I am in the middle of an android project and was trying to create a new package in it.
com.mycompany.myprojectname.new
Well, Eclipse is not letting me to create it and is showing this error:
Invalid package name. 'new' is not a valid Java identifier
I never knew package name has reserved words, which we cannot use. My questions are;
Is this an Eclipse thing? or a Java thing? I tried a pure Java project
(not Android), just to check, but there also I got the same error.
What are other reserved words that is not allowed?
Is there any documentation about this?
Yes, this is a general Java thing.
The list of reserved words can be found here. They are:
abstract continue for new switch
assert default goto package synchronized
boolean do if private this
break double implements protected throw
byte else import public throws
case enum instanceof return transient
catch extends int short try
char final interface static void
class finally long strictfp volatile
const float native super while
Documentation on the fact that reserved words can not be used in package names if found in the package naming tutorial, among other places.
The authoritative source is (as always) the Java Language Specification, specifically:
§ 3.9 Keywords and
§ 3.8 Identifiers
An identifier cannot have the same spelling (Unicode character sequence) as a keyword (§3.9), boolean literal (§3.10.3), or the null literal (§3.10.7), or a compile-time error occurs.
See docs here:
http://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html
In some cases, the internet domain name may not be a valid package
name. This can occur if the domain name contains a hyphen or other
special character, if the package name begins with a digit or other
character that is illegal to use as the beginning of a Java name, or
if the package name contains a reserved Java keyword, such as "int".
new is a java keyword. Use some other word instead of it.
YMMV but be careful naming packages target; may mess with your code repository and/or IDE
Related
I tried to search on stack overflow regarding the issue I am facing but did not find any satisfactory answer. So, please read my question first and suggest.
I have a .co.in domain that I want to use to prepare packages. But as per new java conventions(?) the keyword 'in' can not be used at starting of package name. Hence, I am getting problems in building applications.
I have problem in building android application with flutter due to package name format in.co.mydomain.myapp.
The problem persists in JavaFX applications also where I am using hibernate ORM. When application is run in debug mode, I see HQL queries generated with fully qualified names of entity class resulting into in.co.mydomain.myapp.entities.Student, here also 'in' keyword is a reserved keyword of SQL queries hence it throws errors.
I need expert advice in such a horrible situation I am facing.
The whole thing works without any problem when I rename package to com.mydomain.myapp
In some cases, the internet domain name may not be a valid package name. This can occur if the domain name contains a hyphen or other special character, if the package name begins with a digit or other character that is illegal to use as the beginning of a Java name, or if the package name contains a reserved Java keyword, such as "int". In this event, the suggested convention is to add an underscore.
https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html
So you could use in_.co.mydomain
oracle suggested that,
In some cases, the internet domain name may not be a valid package
name. This can occur if the domain name contains a hyphen or other
special character, if the package name begins with a digit or other
character that is illegal to use as the beginning of a Java name, or
if the package name contains a reserved Java keyword, such as "int".
In this event, the suggested convention is to add an underscore. For
example:
Is there any tricky way to use Java reserved words as variable, method, class, interface, package, or enum constant names?
This is a valid question. Such a thing is possible in other languages. In C#, prefix the identifier with # (as asked before); in Delphi, prefix with &. But Java offers no such feature (partly because it doesn't really need to interact with identifiers defined by other languages the way the .Net world does).
No, there is no way. That's why they're labeled "reserved".
Most often this issue comes up for "class", in this case it is customary to write "clazz".
Strictly speaking you can't, unless you get your hands on a buggy compiler implementation that doesn't adhere to the Java language spec.
But where there's a will, there's a way. Copy the following code into your IDE, switch the source file encoding to UTF-16 and here we go:
public class HelloWorld {
public static void main(String[] args) {
HelloWorld.nеw();
}
public static void nеw () {
System.out.println("Hello,World");
}
}
This code is a well-formed and valid Java class. However, as you have guessed there is a little trick: the 'е' character within "new" identifier does not belong to the ASCII character set, it is actually a cyrrilic 'е' (prounanced 'YE').
Current Java language spec explicitly permits, and this an important point to make, the use of Unicode for naming identifiers. That means that one has an ability to freely call her or his classes in French, Chinise or Russian if they wish. It is also possible to mix and match the alphabets within code. And historically, some letters within Latin and other alphabets are lookalikes.
As a result: no, you can't use the reserved words as identifiers, but you can use identifiers that look exactly like reserved words.
Whether anyone should be doing it is a totally different matter.
No, you can't do this. For more information please go to JLS Sections 3.8, 3.9
The following character sequences,
formed from ASCII letters, are
reserved for use as keywords and
cannot be used as identifiers (§3.8):
Keyword: one of
abstract continue for new switch
assert default if package synchronized
boolean do goto private this
break double implements protected throw
byte else import public throws
case enum instanceof return transient
catch extends int short try
char final interface static void
class finally long strictfp volatile
const float native super while
Yes, there is.
You have to use reserved words from the future.
Like what happened with different methods called assert() in pre-1.4 code.
Hope it helps!
In Scala you can use backticks. For example: myVarialbe.`class`
Not sure what you're trying to do, but $ is a valid character in identifiers, so you could do, say:
int $return = 5;
It looks a little weird, but it does work.
I know it's old question still, might help someone.
It's possible by using GSON's Field Naming Support
eg.
#SerializedName("new")
private String New;
public String getNew ()
{
return New;
}
public void setNew (String aNew)
{
New = aNew;
}
Huh? Why would you want to do that? You can write them in l33t, that will fool the compiler.
class cl4ss {
String r3turn() {
return "but why?";
}
}
There is no way to use reserved words with the javac compiler.
Technically, you can edit the names inside the class file once it's compiled to be anything you want: at that stage, the VM doesn't care, because it's not dealing with source code any more. I believe some obfuscators use this technique.
PL/1 (a 1960's IBM mainframe programming language still around today) rather famously required that while some words act like keywords in certain contexts, all words can be used as identifiers. This isn't even that hard to do in a parser if you set out to be consistent about it. PL/1 was considered to a rather big langauge, and the langauge committee worried that many programmers wouldn't learn all of it, and then would get suprised when they tried to use the keyword from a part they didn't know as an identifier.
So you could write things like:
IF BEGIN=ELSE THEN CALL=3 ELSE CALL FOO(ENDIF) ENDIF
As others have noted here, the ability to do this isn't a recommendation.
The Java designers decided the number of keywords in the langauge was modest, and reserved the set. They even reserved 'GOTO', which isn't actually allowed in any real Java program.
If you really need to use a field/local variable/method named the same as a reserved word, I suggest appending an underscore at the end of the name:
// JPA entity mapping class:
private Boolean void_;
public Boolean getVoid_() { ... }
void setVoid_(Boolean void_) { ... }
It is a more readable choice (IMHO) than appending chars at the beginning of the name (fVoid, aVoid, vVoid, etc.)
The code above is a real world case that happened to me, working with a legacy database, in which the invoice table had a field named void indicating whether the document had been voided or not.
I am porting apitest to jogl, one of the directory is called Null, so I was simply trying to use null as package name and Netbeans complains that it is not valid.
I know I am splitting hairs, but I am just curious.. I didn't find anything here or on google..
Does it have to do with the reflection, the name retrieving, something similar or something else?
Or is it like this by design, same as operator overload?
Edit: I know it is a reserved word, but in code..
An identifier cannot be named null as the language specification mentions:
An identifier cannot have the same spelling (Unicode character sequence) as a keyword (§3.9), boolean literal (§3.10.3), or the null literal (§3.10.7), or a compile-time error occurs.
in this case, null is a literal (null literal)in java language, you can not used as identifier...
According to Oracle:
Naming Convention:
Package names are written in all lower case to avoid conflict with the
names of classes or interfaces.
...
In your case Following reference is important:
In some cases, the internet domain name may not be a valid package
name. This can occur if the domain name contains a hyphen or other
special character, if the package name begins with a digit or other
character that is illegal to use as the beginning of a Java name, or
if the package name contains a reserved Java keyword, such as "int".
In this event, the suggested convention is to add an underscore. For
example:
I believe it is like this only by design.
As per Java Language Specification, 'null' is a keyword and you cannot use any of the keywords to identify any other thing than its prime purpose.
Although you can use Null with capitalized 'N' and similar for all other keywords that does not match case with predefined keywords.
Java discourages use of such poor naming practices.
null is a value given to identifier. package is a identifier. it cannot be a value.
Today I was naming a package in a project which would contain code related to a concept called an "access structure".
Now, naming this package "com.myemployer.project.component.accessstructures" seems unappealing and difficult to read because of the triple "S". (The higher level packages are not actually named "project" and "component").
I was tempted to use "...component.access_structures"
I couldn't find anything mentioned in the Java conventions on Oracle's site . And a brief web search brought up nothing.
What is the official convention for names like this?
From Oracle Docs
Package names are written in all lower case to avoid conflict with the names of classes or interfaces.
Companies use their reversed Internet domain name to begin their package names—for example, com.example.mypackage for a package named mypackage created by a programmer at example.com.
Name collisions that occur within a single company need to be handled by convention within that company, perhaps by including the region or the project name after the company name (for example, com.example.region.mypackage).
In some cases, the internet domain name may not be a valid package name. This can occur if the domain name contains a hyphen or other special character, if the package name begins with a digit or other character that is illegal to use as the beginning of a Java name, or if the package name contains a reserved Java keyword, such as "int". In this event, the suggested convention is to add an underscore
Although this text doesn't specify your exact case, it does say that for an invalid package name we should use an underscore. One could argue that accessStructures is how we would define a method in Java and thus naming a package like that could be confusing.
Overall, it is really up to you.
If you want to keep with this convention, I believe you should name your package:
com.myemployer.project.component.access_structures
Also you can look up synonyms and find alternatives that would give less confusion. Some I quickly found:
accessframework
accessfactory
accessarch (accessarchitecture)
accessconstructs
I dont think there is any standard for that. People follow different naming conventions as per there convenience and readability. But most of the programmers find camel case naming as the most convenient. You can name it like accessStructure
Found one Oracle Doc which recommends to use the package name with all small letters
Package names are written in all lower case to avoid conflict with the
names of classes or interfaces.
According to docs you can't use camelCase for package naming. It's ok to use snake_case for package naming in some cases, but it is more appropriate if you can't use your domain properly, because of the hyphen sign in it or it starts with numbers. But it has to be rather an exception from the rule than the rule.
If I were you I would rephrase it. For example: accessstructures -> accesscore
Is there any tricky way to use Java reserved words as variable, method, class, interface, package, or enum constant names?
This is a valid question. Such a thing is possible in other languages. In C#, prefix the identifier with # (as asked before); in Delphi, prefix with &. But Java offers no such feature (partly because it doesn't really need to interact with identifiers defined by other languages the way the .Net world does).
No, there is no way. That's why they're labeled "reserved".
Most often this issue comes up for "class", in this case it is customary to write "clazz".
Strictly speaking you can't, unless you get your hands on a buggy compiler implementation that doesn't adhere to the Java language spec.
But where there's a will, there's a way. Copy the following code into your IDE, switch the source file encoding to UTF-16 and here we go:
public class HelloWorld {
public static void main(String[] args) {
HelloWorld.nеw();
}
public static void nеw () {
System.out.println("Hello,World");
}
}
This code is a well-formed and valid Java class. However, as you have guessed there is a little trick: the 'е' character within "new" identifier does not belong to the ASCII character set, it is actually a cyrrilic 'е' (prounanced 'YE').
Current Java language spec explicitly permits, and this an important point to make, the use of Unicode for naming identifiers. That means that one has an ability to freely call her or his classes in French, Chinise or Russian if they wish. It is also possible to mix and match the alphabets within code. And historically, some letters within Latin and other alphabets are lookalikes.
As a result: no, you can't use the reserved words as identifiers, but you can use identifiers that look exactly like reserved words.
Whether anyone should be doing it is a totally different matter.
No, you can't do this. For more information please go to JLS Sections 3.8, 3.9
The following character sequences,
formed from ASCII letters, are
reserved for use as keywords and
cannot be used as identifiers (§3.8):
Keyword: one of
abstract continue for new switch
assert default if package synchronized
boolean do goto private this
break double implements protected throw
byte else import public throws
case enum instanceof return transient
catch extends int short try
char final interface static void
class finally long strictfp volatile
const float native super while
Yes, there is.
You have to use reserved words from the future.
Like what happened with different methods called assert() in pre-1.4 code.
Hope it helps!
In Scala you can use backticks. For example: myVarialbe.`class`
Not sure what you're trying to do, but $ is a valid character in identifiers, so you could do, say:
int $return = 5;
It looks a little weird, but it does work.
I know it's old question still, might help someone.
It's possible by using GSON's Field Naming Support
eg.
#SerializedName("new")
private String New;
public String getNew ()
{
return New;
}
public void setNew (String aNew)
{
New = aNew;
}
Huh? Why would you want to do that? You can write them in l33t, that will fool the compiler.
class cl4ss {
String r3turn() {
return "but why?";
}
}
There is no way to use reserved words with the javac compiler.
Technically, you can edit the names inside the class file once it's compiled to be anything you want: at that stage, the VM doesn't care, because it's not dealing with source code any more. I believe some obfuscators use this technique.
PL/1 (a 1960's IBM mainframe programming language still around today) rather famously required that while some words act like keywords in certain contexts, all words can be used as identifiers. This isn't even that hard to do in a parser if you set out to be consistent about it. PL/1 was considered to a rather big langauge, and the langauge committee worried that many programmers wouldn't learn all of it, and then would get suprised when they tried to use the keyword from a part they didn't know as an identifier.
So you could write things like:
IF BEGIN=ELSE THEN CALL=3 ELSE CALL FOO(ENDIF) ENDIF
As others have noted here, the ability to do this isn't a recommendation.
The Java designers decided the number of keywords in the langauge was modest, and reserved the set. They even reserved 'GOTO', which isn't actually allowed in any real Java program.
If you really need to use a field/local variable/method named the same as a reserved word, I suggest appending an underscore at the end of the name:
// JPA entity mapping class:
private Boolean void_;
public Boolean getVoid_() { ... }
void setVoid_(Boolean void_) { ... }
It is a more readable choice (IMHO) than appending chars at the beginning of the name (fVoid, aVoid, vVoid, etc.)
The code above is a real world case that happened to me, working with a legacy database, in which the invoice table had a field named void indicating whether the document had been voided or not.