, you should use the reference type Integer whenever you have to. Integer is nothing more than a boxed int. An Object with a single field containing the specified int value.
Consider this example
public class Ex {
int field1;
Integer field2;
public Ex(){}
}
In this case field1 will be initialized with the value 0 while field2 will be initialized with null. Depending on what the fields represent, both approaches might be advantageous. If field1 represents some kind of UUID, would you want it to be initialized with a zero value?
I wouldn't worry too much about the performance implications of Autoboxing. You can still optimize after you get your code running.
For more information take a look at the documentation
https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Integer.html
You always use int, pretty much.
Integer should rarely be used; it is an intermediate type that the compiler takes care of for you. The one place where Integer is likely to appear is in generics, as int is simply not legal there. Here is an example:
List<Integer> indices = new ArrayList<Integer>();
int v = 10;
indices.add(v);
The above works: It compiles with no errors and does what you think it would (it adds '10' to a list of integer values).
Note that v is of type int, not Integer. That's the correct usage; you could write Integer here and the code works as well, but it wouldn't be particularly idiomatic java. Integer has no advantages over int, only disadvantages; the only time you'd use it, is if int is straight up illegal. Which is why I wrote List<Integer> and not List<int> as the latter is not legal java code (yet - give it a few versions and it may well be legal then, see Project Valhalla).
Note also that the compiler is silently converting your v here; if you look at the compiled code it is as if javac compiled indices.add(Integer.valueOf(v)) here. But that's fine, let the compiler do its thing. As a rule what the compiler emits and what hotspot optimizes are aligned; trust that what javac emits will be relatively efficient given the situation.
int is a primitive type, a value type for number literals.
it is used whenever and wherever you just want to do some basic arithmetical operation;
it is a value type, so it's stored in the Stack area of the memory, hence operations on it are much faster;
whenever it's needed, compiler implicitly and automatically casts back and forth (a.k.a Boxing and Unboxing) from int to Integer and vice versa;
Integer is a Class, which is a reference type and you instantiate an Object of that type.
you create an object of that class, which means, that you also have some methods and operations on that object;
any time you do some arithmetic operation on the instance of Integer, under the hood, it's still implemented by int primitives, and it's just wrapped into box/container;
it is a reference type / object, which is very important, as you can Serialize or Deserialize it;
it also has some very useful utility factory methods, like Integer.valueOf(..) for example, to parse the String as an integer;
it can be well used into declarations of the generic types and it, as a class, supports the hierarchy as well. For instance, it extends Number, and you can make use of this;
it is stored in the Heap area of the memory.
int is a primitive and Integer is an object .
From an memory footprint point of view , primitive consume less memory than object and also primitives are immutable (since they use pass by value ) .
Here is a good article on when to use what :
https://www.baeldung.com/java-primitives-vs-objects
I read this post: Is int an object in Java?.
In the post it is argued that int is not inherited from Object. If so is the case, then why does the code below compile without any error? Given that int is not Object and the signature of format() method is public static String format(String format, Object... args) as shown in documentation: javadoc for String!
public class Testing {
public static void main(String[] args) {
int integer = 7;
String str = String.format("%03d", integer);
System.out.println(str);
}
}
I have also read about "Autoboxing". What does this exactly mean? Are all the primitives replaced by appropriate Object's before compilation? If so, then is there any advantage of memory usage while using large array of int's (int[]) over Integer's (Integer[])? Similar arguments follow for double's etc.
Any insights are welcome.
It is caused by Autoboxing.
Here is a small snippet from the linked Java documentation that explains it better than I could:
Autoboxing is the automatic conversion that the Java compiler makes
between the primitive types and their corresponding object wrapper
classes. For example, converting an int to an Integer, a double to a
Double, and so on. If the conversion goes the other way, this is
called unboxing.
Here is the simplest example of autoboxing:
Character ch = 'a';
The rest of the examples in this section use generics. If you are not
yet familiar with the syntax of generics, see the Generics (Updated)
lesson.
Consider the following code:
List<Integer> li = new ArrayList<>();
for (int i = 1; i < 50; i += 2)
li.add(i);
Although you add the int values as primitive types, rather than
Integer objects, to li, the code compiles. Because li is a list of
Integer objects, not a list of int values, you may wonder why the Java
compiler does not issue a compile-time error. The compiler does not
generate an error because it creates an Integer object from i and adds
the object to li. Thus, the compiler converts the previous code to the
following at runtime:
List<Integer> li = new ArrayList<>();
for (int i = 1; i < 50; i += 2)
li.add(Integer.valueOf(i));
When calling String.format("%d",myInt), myInt is automatically (and implicitly) wrapped in an Integer instance, which extends Object, therefore it compiles.
Concerning the arrays, the conversion from primitiveType[] to WrapperClass[] is not automatic for some reason. If you try to use an array of a primitive type where an array of the wrapper class is expected, it will result in a compile error.
Using Integer creates an overhead compared to using int because you need to assign and store references. However, this overhead is limited when using Integer values between -128 and 127 because these values are pooled (which means that all instances of Integer wrapping a value in this in interval point to a unic reference).
Autoboxing is a help from the compiler, which automatically compiles something like
foo(i);
into
foo(Integer.valueOf(i));
when foo() takes an argument of type Object and you pass it a primitive type (int, in this case). It just makes the code easier to type and read.
And that's what happens here. The String.format() method expects objects as argument. You're passing it a primitive type, so the compiler autoboxes it to an Integer for you.
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How does auto boxing/unboxing work in Java?
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This question is in continuation to a question How can a string be initialized using " "?
I would like to raise your attentation that even Integer, Double, Character, Float, Boolean wrapper class can also be declared in the same way String is declared like:
String s = "Test string"
Integer i = 10; //valid
Double d = 10.00; //valid
Boolean b = true; //valid
Does these class are also given special treatment like the String class.
As I pointed out In my previous answer(How can a string be initialized using " "?)
Yes, to retain primitive types in an OOP, designers made bridge between primitives and Object's with Wrappers and they have a special treatment.
The reason is clearly explained in docs.
There are, however, reasons to use objects in place of primitives, and the Java platform provides wrapper classes for each of the primitive data types. These classes "wrap" the primitive in an object. Often, the wrapping is done by the compiler—if you use a primitive where an object is expected, the compiler boxes the primitive in its wrapper class for you. Similarly, if you use a number object when a primitive is expected, the compiler unboxes the object for you. For more information, see Autoboxing and Unboxing
We use primitives extensively in our programs, So it might be a design decision to allowing syntax like
Integer i = 10; //primitive style
Then memory allocates at compile time itself for i since it is a primitive type, when they found with Wrapper type declarations with an Assignment operator =
Syntax wise ,that is more handy and happy(at least for me :)).
Than writing,
Integer i = new Integer(10); //Object creation style
All these following statements:
Integer i = 10; //valid
Double d = 10.00; //valid
Boolean b = true; //valid
are valid because of autoboxing
Autoboxing is the automatic conversion that the Java compiler makes
between the primitive types and their corresponding object wrapper
classes
Yes primitive Wrapper classes also behave like String class.
You can illustrate like below
Integer i1 = new Integer(10); //valid
Integer i2 =10;
System.out.println(i1==i2); // this one is false
i1=10;
System.out.println(i1==i2); //// this one is true
All wrapper classes of primitive types behave this way. It is called autoboxing and was introduced in java 1.5:
http://docs.oracle.com/javase/tutorial/java/data/autoboxing.html
String str = "Test String";
having a special treatment is irrespective to and the Integer i = 10;
what is the special treatment String s = "Test String";?
these are called String literals gets the memory in String constant pool of jvm.
the one special significance is in terms of Garbage Collection is, the pooled constants are
never be influenced by garbage collection.
Making 'str' as null does not make "Test String" is eligible for garbage collection.
WHY?:: JVM will try to reuse this "Test String" in future. The garbage collection algorithm excludes the objects which are in pooled memory. so normal GC rules won't apply here. check this out: why String literals are not garbage collected
Now how this treatment is quite different to wrapper's auto boxing. automatic boxing is introduced from JDK1.5. auto boxing & auto unboxing
When Integer i = 10; the compiler replaces this statement with Integer i = Integer.valueOf(10);
only the internal cache wrapper objects of JVM are acts like String literals remaining are not. now what are internal cache wrapper objects?
Integer i = 100;
Integer j = 100;
references i & j are given with a single pre existed object's address.
that is why if( i==j) //true
Integer k = 130;
if( i==k) // false
because k's value is beyond the cache range which is -128 to 127 for Integer.
check this:Integer wrapper behaviour when value range is -128 to 127
In the above if we nullify the reference k then its object undergoes for the GC which is not same treatment like String literals.
If we nullify the i or j then corresponding cached object never be
influenced by GC which is same treatment like String literals.
Given that String s has been declared, the following 5 lines of code produce the same result:
int i = Integer.valueOf(s);
int y = Integer.parseInt(s);
int j = Integer.valueOf(s).intValue();
Integer x = Integer.valueOf(s);
Integer k = Integer.valueOf(s).intValue();
Are there circumstances where each one would be the preferred code? It appears that int and Integer are interchangeable and that .intValue() is not needed.
If you require an int, use parseInt(), if you require an Integer use valueOf(). Although they're (sort of) interchangeable now, it still makes more sense to use the one that directly returns the data type that you require. (Historically, they weren't interchangeable at all, this was introduced with auto-boxing and unboxing in Java 5.)
The intValue() method you're using is just converting the Integer class type to the int primitive, so using that and valueOf() is the worst possible combination, you never want to use that. (Nothing bad will happen, it's just longer to read, performs slightly worse and is generally more superfluous.)
If you don't care or don't know, then I'd use parseInt(). Especially as a beginner, it's more common that you want the primitive type rather than the class type.
int and Integer are made to look interchangeable by the magic of auto-boxing and auto-unboxing: In many cases where you need one but have the other, the compiler automagically inserts the necessary code to convert them.
This is useful, but if you know about it, you can avoid it in many places which results in slightly faster code (because there's less conversion to do).
Integer.parseInt() returns an int, so you should use it if you need an int
Integer.valueOf() returns an Integer, so you should use it if you need an Integer
Integer.valueOf().intValue() first creates an Integer and then extracts the int value from it. There's no good reason to use this instead of a simple Integer.parseInt().
The decision between int and Integer is easy to do as well:
generally you'd want to use the primitive type (int), if possible
if you need an Object (for example, if you want to put your number in a Collection), then you need to use the wrapper type (Integer), as the primitive type can't be used here.
int and Integer are not interchangeable.Because of Autoboxing feature fron java 5 onwards, int to Integer conversion is taken care by jvm itself.But we should not use Integer class unnecessarily.Primitive data types are always faster.Wrapper classes should be used only required.
I am confused as to why Integer and int can be used interchangeably in Java even though one is a primitive type and the other is an object?
For example:
Integer b = 42;
int a = b;
Or
int d = 12;
Integer c = d;
The first few sentences of the posted article describe it pretty well:
You can’t put an int (or other primitive value) into a collection. Collections can only hold object references, so you have to box primitive values into the appropriate wrapper class (which is Integer in the case of int). When you take the object out of the collection, you get the Integer that you put in; if you need an int, you must unbox the Integer using the intValue method. All of this boxing and unboxing is a pain, and clutters up your code. The autoboxing and unboxing feature automates the process, eliminating the pain and the clutter.
That is basically it in a nutshell. It allows you take advantage of the Collections Framework for primatives without having to do the work yourself.
The primary disadvantage is that it confuses new programmers, and can lead to messy/confusing code if it's not understood and used correctly.
Java supports autoboxing and automatically wraps primitive values into objects and unwraps objects to primitive values for certain types, like char - Character, int - Integer, double - Double, etc.
Note from Oracle Documentation:
So when should you use autoboxing and unboxing? Use them only when there is an “impedance mismatch” between reference types and primitives, for example, when you have to put numerical values into a collection. It is not appropriate to use autoboxing and unboxing for scientific computing, or other performance-sensitive numerical code. An Integer is not a substitute for an int; autoboxing and unboxing blur the distinction between primitive types and reference types, but they do not eliminate it.
Because of autoboxing and autounboxing http://download.oracle.com/javase/1.5.0/docs/guide/language/autoboxing.html
Using an int and an Integer "interchangeably" is called autoboxing. This feature was introduced in Java 5. Before that, your example code wouldn't have compiled. Instead, you would have to write something like this:
Integer b = Integer.valueOf(42); // or new Integer(42);
int a = b.intValue();
or
int d = 12;
Integer c = Integer.valueOf(d); // or new Integer(d);
That's fairly verbose, which is why autoboxing was introduced. It's a bit of compiler magic to make life easier for the coder.
Technically, int and Integer themselves are not interchangeable and one cannot be used where the other is required. However, autoboxing allows implicit conversion between the two.
As a side note, there is one case where autoboxing (specifically unboxing) fails. If your code tries to autounbox a null value, you will get a NullPointerException at runtime, e.g.:
Integer b = null;
int a = b; // NullPointerException here!
Just something to be aware of...
It's called AutoBoxing. That will explain exactly what it is.
In addition to other answers, because Integer is a wrapper class, that lets you box and unbox an int value. Other info here.
The java language specification states that the java virtual machine must perform automatic boxing/unboxing for integers and a few other types.