I had a function in Java with method signature
public void myMethod (int someInt, String someString)
in my abstract class and I had over-ridden it with method
public void myMethod (Integer someInt, String someString)
The over ride does not work. Is this an inconsistency ? I thought auto-boxing applied to method signature over-ride as well.
int and Integer are two different types. Autoboxing blurs the distinction at the source code level for the convenience of programmers, but does not change the fact that they are in fact two very different types.
As such, you can not #Override a method that takes an int with one that takes an Integer and vice versa.
Note that you should probably think twice before declaring a method to take an Integer instead of an int. Here's an excerpt from Effective Java 2nd Edition, Item 49: Prefer primitives to boxed primitives:
In summary, use primitives in preference to boxed primitive whenever you have the choice. Primitive types are simpler and faster. If you must use boxed primitives, be careful! Autoboxing reduces the verbosity, but not the danger, of using boxed primitives. When your program compares two boxed primitives with the == operator, it does an identity comparison, which is almost certainly not what you want. When your program does mixed-type computations involving boxed and unboxed primitives, it does unboxing, and when your program does unboxing, it can throw NullPointerException. Finally, when your program boxes primitive values, it can result in costly and unnecessary object creations.
There are places where you have no choice but to use boxed primitives, e.g. generics, but otherwise you should seriously consider if a decision to use boxed primitives is justified.
See also
Java Language Guide/Autoboxing
Related questions
What is the difference between an int and an Integer in Java/C#?
Is it guaranteed that new Integer(i) == i in Java? (YES!!! The box is unboxed, not other way around!)
Why does int num = Integer.getInteger("123") throw NullPointerException? (!!!)
Why null == 0 throws NullPointerException in Java?
No, these two signatures define two different methods.
They are absolutely not overridden but overload since the parameters are different. And JVM will choose the method to launch base on this:
widen - boxing - var args...
For example, you have three methods
void add(long n) {} // call this method 1
void add(int... n) {} // call this method 2
void add(Integer n) {} // call this method 3
and when you invoke:
int a = 5;
add(a);
the method 1 will be invoked.
The reason the override doesn't work is because Integerand int are two different things. Integer is an object, whereas int is a primitive type. Java does the implicit typecasting for you. For example:
int myInteger = new Integer(5);
Will create a primitive int type called myInteger with a value of 5. As the Javadoc says,
"The Integer class wraps a value of
the primitive type int in an
object."
You are correct that this scenario will not work because Java provides the functionality of the autoboxing, so at runtime JVM can not decide which method to call because it can call both method as both method fits for the argument type. So I think it will give an error or will choose either method randomly..... Just run it and see.....
int and Integer are two different types in JAVA.Although autoboxing specifies the distinction at the source code level, but does not change the eternal fact that they are in fact two very different types.
Related
What is the difference between Integer.class, Integer.TYPE and int.class?
acc to me
Integer.class is a reference of Integer (Wrapper) Class object
but what is then int.class as int is not a class, it's a primitive type. And what does Integer.TYPE refer to?
Integer.class is, as you say, a reference to the Class object for the Integer type.
int.class is, similarity, a reference to the Class object for the int type. You're right that this doesn't sound right; the primitives all have a Class object as a special case. It's useful for reflection, if you want to tell the difference between foo(Integer value) and foo(int value).
Integer.TYPE (not Integer.type, mind you) is just a shortcut for int.class.
You can get a sense of this with a simple program:
public class IntClasses {
public static void main(String[] args) {
Class<Integer> a = int.class;
Class<Integer> b = Integer.TYPE;
Class<Integer> c = Integer.class;
System.out.println(System.identityHashCode(a));
System.out.println(System.identityHashCode(b));
System.out.println(System.identityHashCode(c));
}
}
Example output (it'll be different each time, but the first two will always be the same, and the third will virtually always be different):
366712642
366712642
1829164700
From java.lang.Class.isPrimitive API
There are nine predefined Class objects to represent the eight primitive types and void. These are created by the Java Virtual Machine, and have the same names as the primitive types that they represent, namely boolean, byte, char, short, int, long, float, and double.
These objects may only be accessed via the following public static final variables java.lang.Boolean.TYPE, java.lang.Integer.TYPE etc
Java handles primitive types versus class types in a schizophrenic way by defining two types for each primitive.
For instance int is the primitive type and Integer the class type. When you use generics, you are forced to use a non-primitive type so ArrayList<Integer> is allowed but ArrayList<int> not.
Since you sometimes want to perform reflection, this duality results in two classes (how else can you inspect a method public int foo ();).
Say you have a class:
public class Foo {
private Integer value;
public int value1 () {
return value;
}
public Integer value2 () {
return value;
}
}
The two methods will not always return the same value, since value2() can return null and value1() will throw a runtime error.
In plain terms :
int -- > Are primitives..for simple math operations. You cannot add them
to a collection.
Integer --> Are objects in themselves.. are wrappers to ints. i.e,
they can be used with collections (as they are objects). They are
collected as normal objects by the GC.
EDIT :
public static void main(String[] args) {
int i = 5;
System.out.println(int.class);
Integer i1 = new Integer(5);
System.out.println(Integer.TYPE);
}
O/P : int
int
So, basically, both return an int. Integer.TYPE just returns the primitive type of the Integer class. It is true for any wrapper class
To me, the easiest way to understand int.class and Integer.class is to stop thinking Integer is a wrapper (which kind of implies it is "special"). In fact, it is easier, and probably more appropriate, to think of int as a special case.
Integer is just a normal Java class, nothing different from e.g. String. It derives from Object, operates like Object, At runtime you can create an instance of Integer, the instance takes an object-like memory layout, e.g. with a pointer at the beginning pointing to the Integer.class, which is what enables the polymorphic runtime behavior of java.
There is really nothing special yet about Integer. if you imagine a Java without boolean, int, long these primitives, but only with Integer, Boolean, Long etc, the type system is actually very consistent.
Conceptually, you can think of int as a special class introduced later for performance reasons. Initially, it has nothing to do with Integer. At the time when Java was created, maintaining an object-like memory layout for plain numbers is very expensive for arithmetic heavy programs. And most of the arithmetic operations do not even involve polymorphic dynamic dispatching at all. E.g. it is a lot less common to invoke methods such as toString on a number.
int is special in the sense that it is a class whose "instances" are laid out in memory with the common object structure stripped off - just four consecutive bytes with no extra meta data.
As a result, you cannot do 123.getClass() because the runtime memory layout of int 123 does not have a class pointer. int.class does exist, it is completely unrelated to Integer.class (yet). In a sense, int is more similar to Void, as in Void.class does exist, but you can never have object o where o.class == Void.class.
Java could just settle here, int is int, Integer is Integer. But what people realize is that although less common, it is still very useful to be able treat int as a normal Java object otherwise you will always have to maintain two sets of your methods at least - one that takes normal objects, and another one that takes primitives - even though performance is not your concern. In these scenarios, int is actually behaving like Integer. Java allows this conversion to automatically happen through the processes of auto-boxing, hence effectively making int and Integer related.
But int is still int, and Integer is still Integer (e.g. int.class != Integer.class). But one extra field is introduced to Integer.class to indicate it relates to int.class, that is Integer.TYPE. So Integer.TYPE == int.class. To me Integer.TYPE is just to capture the notional that Integer and int are related. How to use it, or whether it is even useful, is up to you.
In practice, int and Integer are still separate types where it matters. For example:
void accept (int value);
void accept (Integer value);
are still considered two overloads. Hence when working with reflection, you can use int.class and Integer.class to differentiate between the two.
When not working with reflection or some form of meta programming, because you cannot navigate to int.class from an object, it is relatively rare to see int.class to be used in your code. Sometimes it feels confusing because auto-boxing syntax seems to suggest you should be getting int.class:
int value = 1;
Class c = ((Object)value).getClass();
But that is just an illusion, as the moment you do ((Object)value), you are actually creating a new Integer instance with the same value.
The only time I need to explicitly work with int.class and Integer.class is to build a generics api <T> T getValue(String name, Class<T> type); where I need to differentiate if the api is allowed to return null:
int value = getValue("key", Integer.class); // will potentially throw NPE
int value = getValue("key", int.class); // will never throw NPE
In some code I see this:
private void compute(Long a, Long b, Long c) {
long result = a-(b+c);
...
It seems a bit strange that the result is stored in a primitive long instead of a Long object corresponding to its operands.
Are there any reason that a result should be stored as a primitive?
It seems a bit strange that the result is stored in a primitive long instead of a Long object corresponding to its operands.
No, what is "strange" is that you can use the + and - operators on Long objects. Before Java 5, this would have been a syntax error. Then autoboxing/unboxing was introduced. What you're seeing in this code is autounboxing: the operators require primtives, so the compiler automatically inserts a call to longValue() on the objects. The arithmetic is then performed on primitive long values, and the result is also a long that can be stored without further conversion on the variable.
As for why the code does this, the real question is why someone would use the Long type instead of long. Possible reasons:
The values come from some library/API that delivers Long values.
The values are stored in collections (List, Map), which cannot hold primitives.
Sloppiness or cargo cult programming.
The ability to have null values is required, e.g. to signal unavailable or uninitialized data.
Note that the ability of Long to hold null values means that the calculation (or more specifically, the longValue() calls inserted by the compiler) can fail with a NullPointerException - a possibility the code should deal with somehow.
The reason is obvious: result is declared as primitive.
The arithmetic operators + and - are not defined for boxed types (e.g. Long) but for primitive types (e.g. long).
The result is also a long. See Autoboxing and Unboxing tutorial
Autoboxing this into a Long would result in a small performance cost. It is also unnecessary because
We know it will be non-null (if a, b or c were null, a NullPointerException would occur).
It would be autoboxed implicitly if we use it later where a Long is required.
Based on your needs.I mean the decelaration.
Autoboxing and unboxing can happen anywhere where an object is expected and primitive type is available
Usually you should prefer using primitives, especially if you are certain they cannot be null. If you insist on using the boxed types always think extra hard about what happens when it is null.
Java will do the boxing and unboxing automatically for you, but staring at an int and wondering why you got a NullPointerException can be fun.
From Java 1.5 onwards, autoboxing and unboxing occurs implicitly whenever needed.
The following line:
long result = a-(b+c);
...asks Java to take the result of the expression using 3 Longs, and then store it in a primitive long. Before Java 5, it would complain about the types not matching - but these days it just assumes you mean what you say and automatically does the conversion from object to primitive type for you.
In this example however, unless there's some other good reason not presented here, there's absolutely no point having the parameters as the boxed, object type in the first place.
As per the javadoc
Boxing conversion converts expressions of primitive
type to corresponding expressions of reference type.
Specifically, the following nine conversions are called the boxing conversions:
From type boolean to type Boolean
From type byte to type Byte
From type short to type Short
From type char to type Character
From type int to type Integer
From type long to type Long
From type float to type Float
From type double to type Double
From the null type to the null type
Ideally, boxing a given primitive value p, would always yield an identical reference.
In practice, this may not be feasible using existing implementation techniques. The
rules above are a pragmatic compromise. The final clause above requires that certain
common values always be boxed into indistinguishable objects. The implementation may
cache these, lazily or eagerly. For other values, this formulation disallows any
assumptions about the identity of the boxed values on the programmer's part. This would
allow (but not require) sharing of some or all of these references.
This ensures that in most common cases, the behavior will be the desired one, without
imposing an undue performance penalty, especially on small devices. Less memory-limited
implementations might, for example, cache all char and short values, as well as int and
long values in the range of -32K to +32K.`
Here is the Oracle Doc source
The answer for your doubt is
autoboxing and autounboxing in Java which converts from primitive to wrapper class objects and vice versa respectively.
autoboxing means internally compiler uses valueOf() method of primitive classes ans autounboxing means internally compiler uses xxxValue() method.
Suppose for
private void compute(Long a, Long b, Long c) {
long result = a-(b+c);
its makes this conversion a.longValue()-(b.longValue()+c.longValue())
Which means even before your statement performs addition the compiler provides the primitives of long type as input to your operands
Remember that this goes in hand as JAVA is statically and strongly typed language.
Hence you get long type output
I hope i cleared your doubt
In Java, I can do the following to succinctly guard against a NullPointerException:
if ("myString".equals(someOtherString))
But I cannot do the same with Integers, e.g.
if (5.equals(someOtherInteger))
I get a compile-time error. Any ideas on why this design decision was made? Or any resources that might explain it? Thanks in advance.
EDIT: someOtherInteger is an Integer, not an int.
String has always been an object in Java. There is no autoboxing for strings, and there can't be in principle. Autoboxing from the primitive int to the Integer object has been introduced fairly recently.
It is valid to ask why trying to access member variables of primitives doesn't invoke autoboxing (95.toString(radix) would actually be pretty convenient), but I imagine that the reason is that it wasn't considered a likely use-case, since since almost every wrappedPrimitive.method() has an equivalent WrapperClass.method( primitive ) version.
equals() is usually unnecessary for primitive types since == is already there. However, you do make a good case for it as a null-guard... 5 == integerInstance will try to unbox the instance, and throw a NullPointerException if the instance is null, unfortunately. (I didn't fully appreciate your point at first.)
That said, it would be really cool if we could hear from someone working on Java either currently or at the introduction of autoboxing about whether they considered this sort of functionality.
The JLS specifies that boxing conversions can only occur during assignment conversions, method invocation conversions, or casting conversions. Since you are neither assigning 5 to a variable, passing it as an argument to a method, nor explicitly casting it to Integer, it will not be autoboxed for you.
Assignment conversion (§5.2, §15.26) converts the type of an
expression to the type of a specified variable.
Assignment conversion may cause an OutOfMemoryError (as a result of
boxing conversion (§5.1.7)), a NullPointerException (as a result of
unboxing conversion (§5.1.8)), or a ClassCastException (as a result of
an unchecked conversion (§5.1.9)) to be thrown at run-time.
Method invocation conversion (§5.3, §15.9, §15.12) is applied to each
argument in a method or constructor invocation and, except in one
case, performs the same conversions that assignment conversion does.
Method invocation conversion may cause an OutOfMemoryError (as a
result of boxing conversion (§5.1.7)), a NullPointerException (as a
result of unboxing conversion (§5.1.8)), or a ClassCastException (as a
result of an unchecked conversion (§5.1.9)) to be thrown at run-time.
Casting contexts allow the use of one of:
...
a boxing conversion (§5.1.7) optionally followed by a widening
reference conversion (§5.1.5)
you can use
if (someOtherInteger!=null && someOtherInteger == 5)
I suspect that autoboxing is not implemented for the literal 5, whereas it is for a string myString, as a safety measure. It's safe to autobox a syntactic structure that is prepended and appended with double quotation marks "", because it's unlikely that the quotation marks are unintended, so the user's intention is clear and type-safety is not compromised.
However, the literal 5 could be a typo on the user's part - or it could be intended to be a string, rather than an integer. Therefore, to maintain the benefit that variables must be declared before use in object-oriented programming in order to prevent typos (among many other advantages) (even if it's implicit, as in the case of autoboxing), 5 is not autoboxed.
Here is a bit of reading on the different comparisons:
http://www.leepoint.net/notes-java/data/expressions/22compareobjects.html
Not sure if it was a built in design to reject int
If you do
Integer s=5;
Integer d=5;
if(d.equals(s)){
System.out.println("Fun");
}
It works just fine.
int is a primitive type it doesn't support any methods itself. To compare 2 ints you simply use the == convention as in:
if(a == b)
There is an Integer class that is a wrapper for an int that supports some other method calls
Edit:
Based on your edit you want to compare to Integer but the problem is the literal 5 isn't an Integer you have to create a new integer for it.
Integer myInt = 5;
if(myInt.equals(someOtherInteger)) ...
This design is inherent in the fact that primitives don't have any methods. As to whether primitives should support methods (or simply not exist) is integral to the debate as to whether Java is a Pure Object Oriented Language or not (many say no due to the fact that primitives exist).
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.
The following code throws NullPointerException:
int num = Integer.getInteger("123");
Is my compiler invoking getInteger on null since it's static? That doesn't make any sense!
What's happening?
The Big Picture
There are two issues at play here:
Integer getInteger(String) doesn't do what you think it does
It returns null in this case
the assignment from Integer to int causes auto-unboxing
Since the Integer is null, NullPointerException is thrown
To parse (String) "123" to (int) 123, you can use e.g. int Integer.parseInt(String).
References
Java Language Guide/Autoboxing
Integer API references
static int parseInt(String)
static Integer getInteger(String)
On Integer.getInteger
Here's what the documentation have to say about what this method does:
public static Integer getInteger(String nm): Determines the integer value of the system property with the specified name. If there is no property with the specified name, if the specified name is empty or null, or if the property does not have the correct numeric format, then null is returned.
In other words, this method has nothing to do with parsing a String to an int/Integer value, but rather, it has to do with System.getProperty method.
Admittedly this can be quite a surprise. It's unfortunate that the library has surprises like this, but it does teach you a valuable lesson: always look up the documentation to confirm what a method does.
Coincindentally, a variation of this problem was featured in Return of the Puzzlers: Schlock and Awe (TS-5186), Josh Bloch and Neal Gafter's 2009 JavaOne Technical Session presentation. Here's the concluding slide:
The Moral
Strange and terrible methods lurk in libraries
Some have innocuous sounding names
If your code misbehaves
Make sure you're calling the right methods
Read the library documentation
For API designers
Don't violate the principle of least astonishment
Don't violate the abstraction hierarchy
Don't use similar names for wildly different behaviors
For completeness, there are also these methods that are analogous to Integer.getInteger:
Boolean.getBoolean(String)
Long.getLong(String)
Related questions
Most Astonishing Violation of the Principle of Least Astonishment
Most awkward/misleading method in Java Base API ?
On autounboxing
The other issue, of course, is how the NullPointerException gets thrown. To focus on this issue, we can simplify the snippet as follows:
Integer someInteger = null;
int num = someInteger; // throws NullPointerException!!!
Here's a quote from Effective Java 2nd Edition, Item 49: Prefer primitive types to boxed primitives:
In summary, use primitives in preference to boxed primitive whenever you have the choice. Primitive types are simpler and faster. If you must use boxed primitives, be careful! Autoboxing reduces the verbosity, but not the danger, of using boxed primitives. When your program compares two boxed primitives with the == operator, it does an identity comparison, which is almost certainly not what you want. When your program does mixed-type computations involving boxed and unboxed primitives, it does unboxing, and when your program does unboxing, it can throw NullPointerException. Finally, when your program boxes primitive values, it can result in costly and unnecessary object creations.
There are places where you have no choice but to use boxed primitives, e.g. generics, but otherwise you should seriously consider if a decision to use boxed primitives is justified.
Related questions
What is the difference between an int and an Integer in Java/C#?
Why does autoboxing in Java allow me to have 3 possible values for a boolean?
Is it guaranteed that new Integer(i) == i in Java? (YES!!!)
When comparing two Integers in Java does auto-unboxing occur? (NO!!!)
Java noob: generics over objects only? (yes, unfortunately)
From http://konigsberg.blogspot.com/2008/04/integergetinteger-are-you-kidding-me.html:
getInteger 'Determines the integer value of the system property with the specified name.'
You want this:
Integer.parseInt("123")
Please check documentation of the method getInteger().
In this method, the String parameter is a system property that determines the integer value of the system property with the specified name. "123" is not the name of any system property, as discussed here.
If you want to convert this String to int, then use the method as
int num = Integer.parseInt("123").